make Version 3.77.
make
override Directive
make
make to Update Archive Files
make
@shorttitlepage GNU Make Copyright (C) 1988, '89, '90, '91, '92, '93, '94, '95, '96, '97 Free Software Foundation, Inc.
Published by the Free Software Foundation
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Boston, MA 02111-1307 USA
Printed copies are available for $20 each.
ISBN 1-882114-80-9
Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies.
Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one.
Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that this permission notice may be stated in a translation approved by the Free Software Foundation.
Cover art by Etienne Suvasa.
make
The make utility automatically determines which pieces of a large
program need to be recompiled, and issues commands to recompile them.
This manual describes GNU make, which was implemented by Richard
Stallman and Roland McGrath. GNU make conforms to section 6.2 of
IEEE Standard 1003.2-1992 (POSIX.2).
Our examples show C programs, since they are most common, but you can use
make with any programming language whose compiler can be run with a
shell command. Indeed, make is not limited to programs. You can
use it to describe any task where some files must be updated automatically
from others whenever the others change.
To prepare to use make, you must write a file called
the makefile that describes the relationships among files
in your program and provides commands for updating each file.
In a program, typically, the executable file is updated from object
files, which are in turn made by compiling source files.
Once a suitable makefile exists, each time you change some source files, this simple shell command:
make
suffices to perform all necessary recompilations. The make program
uses the makefile data base and the last-modification times of the files to
decide which of the files need to be updated. For each of those files, it
issues the commands recorded in the data base.
You can provide command line arguments to make to control which
files should be recompiled, or how. See section How to Run make.
If you are new to make, or are looking for a general
introduction, read the first few sections of each chapter, skipping the
later sections. In each chapter, the first few sections contain
introductory or general information and the later sections contain
specialized or technical information.
The exception is section An Introduction to Makefiles,
all of which is introductory.
If you are familiar with other make programs, see section Features of GNU make, which lists the enhancements GNU
make has, and section Incompatibilities and Missing Features, which explains the few things GNU make lacks that
others have.
For a quick summary, see section Summary of Options, section Quick Reference, and section Special Built-in Target Names.
If you have problems with GNU make or think you've found a bug,
please report it to the developers; we cannot promise to do anything but
we might well want to fix it.
Before reporting a bug, make sure you've actually found a real bug. Carefully reread the documentation and see if it really says you can do what you're trying to do. If it's not clear whether you should be able to do something or not, report that too; it's a bug in the documentation!
Before reporting a bug or trying to fix it yourself, try to isolate it
to the smallest possible makefile that reproduces the problem. Then
send us the makefile and the exact results make gave you. Also
say what you expected to occur; this will help us decide whether the
problem was really in the documentation.
Once you've got a precise problem, please send electronic mail to:
bug-make@gnu.org
Please include the version number of make you are using. You can
get this information with the command `make --version'.
Be sure also to include the type of machine and operating system you are
using. If possible, include the contents of the file `config.h'
that is generated by the configuration process.
You need a file called a makefile to tell make what to do.
Most often, the makefile tells make how to compile and link a
program.
In this chapter, we will discuss a simple makefile that describes how to
compile and link a text editor which consists of eight C source files
and three header files. The makefile can also tell make how to
run miscellaneous commands when explicitly asked (for example, to remove
certain files as a clean-up operation). To see a more complex example
of a makefile, see section Complex Makefile Example.
When make recompiles the editor, each changed C source file
must be recompiled. If a header file has changed, each C source file
that includes the header file must be recompiled to be safe. Each
compilation produces an object file corresponding to the source file.
Finally, if any source file has been recompiled, all the object files,
whether newly made or saved from previous compilations, must be linked
together to produce the new executable editor.
A simple makefile consists of "rules" with the following shape:
target ... : dependencies ...
command
...
...
A target is usually the name of a file that is generated by a program; examples of targets are executable or object files. A target can also be the name of an action to carry out, such as `clean' (see section Phony Targets).
A dependency is a file that is used as input to create the target. A target often depends on several files.
A command is an action that make carries out.
A rule may have more than one command, each on its own line.
Please note: you need to put a tab character at the beginning of
every command line! This is an obscurity that catches the unwary.
Usually a command is in a rule with dependencies and serves to create a target file if any of the dependencies change. However, the rule that specifies commands for the target need not have dependencies. For example, the rule containing the delete command associated with the target `clean' does not have dependencies.
A rule, then, explains how and when to remake certain files
which are the targets of the particular rule. make carries out
the commands on the dependencies to create or update the target. A
rule can also explain how and when to carry out an action.
See section Writing Rules.
A makefile may contain other text besides rules, but a simple makefile need only contain rules. Rules may look somewhat more complicated than shown in this template, but all fit the pattern more or less.
Here is a straightforward makefile that describes the way an
executable file called edit depends on eight object files
which, in turn, depend on eight C source and three header files.
In this example, all the C files include `defs.h', but only those defining editing commands include `command.h', and only low level files that change the editor buffer include `buffer.h'.
edit : main.o kbd.o command.o display.o \
insert.o search.o files.o utils.o
cc -o edit main.o kbd.o command.o display.o \
insert.o search.o files.o utils.o
main.o : main.c defs.h
cc -c main.c
kbd.o : kbd.c defs.h command.h
cc -c kbd.c
command.o : command.c defs.h command.h
cc -c command.c
display.o : display.c defs.h buffer.h
cc -c display.c
insert.o : insert.c defs.h buffer.h
cc -c insert.c
search.o : search.c defs.h buffer.h
cc -c search.c
files.o : files.c defs.h buffer.h command.h
cc -c files.c
utils.o : utils.c defs.h
cc -c utils.c
clean :
rm edit main.o kbd.o command.o display.o \
insert.o search.o files.o utils.o
We split each long line into two lines using backslash-newline; this is like using one long line, but is easier to read.
To use this makefile to create the executable file called `edit', type:
make
To use this makefile to delete the executable file and all the object files from the directory, type:
make clean
In the example makefile, the targets include the executable file `edit', and the object files `main.o' and `kbd.o'. The dependencies are files such as `main.c' and `defs.h'. In fact, each `.o' file is both a target and a dependency. Commands include `cc -c main.c' and `cc -c kbd.c'.
When a target is a file, it needs to be recompiled or relinked if any of its dependencies change. In addition, any dependencies that are themselves automatically generated should be updated first. In this example, `edit' depends on each of the eight object files; the object file `main.o' depends on the source file `main.c' and on the header file `defs.h'.
A shell command follows each line that contains a target and
dependencies. These shell commands say how to update the target file.
A tab character must come at the beginning of every command line to
distinguish commands lines from other lines in the makefile. (Bear in
mind that make does not know anything about how the commands
work. It is up to you to supply commands that will update the target
file properly. All make does is execute the commands in the rule
you have specified when the target file needs to be updated.)
The target `clean' is not a file, but merely the name of an
action. Since you
normally
do not want to carry out the actions in this rule, `clean' is not a dependency of any other rule.
Consequently, make never does anything with it unless you tell
it specifically. Note that this rule not only is not a dependency, it
also does not have any dependencies, so the only purpose of the rule
is to run the specified commands. Targets that do not refer to files
but are just actions are called phony targets. See section Phony Targets, for information about this kind of target. See section Errors in Commands, to see how to cause make to ignore errors
from rm or any other command.
make Processes a Makefile
By default, make starts with the first target (not targets whose
names start with `.'). This is called the default goal.
(Goals are the targets that make strives ultimately to
update. See section Arguments to Specify the Goals.)
In the simple example of the previous section, the default goal is to update the executable program `edit'; therefore, we put that rule first.
Thus, when you give the command:
make
make reads the makefile in the current directory and begins by
processing the first rule. In the example, this rule is for relinking
`edit'; but before make can fully process this rule, it
must process the rules for the files that `edit' depends on,
which in this case are the object files. Each of these files is
processed according to its own rule. These rules say to update each
`.o' file by compiling its source file. The recompilation must
be done if the source file, or any of the header files named as
dependencies, is more recent than the object file, or if the object
file does not exist.
The other rules are processed because their targets appear as
dependencies of the goal. If some other rule is not depended on by the
goal (or anything it depends on, etc.), that rule is not processed,
unless you tell make to do so (with a command such as
make clean).
Before recompiling an object file, make considers updating its
dependencies, the source file and header files. This makefile does not
specify anything to be done for them--the `.c' and `.h' files
are not the targets of any rules--so make does nothing for these
files. But make would update automatically generated C programs,
such as those made by Bison or Yacc, by their own rules at this time.
After recompiling whichever object files need it, make decides
whether to relink `edit'. This must be done if the file
`edit' does not exist, or if any of the object files are newer than
it. If an object file was just recompiled, it is now newer than
`edit', so `edit' is relinked.
Thus, if we change the file `insert.c' and run make,
make will compile that file to update `insert.o', and then
link `edit'. If we change the file `command.h' and run
make, make will recompile the object files `kbd.o',
`command.o' and `files.o' and then link the file `edit'.
In our example, we had to list all the object files twice in the rule for `edit' (repeated here):
edit : main.o kbd.o command.o display.o \
insert.o search.o files.o utils.o
cc -o edit main.o kbd.o command.o display.o \
insert.o search.o files.o utils.o
Such duplication is error-prone; if a new object file is added to the system, we might add it to one list and forget the other. We can eliminate the risk and simplify the makefile by using a variable. Variables allow a text string to be defined once and substituted in multiple places later (see section How to Use Variables).
It is standard practice for every makefile to have a variable named
objects, OBJECTS, objs, OBJS, obj,
or OBJ which is a list of all object file names. We would
define such a variable objects with a line like this in the
makefile:
objects = main.o kbd.o command.o display.o \
insert.o search.o files.o utils.o
Then, each place we want to put a list of the object file names, we can substitute the variable's value by writing `$(objects)' (see section How to Use Variables).
Here is how the complete simple makefile looks when you use a variable for the object files:
objects = main.o kbd.o command.o display.o \
insert.o search.o files.o utils.o
edit : $(objects)
cc -o edit $(objects)
main.o : main.c defs.h
cc -c main.c
kbd.o : kbd.c defs.h command.h
cc -c kbd.c
command.o : command.c defs.h command.h
cc -c command.c
display.o : display.c defs.h buffer.h
cc -c display.c
insert.o : insert.c defs.h buffer.h
cc -c insert.c
search.o : search.c defs.h buffer.h
cc -c search.c
files.o : files.c defs.h buffer.h command.h
cc -c files.c
utils.o : utils.c defs.h
cc -c utils.c
clean :
rm edit $(objects)
make Deduce the Commands
It is not necessary to spell out the commands for compiling the individual
C source files, because make can figure them out: it has an
implicit rule for updating a `.o' file from a correspondingly
named `.c' file using a `cc -c' command. For example, it will
use the command `cc -c main.c -o main.o' to compile `main.c' into
`main.o'. We can therefore omit the commands from the rules for the
object files. See section Using Implicit Rules.
When a `.c' file is used automatically in this way, it is also automatically added to the list of dependencies. We can therefore omit the `.c' files from the dependencies, provided we omit the commands.
Here is the entire example, with both of these changes, and a variable
objects as suggested above:
objects = main.o kbd.o command.o display.o \
insert.o search.o files.o utils.o
edit : $(objects)
cc -o edit $(objects)
main.o : defs.h
kbd.o : defs.h command.h
command.o : defs.h command.h
display.o : defs.h buffer.h
insert.o : defs.h buffer.h
search.o : defs.h buffer.h
files.o : defs.h buffer.h command.h
utils.o : defs.h
.PHONY : clean
clean :
-rm edit $(objects)
This is how we would write the makefile in actual practice. (The complications associated with `clean' are described elsewhere. See section Phony Targets, and section Errors in Commands.)
Because implicit rules are so convenient, they are important. You will see them used frequently.
When the objects of a makefile are created only by implicit rules, an alternative style of makefile is possible. In this style of makefile, you group entries by their dependencies instead of by their targets. Here is what one looks like:
objects = main.o kbd.o command.o display.o \
insert.o search.o files.o utils.o
edit : $(objects)
cc -o edit $(objects)
$(objects) : defs.h
kbd.o command.o files.o : command.h
display.o insert.o search.o files.o : buffer.h
Here `defs.h' is given as a dependency of all the object files; `command.h' and `buffer.h' are dependencies of the specific object files listed for them.
Whether this is better is a matter of taste: it is more compact, but some people dislike it because they find it clearer to put all the information about each target in one place.
Compiling a program is not the only thing you might want to write rules for. Makefiles commonly tell how to do a few other things besides compiling a program: for example, how to delete all the object files and executables so that the directory is `clean'.
Here is how we
could write a make rule for cleaning our example editor:
clean:
rm edit $(objects)
In practice, we might want to write the rule in a somewhat more complicated manner to handle unanticipated situations. We would do this:
.PHONY : clean
clean :
-rm edit $(objects)
This prevents make from getting confused by an actual file
called `clean' and causes it to continue in spite of errors from
rm. (See section Phony Targets, and section Errors in Commands.)
A rule such as this should not be placed at the beginning of the
makefile, because we do not want it to run by default! Thus, in the
example makefile, we want the rule for edit, which recompiles
the editor, to remain the default goal.
Since clean is not a dependency of edit, this rule will not
run at all if we give the command `make' with no arguments. In
order to make the rule run, we have to type `make clean'.
See section How to Run make.
The information that tells make how to recompile a system comes from
reading a data base called the makefile.
Makefiles contain five kinds of things: explicit rules, implicit rules, variable definitions, directives, and comments. Rules, variables, and directives are described at length in later chapters.
objects
as a list of all object files (see section Variables Make Makefiles Simpler).
make to do something special while
reading the makefile. These include:
define directive, and perhaps within commands (where the shell
decides what is a comment). A line containing just a comment (with
perhaps spaces before it) is effectively blank, and is ignored.
By default, when make looks for the makefile, it tries the
following names, in order: `GNUmakefile', `makefile'
and `Makefile'.
Normally you should call your makefile either `makefile' or
`Makefile'. (We recommend `Makefile' because it appears
prominently near the beginning of a directory listing, right near other
important files such as `README'.) The first name checked,
`GNUmakefile', is not recommended for most makefiles. You should
use this name if you have a makefile that is specific to GNU
make, and will not be understood by other versions of
make. Other make programs look for `makefile' and
`Makefile', but not `GNUmakefile'.
If make finds none of these names, it does not use any makefile.
Then you must specify a goal with a command argument, and make
will attempt to figure out how to remake it using only its built-in
implicit rules. See section Using Implicit Rules.
If you want to use a nonstandard name for your makefile, you can specify
the makefile name with the `-f' or `--file' option. The
arguments `-f name' or `--file=name' tell
make to read the file name as the makefile. If you use
more than one `-f' or `--file' option, you can specify several
makefiles. All the makefiles are effectively concatenated in the order
specified. The default makefile names `GNUmakefile',
`makefile' and `Makefile' are not checked automatically if you
specify `-f' or `--file'.
The include directive tells make to suspend reading the
current makefile and read one or more other makefiles before continuing.
The directive is a line in the makefile that looks like this:
include filenames...
filenames can contain shell file name patterns.
Extra spaces are allowed and ignored at the beginning of the line, but
a tab is not allowed. (If the line begins with a tab, it will be
considered a command line.) Whitespace is required between
include and the file names, and between file names; extra
whitespace is ignored there and at the end of the directive. A
comment starting with `#' is allowed at the end of the line. If
the file names contain any variable or function references, they are
expanded. See section How to Use Variables.
For example, if you have three `.mk' files, `a.mk',
`b.mk', and `c.mk', and $(bar) expands to
bish bash, then the following expression
include foo *.mk $(bar)
is equivalent to
include foo a.mk b.mk c.mk bish bash
When make processes an include directive, it suspends
reading of the containing makefile and reads from each listed file in
turn. When that is finished, make resumes reading the
makefile in which the directive appears.
One occasion for using include directives is when several programs,
handled by individual makefiles in various directories, need to use a
common set of variable definitions
(see section Setting Variables) or pattern rules
(see section Defining and Redefining Pattern Rules).
Another such occasion is when you want to generate dependencies from
source files automatically; the dependencies can be put in a file that
is included by the main makefile. This practice is generally cleaner
than that of somehow appending the dependencies to the end of the main
makefile as has been traditionally done with other versions of
make. See section Generating Dependencies Automatically.
If the specified name does not start with a slash, and the file is not found in the current directory, several other directories are searched. First, any directories you have specified with the `-I' or `--include-dir' option are searched (see section Summary of Options). Then the following directories (if they exist) are searched, in this order: `prefix/include' (normally `/usr/local/include' (1)) `/usr/gnu/include', `/usr/local/include', `/usr/include'.
If an included makefile cannot be found in any of these directories, a
warning message is generated, but it is not an immediately fatal error;
processing of the makefile containing the include continues.
Once it has finished reading makefiles, make will try to remake
any that are out of date or don't exist.
See section How Makefiles Are Remade.
Only after it has tried to find a way to remake a makefile and failed,
will make diagnose the missing makefile as a fatal error.
If you want make to simply ignore a makefile which does not exist
and cannot be remade, with no error message, use the -include
directive instead of include, like this:
-include filenames...
This is acts like include in every way except that there is no
error (not even a warning) if any of the filenames do not exist.
For compatibility with some other make implementations,
sinclude is another name for -include.
MAKEFILES
If the environment variable MAKEFILES is defined, make
considers its value as a list of names (separated by whitespace) of
additional makefiles to be read before the others. This works much like
the include directive: various directories are searched for those
files (see section Including Other Makefiles). In addition, the
default goal is never taken from one of these makefiles and it is not an
error if the files listed in MAKEFILES are not found.
The main use of MAKEFILES is in communication between recursive
invocations of make (see section Recursive Use of make). It usually is not desirable to set the environment
variable before a top-level invocation of make, because it is
usually better not to mess with a makefile from outside. However, if
you are running make without a specific makefile, a makefile in
MAKEFILES can do useful things to help the built-in implicit
rules work better, such as defining search paths (see section Searching Directories for Dependencies).
Some users are tempted to set MAKEFILES in the environment
automatically on login, and program makefiles to expect this to be done.
This is a very bad idea, because such makefiles will fail to work if run by
anyone else. It is much better to write explicit include directives
in the makefiles. See section Including Other Makefiles.
Sometimes makefiles can be remade from other files, such as RCS or SCCS
files. If a makefile can be remade from other files, you probably want
make to get an up-to-date version of the makefile to read in.
To this end, after reading in all makefiles, make will consider
each as a goal target and attempt to update it. If a makefile has a
rule which says how to update it (found either in that very makefile or
in another one) or if an implicit rule applies to it (see section Using Implicit Rules), it will be updated if necessary. After
all makefiles have been checked, if any have actually been changed,
make starts with a clean slate and reads all the makefiles over
again. (It will also attempt to update each of them over again, but
normally this will not change them again, since they are already up to
date.)
If the makefiles specify a double-colon rule to remake a file with
commands but no dependencies, that file will always be remade
(see section Double-Colon Rules). In the case of makefiles, a makefile that has a
double-colon rule with commands but no dependencies will be remade every
time make is run, and then again after make starts over
and reads the makefiles in again. This would cause an infinite loop:
make would constantly remake the makefile, and never do anything
else. So, to avoid this, make will not attempt to
remake makefiles which are specified as double-colon targets but have no
dependencies.
If you do not specify any makefiles to be read with `-f' or
`--file' options, make will try the default makefile names;
see section What Name to Give Your Makefile. Unlike
makefiles explicitly requested with `-f' or `--file' options,
make is not certain that these makefiles should exist. However,
if a default makefile does not exist but can be created by running
make rules, you probably want the rules to be run so that the
makefile can be used.
Therefore, if none of the default makefiles exists, make will try
to make each of them in the same order in which they are searched for
(see section What Name to Give Your Makefile)
until it succeeds in making one, or it runs out of names to try. Note
that it is not an error if make cannot find or make any makefile;
a makefile is not always necessary.
When you use the `-t' or `--touch' option (see section Instead of Executing the Commands), you would not want to use an out-of-date makefile to decide which targets to touch. So the `-t' option has no effect on updating makefiles; they are really updated even if `-t' is specified. Likewise, `-q' (or `--question') and `-n' (or `--just-print') do not prevent updating of makefiles, because an out-of-date makefile would result in the wrong output for other targets. Thus, `make -f mfile -n foo' will update `mfile', read it in, and then print the commands to update `foo' and its dependencies without running them. The commands printed for `foo' will be those specified in the updated contents of `mfile'.
However, on occasion you might actually wish to prevent updating of even the makefiles. You can do this by specifying the makefiles as goals in the command line as well as specifying them as makefiles. When the makefile name is specified explicitly as a goal, the options `-t' and so on do apply to them.
Thus, `make -f mfile -n mfile foo' would read the makefile `mfile', print the commands needed to update it without actually running them, and then print the commands needed to update `foo' without running them. The commands for `foo' will be those specified by the existing contents of `mfile'.
Sometimes it is useful to have a makefile that is mostly just like
another makefile. You can often use the `include' directive to
include one in the other, and add more targets or variable definitions.
However, if the two makefiles give different commands for the same
target, make will not let you just do this. But there is another way.
In the containing makefile (the one that wants to include the other),
you can use a match-anything pattern rule to say that to remake any
target that cannot be made from the information in the containing
makefile, make should look in another makefile.
See section Defining and Redefining Pattern Rules, for more information on pattern rules.
For example, if you have a makefile called `Makefile' that says how to make the target `foo' (and other targets), you can write a makefile called `GNUmakefile' that contains:
foo:
frobnicate > foo
%: force
@$(MAKE) -f Makefile $@
force: ;
If you say `make foo', make will find `GNUmakefile',
read it, and see that to make `foo', it needs to run the command
`frobnicate > foo'. If you say `make bar', make will
find no way to make `bar' in `GNUmakefile', so it will use the
commands from the pattern rule: `make -f Makefile bar'. If
`Makefile' provides a rule for updating `bar', make
will apply the rule. And likewise for any other target that
`GNUmakefile' does not say how to make.
The way this works is that the pattern rule has a pattern of just
`%', so it matches any target whatever. The rule specifies a
dependency `force', to guarantee that the commands will be run even
if the target file already exists. We give `force' target empty
commands to prevent make from searching for an implicit rule to
build it--otherwise it would apply the same match-anything rule to
`force' itself and create a dependency loop!
A rule appears in the makefile and says when and how to remake certain files, called the rule's targets (most often only one per rule). It lists the other files that are the dependencies of the target, and commands to use to create or update the target.
The order of rules is not significant, except for determining the
default goal: the target for make to consider, if you do
not otherwise specify one. The default goal is the target of the first
rule in the first makefile. If the first rule has multiple targets,
only the first target is taken as the default. There are two
exceptions: a target starting with a period is not a default unless it
contains one or more slashes, `/', as well; and, a target that
defines a pattern rule has no effect on the default goal.
(See section Defining and Redefining Pattern Rules.)
Therefore, we usually write the makefile so that the first rule is the one for compiling the entire program or all the programs described by the makefile (often with a target called `all'). See section Arguments to Specify the Goals.
In general, a rule looks like this:
targets : dependencies
command
...
or like this:
targets : dependencies ; command
command
...
The targets are file names, separated by spaces. Wildcard characters may be used (see section Using Wildcard Characters in File Names) and a name of the form `a(m)' represents member m in archive file a (see section Archive Members as Targets). Usually there is only one target per rule, but occasionally there is a reason to have more (see section Multiple Targets in a Rule).
The command lines start with a tab character. The first command may appear on the line after the dependencies, with a tab character, or may appear on the same line, with a semicolon. Either way, the effect is the same. See section Writing the Commands in Rules.
Because dollar signs are used to start variable references, if you really
want a dollar sign in a rule you must write two of them, `$$'
(see section How to Use Variables).
You may split a long line by inserting a backslash
followed by a newline, but this is not required, as make places no
limit on the length of a line in a makefile.
A rule tells make two things: when the targets are out of date,
and how to update them when necessary.
The criterion for being out of date is specified in terms of the
dependencies, which consist of file names separated by spaces.
(Wildcards and archive members (see section Using make to Update Archive Files) are allowed here too.)
A target is out of date if it does not exist or if it is older than any
of the dependencies (by comparison of last-modification times). The
idea is that the contents of the target file are computed based on
information in the dependencies, so if any of the dependencies changes,
the contents of the existing target file are no longer necessarily
valid.
How to update is specified by commands. These are lines to be executed by the shell (normally `sh'), but with some extra features (see section Writing the Commands in Rules).
A single file name can specify many files using wildcard characters.
The wildcard characters in make are `*', `?' and
`[...]', the same as in the Bourne shell. For example, `*.c'
specifies a list of all the files (in the working directory) whose names
end in `.c'.
The character `~' at the beginning of a file name also has special significance. If alone, or followed by a slash, it represents your home directory. For example `~/bin' expands to `/home/you/bin'. If the `~' is followed by a word, the string represents the home directory of the user named by that word. For example `~john/bin' expands to `/home/john/bin'. On systems which don't have a home directory for each user (such as MS-DOS or MS-Windows), this functionality can be simulated by setting the environment variable HOME.
Wildcard expansion happens automatically in targets, in dependencies,
and in commands (where the shell does the expansion). In other
contexts, wildcard expansion happens only if you request it explicitly
with the wildcard function.
The special significance of a wildcard character can be turned off by preceding it with a backslash. Thus, `foo\*bar' would refer to a specific file whose name consists of `foo', an asterisk, and `bar'.
Wildcards can be used in the commands of a rule, where they are expanded by the shell. For example, here is a rule to delete all the object files:
clean:
rm -f *.o
Wildcards are also useful in the dependencies of a rule. With the following rule in the makefile, `make print' will print all the `.c' files that have changed since the last time you printed them:
print: *.c
lpr -p $?
touch print
This rule uses `print' as an empty target file; see section Empty Target Files to Record Events. (The automatic variable `$?' is used to print only those files that have changed; see section Automatic Variables.)
Wildcard expansion does not happen when you define a variable. Thus, if you write this:
objects = *.o
then the value of the variable objects is the actual string
`*.o'. However, if you use the value of objects in a target,
dependency or command, wildcard expansion will take place at that time.
To set objects to the expansion, instead use:
objects := $(wildcard *.o)
See section The Function wildcard.
Now here is an example of a naive way of using wildcard expansion, that does not do what you would intend. Suppose you would like to say that the executable file `foo' is made from all the object files in the directory, and you write this:
objects = *.o
foo : $(objects)
cc -o foo $(CFLAGS) $(objects)
The value of objects is the actual string `*.o'. Wildcard
expansion happens in the rule for `foo', so that each existing
`.o' file becomes a dependency of `foo' and will be recompiled if
necessary.
But what if you delete all the `.o' files? When a wildcard matches
no files, it is left as it is, so then `foo' will depend on the
oddly-named file `*.o'. Since no such file is likely to exist,
make will give you an error saying it cannot figure out how to
make `*.o'. This is not what you want!
Actually it is possible to obtain the desired result with wildcard
expansion, but you need more sophisticated techniques, including the
wildcard function and string substitution.
These are described in the following section.
Microsoft operating systems (MS-DOS and MS-Windows) use backslashes to separate directories in pathnames, like so:
c:\foo\bar\baz.c
This is equivalent to the Unix-style `c:/foo/bar/baz.c' (the
`c:' part is the so-called drive letter). When make runs on
these systems, it supports backslashes as well as the Unix-style forward
slashes in pathnames. However, this support does not include the
wildcard expansion, where backslash is a quote character. Therefore,
you must use Unix-style slashes in these cases.
wildcard
Wildcard expansion happens automatically in rules. But wildcard expansion
does not normally take place when a variable is set, or inside the
arguments of a function. If you want to do wildcard expansion in such
places, you need to use the wildcard function, like this:
$(wildcard pattern...)
This string, used anywhere in a makefile, is replaced by a
space-separated list of names of existing files that match one of the
given file name patterns. If no existing file name matches a pattern,
then that pattern is omitted from the output of the wildcard
function. Note that this is different from how unmatched wildcards
behave in rules, where they are used verbatim rather than ignored
(see section Pitfalls of Using Wildcards).
One use of the wildcard function is to get a list of all the C source
files in a directory, like this:
$(wildcard *.c)
We can change the list of C source files into a list of object files by replacing the `.c' suffix with `.o' in the result, like this:
$(patsubst %.c,%.o,$(wildcard *.c))
(Here we have used another function, patsubst.
See section Functions for String Substitution and Analysis.)
Thus, a makefile to compile all C source files in the directory and then link them together could be written as follows:
objects := $(patsubst %.c,%.o,$(wildcard *.c))
foo : $(objects)
cc -o foo $(objects)
(This takes advantage of the implicit rule for compiling C programs, so there is no need to write explicit rules for compiling the files. See section The Two Flavors of Variables, for an explanation of `:=', which is a variant of `='.)
For large systems, it is often desirable to put sources in a separate
directory from the binaries. The directory search features of
make facilitate this by searching several directories
automatically to find a dependency. When you redistribute the files
among directories, you do not need to change the individual rules,
just the search paths.
VPATH: Search Path for All Dependencies
The value of the make variable VPATH specifies a list of
directories that make should search. Most often, the
directories are expected to contain dependency files that are not in the
current directory; however, VPATH specifies a search list that
make applies for all files, including files which are targets of
rules.
Thus, if a file that is listed as a target or dependency does not exist
in the current directory, make searches the directories listed in
VPATH for a file with that name. If a file is found in one of
them, that file may become the dependency (see below). Rules may then
specify the names of files in the dependency list as if they all
existed in the current directory. See section Writing Shell Commands with Directory Search.
In the VPATH variable, directory names are separated by colons or
blanks. The order in which directories are listed is the order followed
by make in its search. (On MS-DOS and MS-Windows, semi-colons
are used as separators of directory names in VPATH, since the
colon can be used in the pathname itself, after the drive letter.)
For example,
VPATH = src:../headers
specifies a path containing two directories, `src' and
`../headers', which make searches in that order.
With this value of VPATH, the following rule,
foo.o : foo.c
is interpreted as if it were written like this:
foo.o : src/foo.c
assuming the file `foo.c' does not exist in the current directory but is found in the directory `src'.
vpath Directive
Similar to the VPATH variable, but more selective, is the
vpath directive (note lower case), which allows you to specify a
search path for a particular class of file names: those that match a
particular pattern. Thus you can supply certain search directories for
one class of file names and other directories (or none) for other file
names.
There are three forms of the vpath directive:
vpath pattern directories
VPATH variable.
vpath pattern
vpath
vpath directives.
A vpath pattern is a string containing a `%' character. The
string must match the file name of a dependency that is being searched
for, the `%' character matching any sequence of zero or more
characters (as in pattern rules; see section Defining and Redefining Pattern Rules). For example, %.h matches files that
end in .h. (If there is no `%', the pattern must match the
dependency exactly, which is not useful very often.)
`%' characters in a vpath directive's pattern can be quoted
with preceding backslashes (`\'). Backslashes that would otherwise
quote `%' characters can be quoted with more backslashes.
Backslashes that quote `%' characters or other backslashes are
removed from the pattern before it is compared to file names. Backslashes
that are not in danger of quoting `%' characters go unmolested.
When a dependency fails to exist in the current directory, if the
pattern in a vpath directive matches the name of the
dependency file, then the directories in that directive are searched
just like (and before) the directories in the VPATH variable.
For example,
vpath %.h ../headers
tells make to look for any dependency whose name ends in `.h'
in the directory `../headers' if the file is not found in the current
directory.
If several vpath patterns match the dependency file's name, then
make processes each matching vpath directive one by one,
searching all the directories mentioned in each directive. make
handles multiple vpath directives in the order in which they
appear in the makefile; multiple directives with the same pattern are
independent of each other.
Thus,
vpath %.c foo vpath % blish vpath %.c bar
will look for a file ending in `.c' in `foo', then `blish', then `bar', while
vpath %.c foo:bar vpath % blish
will look for a file ending in `.c' in `foo', then `bar', then `blish'.
When a dependency is found through directory search, regardless of type
(general or selective), the pathname located may not be the one that
make actually provides you in the dependency list. Sometimes
the path discovered through directory search is thrown away.
The algorithm make uses to decide whether to keep or abandon a
path found via directory search is as follows:
make doesn't need to rebuild
the target then you use the path found via directory search.
make must rebuild, then the target is rebuilt locally,
not in the directory found via directory search.
This algorithm may seem complex, but in practice it is quite often exactly what you want.
Other versions of make use a simpler algorithm: if the file does
not exist, and it is found via directory search, then that pathname is
always used whether or not the target needs to be built. Thus, if the
target is rebuilt it is created at the pathname discovered during
directory search.
If, in fact, this is the behavior you want for some or all of your
directories, you can use the GPATH variable to indicate this to
make.
GPATH has the same syntax and format as VPATH (that is, a
space- or colon-delimited list of pathnames). If an out-of-date target
is found by directory search in a directory that also appears in
GPATH, then that pathname is not thrown away. The target is
rebuilt using the expanded path.
When a dependency is found in another directory through directory search,
this cannot change the commands of the rule; they will execute as written.
Therefore, you must write the commands with care so that they will look for
the dependency in the directory where make finds it.
This is done with the automatic variables such as `$^' (see section Automatic Variables). For instance, the value of `$^' is a list of all the dependencies of the rule, including the names of the directories in which they were found, and the value of `$@' is the target. Thus:
foo.o : foo.c
cc -c $(CFLAGS) $^ -o $@
(The variable CFLAGS exists so you can specify flags for C
compilation by implicit rules; we use it here for consistency so it will
affect all C compilations uniformly;
see section Variables Used by Implicit Rules.)
Often the dependencies include header files as well, which you do not want to mention in the commands. The automatic variable `$<' is just the first dependency:
VPATH = src:../headers
foo.o : foo.c defs.h hack.h
cc -c $(CFLAGS) $< -o $@
The search through the directories specified in VPATH or with
vpath also happens during consideration of implicit rules
(see section Using Implicit Rules).
For example, when a file `foo.o' has no explicit rule, make
considers implicit rules, such as the built-in rule to compile
`foo.c' if that file exists. If such a file is lacking in the
current directory, the appropriate directories are searched for it. If
`foo.c' exists (or is mentioned in the makefile) in any of the
directories, the implicit rule for C compilation is applied.
The commands of implicit rules normally use automatic variables as a matter of necessity; consequently they will use the file names found by directory search with no extra effort.
Directory search applies in a special way to libraries used with the linker. This special feature comes into play when you write a dependency whose name is of the form `-lname'. (You can tell something strange is going on here because the dependency is normally the name of a file, and the file name of the library looks like `libname.a', not like `-lname'.)
When a dependency's name has the form `-lname', make
handles it specially by searching for the file `libname.a' in
the current directory, in directories specified by matching vpath
search paths and the VPATH search path, and then in the
directories `/lib', `/usr/lib', and `prefix/lib'
(normally `/usr/local/lib', but MS-DOS/MS-Windows versions of
make behave as if prefix is defined to be the root of the
DJGPP installation tree).
For example,
foo : foo.c -lcurses
cc $^ -o $@
would cause the command `cc foo.c /usr/lib/libcurses.a -o foo' to be executed when `foo' is older than `foo.c' or than `/usr/lib/libcurses.a'.
A phony target is one that is not really the name of a file. It is just a name for some commands to be executed when you make an explicit request. There are two reasons to use a phony target: to avoid a conflict with a file of the same name, and to improve performance.
If you write a rule whose commands will not create the target file, the commands will be executed every time the target comes up for remaking. Here is an example:
clean:
rm *.o temp
Because the rm command does not create a file named `clean',
probably no such file will ever exist. Therefore, the rm command
will be executed every time you say `make clean'.
The phony target will cease to work if anything ever does create a file
named `clean' in this directory. Since it has no dependencies, the
file `clean' would inevitably be considered up to date, and its
commands would not be executed. To avoid this problem, you can explicitly
declare the target to be phony, using the special target .PHONY
(see section Special Built-in Target Names) as follows:
.PHONY : clean
Once this is done, `make clean' will run the commands regardless of whether there is a file named `clean'.
Since it knows that phony targets do not name actual files that could be
remade from other files, make skips the implicit rule search for
phony targets (see section Using Implicit Rules). This is why declaring a target
phony is good for performance, even if you are not worried about the
actual file existing.
Thus, you first write the line that states that clean is a
phony target, then you write the rule, like this:
.PHONY: clean
clean:
rm *.o temp
A phony target should not be a dependency of a real target file; if it
is, its commands are run every time make goes to update that
file. As long as a phony target is never a dependency of a real
target, the phony target commands will be executed only when the phony
target is a specified goal (see section Arguments to Specify the Goals).
Phony targets can have dependencies. When one directory contains multiple programs, it is most convenient to describe all of the programs in one makefile `./Makefile'. Since the target remade by default will be the first one in the makefile, it is common to make this a phony target named `all' and give it, as dependencies, all the individual programs. For example:
all : prog1 prog2 prog3
.PHONY : all
prog1 : prog1.o utils.o
cc -o prog1 prog1.o utils.o
prog2 : prog2.o
cc -o prog2 prog2.o
prog3 : prog3.o sort.o utils.o
cc -o prog3 prog3.o sort.o utils.o
Now you can say just `make' to remake all three programs, or specify as arguments the ones to remake (as in `make prog1 prog3').
When one phony target is a dependency of another, it serves as a subroutine of the other. For example, here `make cleanall' will delete the object files, the difference files, and the file `program':
.PHONY: cleanall cleanobj cleandiff
cleanall : cleanobj cleandiff
rm program
cleanobj :
rm *.o
cleandiff :
rm *.diff
If a rule has no dependencies or commands, and the target of the rule
is a nonexistent file, then make imagines this target to have
been updated whenever its rule is run. This implies that all targets
depending on this one will always have their commands run.
An example will illustrate this:
clean: FORCE
rm $(objects)
FORCE:
Here the target `FORCE' satisfies the special conditions, so the target `clean' that depends on it is forced to run its commands. There is nothing special about the name `FORCE', but that is one name commonly used this way.
As you can see, using `FORCE' this way has the same results as using `.PHONY: clean'.
Using `.PHONY' is more explicit and more efficient. However,
other versions of make do not support `.PHONY'; thus
`FORCE' appears in many makefiles. See section Phony Targets.
The empty target is a variant of the phony target; it is used to hold commands for an action that you request explicitly from time to time. Unlike a phony target, this target file can really exist; but the file's contents do not matter, and usually are empty.
The purpose of the empty target file is to record, with its
last-modification time, when the rule's commands were last executed. It
does so because one of the commands is a touch command to update the
target file.
The empty target file must have some dependencies. When you ask to remake the empty target, the commands are executed if any dependency is more recent than the target; in other words, if a dependency has changed since the last time you remade the target. Here is an example:
print: foo.c bar.c
lpr -p $?
touch print
With this rule, `make print' will execute the lpr command if
either source file has changed since the last `make print'. The
automatic variable `$?' is used to print only those files that have
changed (see section Automatic Variables).
Certain names have special meanings if they appear as targets.
.PHONY
.PHONY are considered to
be phony targets. When it is time to consider such a target,
make will run its commands unconditionally, regardless of
whether a file with that name exists or what its last-modification
time is. See section Phony Targets.
.SUFFIXES
.SUFFIXES are the list
of suffixes to be used in checking for suffix rules.
See section Old-Fashioned Suffix Rules.
.DEFAULT
.DEFAULT are used for any target for
which no rules are found (either explicit rules or implicit rules).
See section Defining Last-Resort Default Rules. If .DEFAULT commands are specified, every
file mentioned as a dependency, but not as a target in a rule, will have
these commands executed on its behalf. See section Implicit Rule Search Algorithm.
.PRECIOUS
.PRECIOUS depends on are given the following
special treatment: if make is killed or interrupted during the
execution of their commands, the target is not deleted.
See section Interrupting or Killing make.
Also, if the target is an intermediate file, it will not be deleted
after it is no longer needed, as is normally done.
See section Chains of Implicit Rules.
You can also list the target pattern of an implicit rule (such as
`%.o') as a dependency file of the special target .PRECIOUS
to preserve intermediate files created by rules whose target patterns
match that file's name.
.INTERMEDIATE
.INTERMEDIATE depends on are treated as
intermediate files. See section Chains of Implicit Rules.
.INTERMEDIATE with no dependencies marks all file targets
mentioned in the makefile as intermediate.
.SECONDARY
.SECONDARY depends on are treated as
intermediate files, except that they are never automatically deleted.
See section Chains of Implicit Rules.
.SECONDARY with no dependencies marks all file targets mentioned
in the makefile as secondary.
.IGNORE
.IGNORE, then make will
ignore errors in execution of the commands run for those particular
files. The commands for .IGNORE are not meaningful.
If mentioned as a target with no dependencies, .IGNORE says to
ignore errors in execution of commands for all files. This usage of
`.IGNORE' is supported only for historical compatibility. Since
this affects every command in the makefile, it is not very useful; we
recommend you use the more selective ways to ignore errors in specific
commands. See section Errors in Commands.
.SILENT
.SILENT, then make will
not the print commands to remake those particular files before executing
them. The commands for .SILENT are not meaningful.
If mentioned as a target with no dependencies, .SILENT says not
to print any commands before executing them. This usage of
`.SILENT' is supported only for historical compatibility. We
recommend you use the more selective ways to silence specific commands.
See section Command Echoing. If you want to silence all commands
for a particular run of make, use the `-s' or
`--silent' option (see section Summary of Options).
.EXPORT_ALL_VARIABLES
make to
export all variables to child processes by default.
See section Communicating Variables to a Sub-make.
Any defined implicit rule suffix also counts as a special target if it appears as a target, and so does the concatenation of two suffixes, such as `.c.o'. These targets are suffix rules, an obsolete way of defining implicit rules (but a way still widely used). In principle, any target name could be special in this way if you break it in two and add both pieces to the suffix list. In practice, suffixes normally begin with `.', so these special target names also begin with `.'. See section Old-Fashioned Suffix Rules.
A rule with multiple targets is equivalent to writing many rules, each with one target, and all identical aside from that. The same commands apply to all the targets, but their effects may vary because you can substitute the actual target name into the command using `$@'. The rule contributes the same dependencies to all the targets also.
This is useful in two cases.
kbd.o command.o files.o: command.hgives an additional dependency to each of the three object files mentioned.
bigoutput littleoutput : text.g
generate text.g -$(subst output,,$@) > $@
is equivalent to
bigoutput : text.g
generate text.g -big > bigoutput
littleoutput : text.g
generate text.g -little > littleoutput
Here we assume the hypothetical program generate makes two
types of output, one if given `-big' and one if given
`-little'.
See section Functions for String Substitution and Analysis,
for an explanation of the subst function.
Suppose you would like to vary the dependencies according to the target, much as the variable `$@' allows you to vary the commands. You cannot do this with multiple targets in an ordinary rule, but you can do it with a static pattern rule. See section Static Pattern Rules.
One file can be the target of several rules. All the dependencies mentioned in all the rules are merged into one list of dependencies for the target. If the target is older than any dependency from any rule, the commands are executed.
There can only be one set of commands to be executed for a file.
If more than one rule gives commands for the same file,
make uses the last set given and prints an error message.
(As a special case, if the file's name begins with a dot, no
error message is printed. This odd behavior is only for
compatibility with other implementations of make.)
There is no reason to
write your makefiles this way; that is why make gives you
an error message.
An extra rule with just dependencies can be used to give a few extra
dependencies to many files at once. For example, one usually has a
variable named objects containing a list of all the compiler output
files in the system being made. An easy way to say that all of them must
be recompiled if `config.h' changes is to write the following:
objects = foo.o bar.o foo.o : defs.h bar.o : defs.h test.h $(objects) : config.h
This could be inserted or taken out without changing the rules that really specify how to make the object files, making it a convenient form to use if you wish to add the additional dependency intermittently.
Another wrinkle is that the additional dependencies could be specified with
a variable that you set with a command argument to make
(see section Overriding Variables). For example,
extradeps= $(objects) : $(extradeps)
means that the command `make extradeps=foo.h' will consider `foo.h' as a dependency of each object file, but plain `make' will not.
If none of the explicit rules for a target has commands, then make
searches for an applicable implicit rule to find some commands
see section Using Implicit Rules).
Static pattern rules are rules which specify multiple targets and construct the dependency names for each target based on the target name. They are more general than ordinary rules with multiple targets because the targets do not have to have identical dependencies. Their dependencies must be analogous, but not necessarily identical.
Here is the syntax of a static pattern rule:
targets ...: target-pattern: dep-patterns ...
commands
...
The targets list specifies the targets that the rule applies to. The targets can contain wildcard characters, just like the targets of ordinary rules (see section Using Wildcard Characters in File Names).
The target-pattern and dep-patterns say how to compute the dependencies of each target. Each target is matched against the target-pattern to extract a part of the target name, called the stem. This stem is substituted into each of the dep-patterns to make the dependency names (one from each dep-pattern).
Each pattern normally contains the character `%' just once. When the target-pattern matches a target, the `%' can match any part of the target name; this part is called the stem. The rest of the pattern must match exactly. For example, the target `foo.o' matches the pattern `%.o', with `foo' as the stem. The targets `foo.c' and `foo.out' do not match that pattern.
The dependency names for each target are made by substituting the stem for the `%' in each dependency pattern. For example, if one dependency pattern is `%.c', then substitution of the stem `foo' gives the dependency name `foo.c'. It is legitimate to write a dependency pattern that does not contain `%'; then this dependency is the same for all targets.
`%' characters in pattern rules can be quoted with preceding backslashes (`\'). Backslashes that would otherwise quote `%' characters can be quoted with more backslashes. Backslashes that quote `%' characters or other backslashes are removed from the pattern before it is compared to file names or has a stem substituted into it. Backslashes that are not in danger of quoting `%' characters go unmolested. For example, the pattern `the\%weird\\%pattern\\' has `the%weird\' preceding the operative `%' character, and `pattern\\' following it. The final two backslashes are left alone because they cannot affect any `%' character.
Here is an example, which compiles each of `foo.o' and `bar.o' from the corresponding `.c' file:
objects = foo.o bar.o
all: $(objects)
$(objects): %.o: %.c
$(CC) -c $(CFLAGS) $< -o $@
Here `$<' is the automatic variable that holds the name of the dependency and `$@' is the automatic variable that holds the name of the target; see section Automatic Variables.
Each target specified must match the target pattern; a warning is issued
for each target that does not. If you have a list of files, only some of
which will match the pattern, you can use the filter function to
remove nonmatching file names (see section Functions for String Substitution and Analysis):
files = foo.elc bar.o lose.o
$(filter %.o,$(files)): %.o: %.c
$(CC) -c $(CFLAGS) $< -o $@
$(filter %.elc,$(files)): %.elc: %.el
emacs -f batch-byte-compile $<
In this example the result of `$(filter %.o,$(files))' is `bar.o lose.o', and the first static pattern rule causes each of these object files to be updated by compiling the corresponding C source file. The result of `$(filter %.elc,$(files))' is `foo.elc', so that file is made from `foo.el'.
Another example shows how to use $* in static pattern rules:
bigoutput littleoutput : %output : text.g
generate text.g -$* > $@
When the generate command is run, $* will expand to the
stem, either `big' or `little'.
A static pattern rule has much in common with an implicit rule defined as a
pattern rule (see section Defining and Redefining Pattern Rules).
Both have a pattern for the target and patterns for constructing the
names of dependencies. The difference is in how make decides
when the rule applies.
An implicit rule can apply to any target that matches its pattern, but it does apply only when the target has no commands otherwise specified, and only when the dependencies can be found. If more than one implicit rule appears applicable, only one applies; the choice depends on the order of rules.
By contrast, a static pattern rule applies to the precise list of targets that you specify in the rule. It cannot apply to any other target and it invariably does apply to each of the targets specified. If two conflicting rules apply, and both have commands, that's an error.
The static pattern rule can be better than an implicit rule for these reasons:
make to use the wrong implicit rule. The choice
might depend on the order in which the implicit rule search is done.
With static pattern rules, there is no uncertainty: each rule applies
to precisely the targets specified.
Double-colon rules are rules written with `::' instead of `:' after the target names. They are handled differently from ordinary rules when the same target appears in more than one rule.
When a target appears in multiple rules, all the rules must be the same type: all ordinary, or all double-colon. If they are double-colon, each of them is independent of the others. Each double-colon rule's commands are executed if the target is older than any dependencies of that rule. This can result in executing none, any, or all of the double-colon rules.
Double-colon rules with the same target are in fact completely separate from one another. Each double-colon rule is processed individually, just as rules with different targets are processed.
The double-colon rules for a target are executed in the order they appear in the makefile. However, the cases where double-colon rules really make sense are those where the order of executing the commands would not matter.
Double-colon rules are somewhat obscure and not often very useful; they provide a mechanism for cases in which the method used to update a target differs depending on which dependency files caused the update, and such cases are rare.
Each double-colon rule should specify commands; if it does not, an implicit rule will be used if one applies. See section Using Implicit Rules.
In the makefile for a program, many of the rules you need to write often
say only that some object file depends on some header
file. For example, if `main.c' uses `defs.h' via an
#include, you would write:
main.o: defs.h
You need this rule so that make knows that it must remake
`main.o' whenever `defs.h' changes. You can see that for a
large program you would have to write dozens of such rules in your
makefile. And, you must always be very careful to update the makefile
every time you add or remove an #include.
To avoid this hassle, most modern C compilers can write these rules for
you, by looking at the #include lines in the source files.
Usually this is done with the `-M' option to the compiler.
For example, the command:
cc -M main.c
generates the output:
main.o : main.c defs.h
Thus you no longer have to write all those rules yourself. The compiler will do it for you.
Note that such a dependency constitutes mentioning `main.o' in a
makefile, so it can never be considered an intermediate file by implicit
rule search. This means that make won't ever remove the file
after using it; see section Chains of Implicit Rules.
With old make programs, it was traditional practice to use this
compiler feature to generate dependencies on demand with a command like
`make depend'. That command would create a file `depend'
containing all the automatically-generated dependencies; then the
makefile could use include to read them in (see section Including Other Makefiles).
In GNU make, the feature of remaking makefiles makes this
practice obsolete--you need never tell make explicitly to
regenerate the dependencies, because it always regenerates any makefile
that is out of date. See section How Makefiles Are Remade.
The practice we recommend for automatic dependency generation is to have one makefile corresponding to each source file. For each source file `name.c' there is a makefile `name.d' which lists what files the object file `name.o' depends on. That way only the source files that have changed need to be rescanned to produce the new dependencies.
Here is the pattern rule to generate a file of dependencies (i.e., a makefile) called `name.d' from a C source file called `name.c':
%.d: %.c
$(SHELL) -ec '$(CC) -M $(CPPFLAGS) $< \
| sed '\''s/\($*\)\.o[ :]*/\1.o $@ : /g'\'' > $@; \
[ -s $@ ] || rm -f $@'
See section Defining and Redefining Pattern Rules, for information on defining pattern rules. The
`-e' flag to the shell makes it exit immediately if the
$(CC) command fails (exits with a nonzero status). Normally the
shell exits with the status of the last command in the pipeline
(sed in this case), so make would not notice a nonzero
status from the compiler.
With the GNU C compiler, you may wish to use the `-MM' flag instead of `-M'. This omits dependencies on system header files. See section `Options Controlling the Preprocessor' in Using GNU CC, for details.
The purpose of the sed command is to translate (for example):
main.o : main.c defs.h
into:
main.o main.d : main.c defs.h
This makes each `.d' file depend on all the source and header files
that the corresponding `.o' file depends on. make then
knows it must regenerate the dependencies whenever any of the source or
header files changes.
Once you've defined the rule to remake the `.d' files,
you then use the include directive to read them all in.
See section Including Other Makefiles. For example:
sources = foo.c bar.c include $(sources:.c=.d)
(This example uses a substitution variable reference to translate the
list of source files `foo.c bar.c' into a list of dependency
makefiles, `foo.d bar.d'. See section Substitution References, for full
information on substitution references.) Since the `.d' files are
makefiles like any others, make will remake them as necessary
with no further work from you. See section How Makefiles Are Remade.
The commands of a rule consist of shell command lines to be executed one by one. Each command line must start with a tab, except that the first command line may be attached to the target-and-dependencies line with a semicolon in between. Blank lines and lines of just comments may appear among the command lines; they are ignored. (But beware, an apparently "blank" line that begins with a tab is not blank! It is an empty command; see section Using Empty Commands.)
Users use many different shell programs, but commands in makefiles are always interpreted by `/bin/sh' unless the makefile specifies otherwise. See section Command Execution.
The shell that is in use determines whether comments can be written on command lines, and what syntax they use. When the shell is `/bin/sh', a `#' starts a comment that extends to the end of the line. The `#' does not have to be at the beginning of a line. Text on a line before a `#' is not part of the comment.
Normally make prints each command line before it is executed.
We call this echoing because it gives the appearance that you
are typing the commands yourself.
When a line starts with `@', the echoing of that line is suppressed.
The `@' is discarded before the command is passed to the shell.
Typically you would use this for a command whose only effect is to print
something, such as an echo command to indicate progress through
the makefile:
@echo About to make distribution files
When make is given the flag `-n' or `--just-print',
echoing is all that happens, no execution. See section Summary of Options. In this case and only this case, even the
commands starting with `@' are printed. This flag is useful for
finding out which commands make thinks are necessary without
actually doing them.
The `-s' or `--silent'
flag to make prevents all echoing, as if all commands
started with `@'. A rule in the makefile for the special target
.SILENT without dependencies has the same effect
(see section Special Built-in Target Names).
.SILENT is essentially obsolete since `@' is more flexible.
When it is time to execute commands to update a target, they are executed
by making a new subshell for each line. (In practice, make may
take shortcuts that do not affect the results.)
Please note: this implies that shell commands such as cd
that set variables local to each process will not affect the following
command lines. (2) If you want to use cd
to affect the next command, put the two on a single line with a
semicolon between them. Then make will consider them a single
command and pass them, together, to a shell which will execute them in
sequence. For example:
foo : bar/lose
cd bar; gobble lose > ../foo
If you would like to split a single shell command into multiple lines of text, you must use a backslash at the end of all but the last subline. Such a sequence of lines is combined into a single line, by deleting the backslash-newline sequences, before passing it to the shell. Thus, the following is equivalent to the preceding example:
foo : bar/lose
cd bar; \
gobble lose > ../foo
The program used as the shell is taken from the variable SHELL.
By default, the program `/bin/sh' is used.
On MS-DOS, if SHELL is not set, the value of the variable
COMSPEC (which is always set) is used instead.
The processing of lines that set the variable SHELL in Makefiles
is different on MS-DOS. The stock shell, `command.com', is
ridiculously limited in its functionality and many users of make
tend to install a replacement shell. Therefore, on MS-DOS, make
examines the value of SHELL, and changes its behavior based on
whether it points to a Unix-style or DOS-style shell. This allows
reasonable functionality even if SHELL points to
`command.com'.
If SHELL points to a Unix-style shell, make on MS-DOS
additionally checks whether that shell can indeed be found; if not, it
ignores the line that sets SHELL. In MS-DOS, GNU make
searches for the shell in the following places:
SHELL. For
example, if the makefile specifies `SHELL = /bin/sh', make
will look in the directory `/bin' on the current drive.
PATH variable, in order.
In every directory it examines, make will first look for the
specific file (`sh' in the example above). If this is not found,
it will also look in that directory for that file with one of the known
extensions which identify executable files. For example `.exe',
`.com', `.bat', `.btm', `.sh', and some others.
If any of these attempts is successful, the value of SHELL will
be set to the full pathname of the shell as found. However, if none of
these is found, the value of SHELL will not be changed, and thus
the line that sets it will be effectively ignored. This is so
make will only support features specific to a Unix-style shell if
such a shell is actually installed on the system where make runs.
Note that this extended search for the shell is limited to the cases
where SHELL is set from the Makefile; if it is set in the
environment or command line, you are expected to set it to the full
pathname of the shell, exactly as things are on Unix.
The effect of the above DOS-specific processing is that a Makefile that
says `SHELL = /bin/sh' (as many Unix makefiles do), will work
on MS-DOS unaltered if you have e.g. `sh.exe' installed in some
directory along your PATH.
Unlike most variables, the variable SHELL is never set from the
environment. This is because the SHELL environment variable is
used to specify your personal choice of shell program for interactive
use. It would be very bad for personal choices like this to affect the
functioning of makefiles. See section Variables from the Environment. However, on MS-DOS and MS-Windows the value of
SHELL in the environment is used, since on those systems
most users do not set this variable, and therefore it is most likely set
specifically to be used by make. On MS-DOS, if the setting of
SHELL is not suitable for make, you can set the variable
MAKESHELL to the shell that make should use; this will
override the value of SHELL.
GNU make knows how to execute several commands at once.
Normally, make will execute only one command at a time, waiting
for it to finish before executing the next. However, the `-j' or
`--jobs' option tells make to execute many commands
simultaneously.
On MS-DOS, the `-j' option has no effect, since that system doesn't support multi-processing.
If the `-j' option is followed by an integer, this is the number of commands to execute at once; this is called the number of job slots. If there is nothing looking like an integer after the `-j' option, there is no limit on the number of job slots. The default number of job slots is one, which means serial execution (one thing at a time).
One unpleasant consequence of running several commands simultaneously is that output from all of the commands comes when the commands send it, so messages from different commands may be interspersed.
Another problem is that two processes cannot both take input from the
same device; so to make sure that only one command tries to take input
from the terminal at once, make will invalidate the standard
input streams of all but one running command. This means that
attempting to read from standard input will usually be a fatal error (a
`Broken pipe' signal) for most child processes if there are
several.
It is unpredictable which command will have a valid standard input stream
(which will come from the terminal, or wherever you redirect the standard
input of make). The first command run will always get it first, and
the first command started after that one finishes will get it next, and so
on.
We will change how this aspect of make works if we find a better
alternative. In the mean time, you should not rely on any command using
standard input at all if you are using the parallel execution feature; but
if you are not using this feature, then standard input works normally in
all commands.
If a command fails (is killed by a signal or exits with a nonzero
status), and errors are not ignored for that command
(see section Errors in Commands),
the remaining command lines to remake the same target will not be run.
If a command fails and the `-k' or `--keep-going'
option was not given
(see section Summary of Options),
make aborts execution. If make
terminates for any reason (including a signal) with child processes
running, it waits for them to finish before actually exiting.
When the system is heavily loaded, you will probably want to run fewer jobs
than when it is lightly loaded. You can use the `-l' option to tell
make to limit the number of jobs to run at once, based on the load
average. The `-l' or `--max-load'
option is followed by a floating-point number. For
example,
-l 2.5
will not let make start more than one job if the load average is
above 2.5. The `-l' option with no following number removes the
load limit, if one was given with a previous `-l' option.
More precisely, when make goes to start up a job, and it already has
at least one job running, it checks the current load average; if it is not
lower than the limit given with `-l', make waits until the load
average goes below that limit, or until all the other jobs finish.
By default, there is no load limit.
After each shell command returns, make looks at its exit status.
If the command completed successfully, the next command line is executed
in a new shell; after the last command line is finished, the rule is
finished.
If there is an error (the exit status is nonzero), make gives up on
the current rule, and perhaps on all rules.
Sometimes the failure of a certain command does not indicate a problem.
For example, you may use the mkdir command to ensure that a
directory exists. If the directory already exists, mkdir will
report an error, but you probably want make to continue regardless.
To ignore errors in a command line, write a `-' at the beginning of the line's text (after the initial tab). The `-' is discarded before the command is passed to the shell for execution.
For example,
clean:
-rm -f *.o
This causes rm to continue even if it is unable to remove a file.
When you run make with the `-i' or `--ignore-errors'
flag, errors are ignored in all commands of all rules. A rule in the
makefile for the special target .IGNORE has the same effect, if
there are no dependencies. These ways of ignoring errors are obsolete
because `-' is more flexible.
When errors are to be ignored, because of either a `-' or the
`-i' flag, make treats an error return just like success,
except that it prints out a message that tells you the status code
the command exited with, and says that the error has been ignored.
When an error happens that make has not been told to ignore,
it implies that the current target cannot be correctly remade, and neither
can any other that depends on it either directly or indirectly. No further
commands will be executed for these targets, since their preconditions
have not been achieved.
Normally make gives up immediately in this circumstance, returning a
nonzero status. However, if the `-k' or `--keep-going'
flag is specified, make
continues to consider the other dependencies of the pending targets,
remaking them if necessary, before it gives up and returns nonzero status.
For example, after an error in compiling one object file, `make -k'
will continue compiling other object files even though it already knows
that linking them will be impossible. See section Summary of Options.
The usual behavior assumes that your purpose is to get the specified
targets up to date; once make learns that this is impossible, it
might as well report the failure immediately. The `-k' option says
that the real purpose is to test as many of the changes made in the
program as possible, perhaps to find several independent problems so
that you can correct them all before the next attempt to compile. This
is why Emacs' compile command passes the `-k' flag by
default.
Usually when a command fails, if it has changed the target file at all,
the file is corrupted and cannot be used--or at least it is not
completely updated. Yet the file's timestamp says that it is now up to
date, so the next time make runs, it will not try to update that
file. The situation is just the same as when the command is killed by a
signal; see section Interrupting or Killing make. So generally the right thing to do is to
delete the target file if the command fails after beginning to change
the file. make will do this if .DELETE_ON_ERROR appears
as a target. This is almost always what you want make to do, but
it is not historical practice; so for compatibility, you must explicitly
request it.
make
If make gets a fatal signal while a command is executing, it may
delete the target file that the command was supposed to update. This is
done if the target file's last-modification time has changed since
make first checked it.
The purpose of deleting the target is to make sure that it is remade from
scratch when make is next run. Why is this? Suppose you type
Ctrl-c while a compiler is running, and it has begun to write an
object file `foo.o'. The Ctrl-c kills the compiler, resulting
in an incomplete file whose last-modification time is newer than the source
file `foo.c'. But make also receives the Ctrl-c signal
and deletes this incomplete file. If make did not do this, the next
invocation of make would think that `foo.o' did not require
updating--resulting in a strange error message from the linker when it
tries to link an object file half of which is missing.
You can prevent the deletion of a target file in this way by making the
special target .PRECIOUS depend on it. Before remaking a target,
make checks to see whether it appears on the dependencies of
.PRECIOUS, and thereby decides whether the target should be deleted
if a signal happens. Some reasons why you might do this are that the
target is updated in some atomic fashion, or exists only to record a
modification-time (its contents do not matter), or must exist at all
times to prevent other sorts of trouble.
make
Recursive use of make means using make as a command in a
makefile. This technique is useful when you want separate makefiles for
various subsystems that compose a larger system. For example, suppose you
have a subdirectory `subdir' which has its own makefile, and you would
like the containing directory's makefile to run make on the
subdirectory. You can do it by writing this:
subsystem:
cd subdir && $(MAKE)
or, equivalently, this (see section Summary of Options):
subsystem:
$(MAKE) -C subdir
You can write recursive make commands just by copying this example,
but there are many things to know about how they work and why, and about
how the sub-make relates to the top-level make.
For your convenience, GNU make sets the variable CURDIR to
the pathname of the current working directory for you. If -C is
in effect, it will contain the path of the new directory, not the
original. The value has the same precedence it would have if it were
set in the makefile (by default, an environment variable CURDIR
will not override this value). Note that setting this variable has no
effect on the operation of make
MAKE Variable Works
Recursive make commands should always use the variable MAKE,
not the explicit command name `make', as shown here:
subsystem:
cd subdir && $(MAKE)
The value of this variable is the file name with which make was
invoked. If this file name was `/bin/make', then the command executed
is `cd subdir && /bin/make'. If you use a special version of
make to run the top-level makefile, the same special version will be
executed for recursive invocations.
As a special feature, using the variable MAKE in the commands of
a rule alters the effects of the `-t' (`--touch'), `-n'
(`--just-print'), or `-q' (`--question') option.
Using the MAKE variable has the same effect as using a `+'
character at the beginning of the command line. See section Instead of Executing the Commands.
Consider the command `make -t' in the above example. (The `-t' option marks targets as up to date without actually running any commands; see section Instead of Executing the Commands.) Following the usual definition of `-t', a `make -t' command in the example would create a file named `subsystem' and do nothing else. What you really want it to do is run `cd subdir && make -t'; but that would require executing the command, and `-t' says not to execute commands.
The special feature makes this do what you want: whenever a command
line of a rule contains the variable MAKE, the flags `-t',
`-n' and `-q' do not apply to that line. Command lines
containing MAKE are executed normally despite the presence of a
flag that causes most commands not to be run. The usual
MAKEFLAGS mechanism passes the flags to the sub-make
(see section Communicating Options to a Sub-make), so your request to touch the files, or print the
commands, is propagated to the subsystem.
make
Variable values of the top-level make can be passed to the
sub-make through the environment by explicit request. These
variables are defined in the sub-make as defaults, but do not
override what is specified in the makefile used by the sub-make
makefile unless you use the `-e' switch (see section Summary of Options).
To pass down, or export, a variable, make adds the variable
and its value to the environment for running each command. The
sub-make, in turn, uses the environment to initialize its table
of variable values. See section Variables from the Environment.
Except by explicit request, make exports a variable only if it
is either defined in the environment initially or set on the command
line, and if its name consists only of letters, numbers, and underscores.
Some shells cannot cope with environment variable names consisting of
characters other than letters, numbers, and underscores.
The special variables SHELL and MAKEFLAGS are always
exported (unless you unexport them).
MAKEFILES is exported if you set it to anything.
make automatically passes down variable values that were defined
on the command line, by putting them in the MAKEFLAGS variable.
See the next section.
Variables are not normally passed down if they were created by
default by make (see section Variables Used by Implicit Rules). The sub-make will define these for
itself.
If you want to export specific variables to a sub-make, use the
export directive, like this:
export variable ...
If you want to prevent a variable from being exported, use the
unexport directive, like this:
unexport variable ...
As a convenience, you can define a variable and export it at the same time by doing:
export variable = value
has the same result as:
variable = value export variable
and
export variable := value
has the same result as:
variable := value export variable
Likewise,
export variable += value
is just like:
variable += value export variable
See section Appending More Text to Variables.
You may notice that the export and unexport directives
work in make in the same way they work in the shell, sh.
If you want all variables to be exported by default, you can use
export by itself:
export
This tells make that variables which are not explicitly mentioned
in an export or unexport directive should be exported.
Any variable given in an unexport directive will still not
be exported. If you use export by itself to export variables by
default, variables whose names contain characters other than
alphanumerics and underscores will not be exported unless specifically
mentioned in an export directive.
The behavior elicited by an export directive by itself was the
default in older versions of GNU make. If your makefiles depend
on this behavior and you want to be compatible with old versions of
make, you can write a rule for the special target
.EXPORT_ALL_VARIABLES instead of using the export directive.
This will be ignored by old makes, while the export
directive will cause a syntax error.
Likewise, you can use unexport by itself to tell make
not to export variables by default. Since this is the default
behavior, you would only need to do this if export had been used
by itself earlier (in an included makefile, perhaps). You
cannot use export and unexport by themselves to
have variables exported for some commands and not for others. The last
export or unexport directive that appears by itself
determines the behavior for the entire run of make.
As a special feature, the variable MAKELEVEL is changed when it
is passed down from level to level. This variable's value is a string
which is the depth of the level as a decimal number. The value is
`0' for the top-level make; `1' for a sub-make,
`2' for a sub-sub-make, and so on. The incrementation
happens when make sets up the environment for a command.
The main use of MAKELEVEL is to test it in a conditional
directive (see section Conditional Parts of Makefiles); this
way you can write a makefile that behaves one way if run recursively and
another way if run directly by you.
You can use the variable MAKEFILES to cause all sub-make
commands to use additional makefiles. The value of MAKEFILES is
a whitespace-separated list of file names. This variable, if defined in
the outer-level makefile, is passed down through the environment; then
it serves as a list of extra makefiles for the sub-make to read
before the usual or specified ones. See section The Variable MAKEFILES.
make
Flags such as `-s' and `-k' are passed automatically to the
sub-make through the variable MAKEFLAGS. This variable is
set up automatically by make to contain the flag letters that
make received. Thus, if you do `make -ks' then
MAKEFLAGS gets the value `ks'.
As a consequence, every sub-make gets a value for MAKEFLAGS
in its environment. In response, it takes the flags from that value and
processes them as if they had been given as arguments.
See section Summary of Options.
Likewise variables defined on the command line are passed to the
sub-make through MAKEFLAGS. Words in the value of
MAKEFLAGS that contain `=', make treats as variable
definitions just as if they appeared on the command line.
See section Overriding Variables.
The options `-C', `-f', `-o', and `-W' are not put
into MAKEFLAGS; these options are not passed down.
The `-j' option is a special case (see section Parallel Execution).
If you set it to some numeric value, `-j 1' is always put into
MAKEFLAGS instead of the value you specified. This is because if
the `-j' option were passed down to sub-makes, you would
get many more jobs running in parallel than you asked for. If you give
`-j' with no numeric argument, meaning to run as many jobs as
possible in parallel, this is passed down, since multiple infinities are
no more than one.
If you do not want to pass the other flags down, you must change the
value of MAKEFLAGS, like this:
subsystem:
cd subdir && $(MAKE) MAKEFLAGS=
The command line variable definitions really appear in the variable
MAKEOVERRIDES, and MAKEFLAGS contains a reference to this
variable. If you do want to pass flags down normally, but don't want to
pass down the command line variable definitions, you can reset
MAKEOVERRIDES to empty, like this:
MAKEOVERRIDES =
This is not usually useful to do. However, some systems have a small
fixed limit on the size of the environment, and putting so much
information in into the value of MAKEFLAGS can exceed it.
If you see the error message `Arg list too long', this may be the problem.
(For strict compliance with POSIX.2, changing MAKEOVERRIDES does
not affect MAKEFLAGS if the special target `.POSIX' appears
in the makefile. You probably do not care about this.)
A similar variable MFLAGS exists also, for historical
compatibility. It has the same value as MAKEFLAGS except that it
does not contain the command line variable definitions, and it always
begins with a hyphen unless it is empty (MAKEFLAGS begins with a
hyphen only when it begins with an option that has no single-letter
version, such as `--warn-undefined-variables'). MFLAGS was
traditionally used explicitly in the recursive make command, like
this:
subsystem:
cd subdir && $(MAKE) $(MFLAGS)
but now MAKEFLAGS makes this usage redundant. If you want your
makefiles to be compatible with old make programs, use this
technique; it will work fine with more modern make versions too.
The MAKEFLAGS variable can also be useful if you want to have
certain options, such as `-k' (see section Summary of Options), set each time you run make. You simply put a value for
MAKEFLAGS in your environment. You can also set MAKEFLAGS in
a makefile, to specify additional flags that should also be in effect for
that makefile. (Note that you cannot use MFLAGS this way. That
variable is set only for compatibility; make does not interpret a
value you set for it in any way.)
When make interprets the value of MAKEFLAGS (either from the
environment or from a makefile), it first prepends a hyphen if the value
does not already begin with one. Then it chops the value into words
separated by blanks, and parses these words as if they were options given
on the command line (except that `-C', `-f', `-h',
`-o', `-W', and their long-named versions are ignored; and there
is no error for an invalid option).
If you do put MAKEFLAGS in your environment, you should be sure not
to include any options that will drastically affect the actions of
make and undermine the purpose of makefiles and of make
itself. For instance, the `-t', `-n', and `-q' options, if
put in one of these variables, could have disastrous consequences and would
certainly have at least surprising and probably annoying effects.
If you use several levels of recursive make invocations, the
`-w' or `--print-directory' option can make the output a
lot easier to understand by showing each directory as make
starts processing it and as make finishes processing it. For
example, if `make -w' is run in the directory `/u/gnu/make',
make will print a line of the form:
make: Entering directory `/u/gnu/make'.
before doing anything else, and a line of the form:
make: Leaving directory `/u/gnu/make'.
when processing is completed.
Normally, you do not need to specify this option because `make'
does it for you: `-w' is turned on automatically when you use the
`-C' option, and in sub-makes. make will not
automatically turn on `-w' if you also use `-s', which says to
be silent, or if you use `--no-print-directory' to explicitly
disable it.
When the same sequence of commands is useful in making various targets, you
can define it as a canned sequence with the define directive, and
refer to the canned sequence from the rules for those targets. The canned
sequence is actually a variable, so the name must not conflict with other
variable names.
Here is an example of defining a canned sequence of commands:
define run-yacc yacc $(firstword $^) mv y.tab.c $@ endef
Here run-yacc is the name of the variable being defined;
endef marks the end of the definition; the lines in between are the
commands. The define directive does not expand variable references
and function calls in the canned sequence; the `$' characters,
parentheses, variable names, and so on, all become part of the value of the
variable you are defining.
See section Defining Variables Verbatim,
for a complete explanation of define.
The first command in this example runs Yacc on the first dependency of whichever rule uses the canned sequence. The output file from Yacc is always named `y.tab.c'. The second command moves the output to the rule's target file name.
To use the canned sequence, substitute the variable into the commands of a
rule. You can substitute it like any other variable
(see section Basics of Variable References).
Because variables defined by define are recursively expanded
variables, all the variable references you wrote inside the define
are expanded now. For example:
foo.c : foo.y
$(run-yacc)
`foo.y' will be substituted for the variable `$^' when it occurs in
run-yacc's value, and `foo.c' for `$@'.
This is a realistic example, but this particular one is not needed in
practice because make has an implicit rule to figure out these
commands based on the file names involved
(see section Using Implicit Rules).
In command execution, each line of a canned sequence is treated just as
if the line appeared on its own in the rule, preceded by a tab. In
particular, make invokes a separate subshell for each line. You
can use the special prefix characters that affect command lines
(`@', `-', and `+') on each line of a canned sequence.
See section Writing the Commands in Rules.
For example, using this canned sequence:
define frobnicate @echo "frobnicating target $@" frob-step-1 $< -o $@-step-1 frob-step-2 $@-step-1 -o $@ endef
make will not echo the first line, the echo command.
But it will echo the following two command lines.
On the other hand, prefix characters on the command line that refers to a canned sequence apply to every line in the sequence. So the rule:
frob.out: frob.in @$(frobnicate)
does not echo any commands. (See section Command Echoing, for a full explanation of `@'.)
It is sometimes useful to define commands which do nothing. This is done simply by giving a command that consists of nothing but whitespace. For example:
target: ;
defines an empty command string for `target'. You could also use a line beginning with a tab character to define an empty command string, but this would be confusing because such a line looks empty.
You may be wondering why you would want to define a command string that
does nothing. The only reason this is useful is to prevent a target
from getting implicit commands (from implicit rules or the
.DEFAULT special target; see section Using Implicit Rules and
see section Defining Last-Resort Default Rules).
You may be inclined to define empty command strings for targets that are not actual files, but only exist so that their dependencies can be remade. However, this is not the best way to do that, because the dependencies may not be remade properly if the target file actually does exist. See section Phony Targets, for a better way to do this.
A variable is a name defined in a makefile to represent a string
of text, called the variable's value. These values are
substituted by explicit request into targets, dependencies, commands,
and other parts of the makefile. (In some other versions of make,
variables are called macros.)
Variables and functions in all parts of a makefile are expanded when
read, except for the shell commands in rules, the right-hand sides of
variable definitions using `=', and the bodies of variable
definitions using the define directive.
Variables can represent lists of file names, options to pass to compilers, programs to run, directories to look in for source files, directories to write output in, or anything else you can imagine.
A variable name may be any sequence of characters not containing `:',
`#', `=', or leading or trailing whitespace. However,
variable names containing characters other than letters, numbers, and
underscores should be avoided, as they may be given special meanings in the
future, and with some shells they cannot be passed through the environment to a
sub-make
(see section Communicating Variables to a Sub-make).
Variable names are case-sensitive. The names `foo', `FOO', and `Foo' all refer to different variables.
It is traditional to use upper case letters in variable names, but we recommend using lower case letters for variable names that serve internal purposes in the makefile, and reserving upper case for parameters that control implicit rules or for parameters that the user should override with command options (see section Overriding Variables).
A few variables have names that are a single punctuation character or just a few characters. These are the automatic variables, and they have particular specialized uses. See section Automatic Variables.
To substitute a variable's value, write a dollar sign followed by the name
of the variable in parentheses or braces: either `$(foo)' or
`${foo}' is a valid reference to the variable foo. This
special significance of `$' is why you must write `$$' to have
the effect of a single dollar sign in a file name or command.
Variable references can be used in any context: targets, dependencies, commands, most directives, and new variable values. Here is an example of a common case, where a variable holds the names of all the object files in a program:
objects = program.o foo.o utils.o
program : $(objects)
cc -o program $(objects)
$(objects) : defs.h
Variable references work by strict textual substitution. Thus, the rule
foo = c
prog.o : prog.$(foo)
$(foo)$(foo) -$(foo) prog.$(foo)
could be used to compile a C program `prog.c'. Since spaces before
the variable value are ignored in variable assignments, the value of
foo is precisely `c'. (Don't actually write your makefiles
this way!)
A dollar sign followed by a character other than a dollar sign,
open-parenthesis or open-brace treats that single character as the
variable name. Thus, you could reference the variable x with
`$x'. However, this practice is strongly discouraged, except in
the case of the automatic variables (see section Automatic Variables).
There are two ways that a variable in GNU make can have a value;
we call them the two flavors of variables. The two flavors are
distinguished in how they are defined and in what they do when expanded.
The first flavor of variable is a recursively expanded variable.
Variables of this sort are defined by lines using `='
(see section Setting Variables) or by the define directive
(see section Defining Variables Verbatim). The value you specify
is installed verbatim; if it contains references to other variables,
these references are expanded whenever this variable is substituted (in
the course of expanding some other string). When this happens, it is
called recursive expansion.
For example,
foo = $(bar) bar = $(ugh) ugh = Huh? all:;echo $(foo)
will echo `Huh?': `$(foo)' expands to `$(bar)' which expands to `$(ugh)' which finally expands to `Huh?'.
This flavor of variable is the only sort supported by other versions of
make. It has its advantages and its disadvantages. An advantage
(most would say) is that:
CFLAGS = $(include_dirs) -O include_dirs = -Ifoo -Ibar
will do what was intended: when `CFLAGS' is expanded in a command, it will expand to `-Ifoo -Ibar -O'. A major disadvantage is that you cannot append something on the end of a variable, as in
CFLAGS = $(CFLAGS) -O
because it will cause an infinite loop in the variable expansion.
(Actually make detects the infinite loop and reports an error.)
Another disadvantage is that any functions
(see section Functions for Transforming Text)
referenced in the definition will be executed every time the variable is
expanded. This makes make run slower; worse, it causes the
wildcard and shell functions to give unpredictable results
because you cannot easily control when they are called, or even how many
times.
To avoid all the problems and inconveniences of recursively expanded variables, there is another flavor: simply expanded variables.
Simply expanded variables are defined by lines using `:=' (see section Setting Variables). The value of a simply expanded variable is scanned once and for all, expanding any references to other variables and functions, when the variable is defined. The actual value of the simply expanded variable is the result of expanding the text that you write. It does not contain any references to other variables; it contains their values as of the time this variable was defined. Therefore,
x := foo y := $(x) bar x := later
is equivalent to
y := foo bar x := later
When a simply expanded variable is referenced, its value is substituted verbatim.
Here is a somewhat more complicated example, illustrating the use of
`:=' in conjunction with the shell function.
(See section The shell Function.) This example
also shows use of the variable MAKELEVEL, which is changed
when it is passed down from level to level.
(See section Communicating Variables to a Sub-make, for information about MAKELEVEL.)
ifeq (0,${MAKELEVEL})
cur-dir := $(shell pwd)
whoami := $(shell whoami)
host-type := $(shell arch)
MAKE := ${MAKE} host-type=${host-type} whoami=${whoami}
endif
An advantage of this use of `:=' is that a typical `descend into a directory' command then looks like this:
${subdirs}:
${MAKE} cur-dir=${cur-dir}/$@ -C $@ all
Simply expanded variables generally make complicated makefile programming more predictable because they work like variables in most programming languages. They allow you to redefine a variable using its own value (or its value processed in some way by one of the expansion functions) and to use the expansion functions much more efficiently (see section Functions for Transforming Text).
You can also use them to introduce controlled leading whitespace into variable values. Leading whitespace characters are discarded from your input before substitution of variable references and function calls; this means you can include leading spaces in a variable value by protecting them with variable references, like this:
nullstring := space := $(nullstring) # end of the line
Here the value of the variable space is precisely one space. The
comment `# end of the line' is included here just for clarity.
Since trailing space characters are not stripped from variable
values, just a space at the end of the line would have the same effect
(but be rather hard to read). If you put whitespace at the end of a
variable value, it is a good idea to put a comment like that at the end
of the line to make your intent clear. Conversely, if you do not
want any whitespace characters at the end of your variable value, you
must remember not to put a random comment on the end of the line after
some whitespace, such as this:
dir := /foo/bar # directory to put the frobs in
Here the value of the variable dir is `/foo/bar '
(with four trailing spaces), which was probably not the intention.
(Imagine something like `$(dir)/file' with this definition!)
There is another assignment operator for variables, `?='. This is called a conditional variable assignment operator, because it only has an effect if the variable is not yet defined. This statement:
FOO ?= bar
is exactly equivalent to this
(see section The origin Function):
ifeq ($(origin FOO), undefined) FOO = bar endif
Note that a variable set to an empty value is still defined, so `?=' will not set that variable.
This section describes some advanced features you can use to reference variables in more flexible ways.
A substitution reference substitutes the value of a variable with alterations that you specify. It has the form `$(var:a=b)' (or `${var:a=b}') and its meaning is to take the value of the variable var, replace every a at the end of a word with b in that value, and substitute the resulting string.
When we say "at the end of a word", we mean that a must appear either followed by whitespace or at the end of the value in order to be replaced; other occurrences of a in the value are unaltered. For example:
foo := a.o b.o c.o bar := $(foo:.o=.c)
sets `bar' to `a.c b.c c.c'. See section Setting Variables.
A substitution reference is actually an abbreviation for use of the
patsubst expansion function (see section Functions for String Substitution and Analysis). We provide
substitution references as well as patsubst for compatibility with
other implementations of make.
Another type of substitution reference lets you use the full power of
the patsubst function. It has the same form
`$(var:a=b)' described above, except that now
a must contain a single `%' character. This case is
equivalent to `$(patsubst a,b,$(var))'.
See section Functions for String Substitution and Analysis,
for a description of the patsubst function.
For example: foo := a.o b.o c.o bar := $(foo:%.o=%.c)
sets `bar' to `a.c b.c c.c'.
Computed variable names are a complicated concept needed only for sophisticated makefile programming. For most purposes you need not consider them, except to know that making a variable with a dollar sign in its name might have strange results. However, if you are the type that wants to understand everything, or you are actually interested in what they do, read on.
Variables may be referenced inside the name of a variable. This is called a computed variable name or a nested variable reference. For example,
x = y y = z a := $($(x))
defines a as `z': the `$(x)' inside `$($(x))' expands
to `y', so `$($(x))' expands to `$(y)' which in turn expands
to `z'. Here the name of the variable to reference is not stated
explicitly; it is computed by expansion of `$(x)'. The reference
`$(x)' here is nested within the outer variable reference.
The previous example shows two levels of nesting, but any number of levels is possible. For example, here are three levels:
x = y y = z z = u a := $($($(x)))
Here the innermost `$(x)' expands to `y', so `$($(x))' expands to `$(y)' which in turn expands to `z'; now we have `$(z)', which becomes `u'.
References to recursively-expanded variables within a variable name are reexpanded in the usual fashion. For example:
x = $(y) y = z z = Hello a := $($(x))
defines a as `Hello': `$($(x))' becomes `$($(y))'
which becomes `$(z)' which becomes `Hello'.
Nested variable references can also contain modified references and
function invocations (see section Functions for Transforming Text),
just like any other reference.
For example, using the subst function
(see section Functions for String Substitution and Analysis):
x = variable1 variable2 := Hello y = $(subst 1,2,$(x)) z = y a := $($($(z)))
eventually defines a as `Hello'. It is doubtful that anyone
would ever want to write a nested reference as convoluted as this one, but
it works: `$($($(z)))' expands to `$($(y))' which becomes
`$($(subst 1,2,$(x)))'. This gets the value `variable1' from
x and changes it by substitution to `variable2', so that the
entire string becomes `$(variable2)', a simple variable reference
whose value is `Hello'.
A computed variable name need not consist entirely of a single variable reference. It can contain several variable references, as well as some invariant text. For example,
a_dirs := dira dirb 1_dirs := dir1 dir2 a_files := filea fileb 1_files := file1 file2 ifeq "$(use_a)" "yes" a1 := a else a1 := 1 endif ifeq "$(use_dirs)" "yes" df := dirs else df := files endif dirs := $($(a1)_$(df))
will give dirs the same value as a_dirs, 1_dirs,
a_files or 1_files depending on the settings of use_a
and use_dirs.
Computed variable names can also be used in substitution references:
a_objects := a.o b.o c.o 1_objects := 1.o 2.o 3.o sources := $($(a1)_objects:.o=.c)
defines sources as either `a.c b.c c.c' or `1.c 2.c 3.c',
depending on the value of a1.
The only restriction on this sort of use of nested variable references is that they cannot specify part of the name of a function to be called. This is because the test for a recognized function name is done before the expansion of nested references. For example,
ifdef do_sort func := sort else func := strip endif bar := a d b g q c foo := $($(func) $(bar))
attempts to give `foo' the value of the variable `sort a d b g
q c' or `strip a d b g q c', rather than giving `a d b g q c'
as the argument to either the sort or the strip function.
This restriction could be removed in the future if that change is shown
to be a good idea.
You can also use computed variable names in the left-hand side of a
variable assignment, or in a define directive, as in:
dir = foo $(dir)_sources := $(wildcard $(dir)/*.c) define $(dir)_print lpr $($(dir)_sources) endef
This example defines the variables `dir', `foo_sources', and `foo_print'.
Note that nested variable references are quite different from recursively expanded variables (see section The Two Flavors of Variables), though both are used together in complex ways when doing makefile programming.
Variables can get values in several different ways:
make.
See section Overriding Variables.
make variables.
See section Variables from the Environment.
To set a variable from the makefile, write a line starting with the variable name followed by `=' or `:='. Whatever follows the `=' or `:=' on the line becomes the value. For example,
objects = main.o foo.o bar.o utils.o
defines a variable named objects. Whitespace around the variable
name and immediately after the `=' is ignored.
Variables defined with `=' are recursively expanded variables. Variables defined with `:=' are simply expanded variables; these definitions can contain variable references which will be expanded before the definition is made. See section The Two Flavors of Variables.
The variable name may contain function and variable references, which are expanded when the line is read to find the actual variable name to use.
There is no limit on the length of the value of a variable except the
amount of swapping space on the computer. When a variable definition is
long, it is a good idea to break it into several lines by inserting
backslash-newline at convenient places in the definition. This will not
affect the functioning of make, but it will make the makefile easier
to read.
Most variable names are considered to have the empty string as a value if you have never set them. Several variables have built-in initial values that are not empty, but you can set them in the usual ways (see section Variables Used by Implicit Rules). Several special variables are set automatically to a new value for each rule; these are called the automatic variables (see section Automatic Variables).
If you'd like a variable to be set to a value only if it's not already
set, then you can use the shorthand operator `?=' instead of
`='. These two settings of the variable `FOO' are identical
(see section The origin Function):
FOO ?= bar
and
ifeq ($(origin FOO), undefined) FOO = bar endif
Often it is useful to add more text to the value of a variable already defined. You do this with a line containing `+=', like this:
objects += another.o
This takes the value of the variable objects, and adds the text
`another.o' to it (preceded by a single space). Thus:
objects = main.o foo.o bar.o utils.o objects += another.o
sets objects to `main.o foo.o bar.o utils.o another.o'.
Using `+=' is similar to:
objects = main.o foo.o bar.o utils.o objects := $(objects) another.o
but differs in ways that become important when you use more complex values.
When the variable in question has not been defined before, `+=' acts just like normal `=': it defines a recursively-expanded variable. However, when there is a previous definition, exactly what `+=' does depends on what flavor of variable you defined originally. See section The Two Flavors of Variables, for an explanation of the two flavors of variables.
When you add to a variable's value with `+=', make acts
essentially as if you had included the extra text in the initial
definition of the variable. If you defined it first with `:=',
making it a simply-expanded variable, `+=' adds to that
simply-expanded definition, and expands the new text before appending it
to the old value just as `:=' does
(see section Setting Variables, for a full explanation of `:=').
In fact,
variable := value variable += more
is exactly equivalent to:
variable := value variable := $(variable) more
On the other hand, when you use `+=' with a variable that you defined
first to be recursively-expanded using plain `=', make does
something a bit different. Recall that when you define a
recursively-expanded variable, make does not expand the value you set
for variable and function references immediately. Instead it stores the text
verbatim, and saves these variable and function references to be expanded
later, when you refer to the new variable (see section The Two Flavors of Variables). When you use `+=' on a recursively-expanded variable,
it is this unexpanded text to which make appends the new text you
specify.
variable = value variable += more
is roughly equivalent to:
temp = value variable = $(temp) more
except that of course it never defines a variable called temp.
The importance of this comes when the variable's old value contains
variable references. Take this common example:
CFLAGS = $(includes) -O ... CFLAGS += -pg # enable profiling
The first line defines the CFLAGS variable with a reference to another
variable, includes. (CFLAGS is used by the rules for C
compilation; see section Catalogue of Implicit Rules.)
Using `=' for the definition makes CFLAGS a recursively-expanded
variable, meaning `$(includes) -O' is not expanded when
make processes the definition of CFLAGS. Thus, includes
need not be defined yet for its value to take effect. It only has to be
defined before any reference to CFLAGS. If we tried to append to the
value of CFLAGS without using `+=', we might do it like this:
CFLAGS := $(CFLAGS) -pg # enable profiling
This is pretty close, but not quite what we want. Using `:='
redefines CFLAGS as a simply-expanded variable; this means
make expands the text `$(CFLAGS) -pg' before setting the
variable. If includes is not yet defined, we get ` -O
-pg', and a later definition of includes will have no effect.
Conversely, by using `+=' we set CFLAGS to the
unexpanded value `$(includes) -O -pg'. Thus we preserve
the reference to includes, so if that variable gets defined at
any later point, a reference like `$(CFLAGS)' still uses its
value.
override Directive
If a variable has been set with a command argument
(see section Overriding Variables),
then ordinary assignments in the makefile are ignored. If you want to set
the variable in the makefile even though it was set with a command
argument, you can use an override directive, which is a line that
looks like this:
override variable = value
or
override variable := value
To append more text to a variable defined on the command line, use:
override variable += more text
See section Appending More Text to Variables.
The override directive was not invented for escalation in the war
between makefiles and command arguments. It was invented so you can alter
and add to values that the user specifies with command arguments.
For example, suppose you always want the `-g' switch when you run the
C compiler, but you would like to allow the user to specify the other
switches with a command argument just as usual. You could use this
override directive:
override CFLAGS += -g
You can also use override directives with define directives.
This is done as you might expect:
override define foo bar endef
See the next section for information about define.
Another way to set the value of a variable is to use the define
directive. This directive has an unusual syntax which allows newline
characters to be included in the value, which is convenient for defining
canned sequences of commands
(see section Defining Canned Command Sequences).
The define directive is followed on the same line by the name of the
variable and nothing more. The value to give the variable appears on the
following lines. The end of the value is marked by a line containing just
the word endef. Aside from this difference in syntax, define
works just like `=': it creates a recursively-expanded variable
(see section The Two Flavors of Variables).
The variable name may contain function and variable references, which
are expanded when the directive is read to find the actual variable name
to use.
define two-lines echo foo echo $(bar) endef
The value in an ordinary assignment cannot contain a newline; but the
newlines that separate the lines of the value in a define become
part of the variable's value (except for the final newline which precedes
the endef and is not considered part of the value).
The previous example is functionally equivalent to this:
two-lines = echo foo; echo $(bar)
since two commands separated by semicolon behave much like two separate
shell commands. However, note that using two separate lines means
make will invoke the shell twice, running an independent subshell
for each line. See section Command Execution.
If you want variable definitions made with define to take
precedence over command-line variable definitions, you can use the
override directive together with define:
override define two-lines foo $(bar) endef
See section The override Directive.
Variables in make can come from the environment in which
make is run. Every environment variable that make sees when
it starts up is transformed into a make variable with the same name
and value. But an explicit assignment in the makefile, or with a command
argument, overrides the environment. (If the `-e' flag is specified,
then values from the environment override assignments in the makefile.
See section Summary of Options.
But this is not recommended practice.)
Thus, by setting the variable CFLAGS in your environment, you can
cause all C compilations in most makefiles to use the compiler switches you
prefer. This is safe for variables with standard or conventional meanings
because you know that no makefile will use them for other things. (But
this is not totally reliable; some makefiles set CFLAGS explicitly
and therefore are not affected by the value in the environment.)
When make is invoked recursively, variables defined in the
outer invocation can be passed to inner invocations through the
environment (see section Recursive Use of make). By
default, only variables that came from the environment or the command
line are passed to recursive invocations. You can use the
export directive to pass other variables.
See section Communicating Variables to a Sub-make, for full details.
Other use of variables from the environment is not recommended. It is not wise for makefiles to depend for their functioning on environment variables set up outside their control, since this would cause different users to get different results from the same makefile. This is against the whole purpose of most makefiles.
Such problems would be especially likely with the variable SHELL,
which is normally present in the environment to specify the user's choice
of interactive shell. It would be very undesirable for this choice to
affect make. So make ignores the environment value of
SHELL (except on MS-DOS and MS-Windows, where SHELL is
usually not set. See section Command Execution.)
Variable values in make are usually global; that is, they are the
same regardless of where they are evaluated (unless they're reset, of
course). One exception to that is automatic variables
(see section Automatic Variables).
The other exception is target-specific variable values. This
feature allows you to define different values for the same variable,
based on the target that make is currently building. As with
automatic variables, these values are only available within the context
of a target's command script (and in other target-specific assignments).
Set a target-specific variable value like this:
target ... : variable-assignment
or like this:
target ... : override variable-assignment
Multiple target values create a target-specific variable value for each member of the target list individually.
The variable-assignment can be any valid form of assignment; recursive (`='), static (`:='), appending (`+='), or conditional (`?='). All variables that appear within the variable-assignment are evaluated within the context of the target: thus, any previously-defined target-specific variable values will be in effect. Note that this variable is actually distinct from any "global" value: the two variables do not have to have the same flavor (recursive vs. static).
Target-specific variables have the same priority as any other makefile
variable. Variables provided on the command-line (and in the
environment if the `-e' option is in force) will take precedence.
Specifying the override directive will allow the target-specific
variable value to be preferred.
There is one more special feature of target-specific variables: when you define a target-specific variable, that variable value is also in effect for all dependencies of this target (unless those dependencies override it with their own target-specific variable value). So, for example, a statement like this:
prog : CFLAGS = -g prog : prog.o foo.o bar.o
will set CFLAGS to `-g' in the command script for
`prog', but it will also set CFLAGS to `-g' in the
command scripts that create `prog.o', `foo.o', and
`bar.o', and any command scripts which create their dependencies.
In addition to target-specific variable values (see section Target-specific Variable Values), GNU make supports
pattern-specific variable values. In this form, a variable is defined
for any target that matches the pattern specified. Variables defined in
this way are searched after any target-specific variables defined
explicitly for that target, and before target-specific variables defined
for the parent target.
Set a pattern-specific variable value like this:
pattern ... : variable-assignment
or like this:
pattern ... : override variable-assignment
where pattern is a %-pattern. As with target-specific variable
values, multiple pattern values create a pattern-specific variable
value for each pattern individually. The variable-assignment can
be any valid form of assignment. Any command-line variable setting will
take precedence, unless override is specified.
For example:
%.o : CFLAGS = -O
will assign CFLAGS the value of `-O' for all targets
matching the pattern %.o.
A conditional causes part of a makefile to be obeyed or ignored
depending on the values of variables. Conditionals can compare the
value of one variable to another, or the value of a variable to
a constant string. Conditionals control what make actually
"sees" in the makefile, so they cannot be used to control shell
commands at the time of execution.
The following example of a conditional tells make to use one set
of libraries if the CC variable is `gcc', and a different
set of libraries otherwise. It works by controlling which of two
command lines will be used as the command for a rule. The result is
that `CC=gcc' as an argument to make changes not only which
compiler is used but also which libraries are linked.
libs_for_gcc = -lgnu
normal_libs =
foo: $(objects)
ifeq ($(CC),gcc)
$(CC) -o foo $(objects) $(libs_for_gcc)
else
$(CC) -o foo $(objects) $(normal_libs)
endif
This conditional uses three directives: one ifeq, one else
and one endif.
The ifeq directive begins the conditional, and specifies the
condition. It contains two arguments, separated by a comma and surrounded
by parentheses. Variable substitution is performed on both arguments and
then they are compared. The lines of the makefile following the
ifeq are obeyed if the two arguments match; otherwise they are
ignored.
The else directive causes the following lines to be obeyed if the
previous conditional failed. In the example above, this means that the
second alternative linking command is used whenever the first alternative
is not used. It is optional to have an else in a conditional.
The endif directive ends the conditional. Every conditional must
end with an endif. Unconditional makefile text follows.
As this example illustrates, conditionals work at the textual level: the lines of the conditional are treated as part of the makefile, or ignored, according to the condition. This is why the larger syntactic units of the makefile, such as rules, may cross the beginning or the end of the conditional.
When the variable CC has the value `gcc', the above example has
this effect:
foo: $(objects)
$(CC) -o foo $(objects) $(libs_for_gcc)
When the variable CC has any other value, the effect is this:
foo: $(objects)
$(CC) -o foo $(objects) $(normal_libs)
Equivalent results can be obtained in another way by conditionalizing a variable assignment and then using the variable unconditionally:
libs_for_gcc = -lgnu
normal_libs =
ifeq ($(CC),gcc)
libs=$(libs_for_gcc)
else
libs=$(normal_libs)
endif
foo: $(objects)
$(CC) -o foo $(objects) $(libs)
The syntax of a simple conditional with no else is as follows:
conditional-directive text-if-true endif
The text-if-true may be any lines of text, to be considered as part of the makefile if the condition is true. If the condition is false, no text is used instead.
The syntax of a complex conditional is as follows:
conditional-directive text-if-true else text-if-false endif
If the condition is true, text-if-true is used; otherwise, text-if-false is used instead. The text-if-false can be any number of lines of text.
The syntax of the conditional-directive is the same whether the conditional is simple or complex. There are four different directives that test different conditions. Here is a table of them:
ifeq (arg1, arg2)
ifeq 'arg1' 'arg2'
ifeq "arg1" "arg2"
ifeq "arg1" 'arg2'
ifeq 'arg1' "arg2"
strip function (see section Functions for String Substitution and Analysis) to avoid interpreting
whitespace as a non-empty value. For example:
ifeq ($(strip $(foo)),) text-if-empty endifwill evaluate text-if-empty even if the expansion of
$(foo) contains whitespace characters.
ifneq (arg1, arg2)
ifneq 'arg1' 'arg2'
ifneq "arg1" "arg2"
ifneq "arg1" 'arg2'
ifneq 'arg1' "arg2"
ifdef variable-name
ifdef only tests whether a variable has a value. It
does not expand the variable to see if that value is nonempty.
Consequently, tests using ifdef return true for all definitions
except those like foo =. To test for an empty value, use
ifeq ($(foo),). For example,
bar = foo = $(bar) ifdef foo frobozz = yes else frobozz = no endifsets `frobozz' to `yes', while:
foo = ifdef foo frobozz = yes else frobozz = no endifsets `frobozz' to `no'.
ifndef variable-name
Extra spaces are allowed and ignored at the beginning of the conditional directive line, but a tab is not allowed. (If the line begins with a tab, it will be considered a command for a rule.) Aside from this, extra spaces or tabs may be inserted with no effect anywhere except within the directive name or within an argument. A comment starting with `#' may appear at the end of the line.
The other two directives that play a part in a conditional are else
and endif. Each of these directives is written as one word, with no
arguments. Extra spaces are allowed and ignored at the beginning of the
line, and spaces or tabs at the end. A comment starting with `#' may
appear at the end of the line.
Conditionals affect which lines of the makefile make uses. If
the condition is true, make reads the lines of the
text-if-true as part of the makefile; if the condition is false,
make ignores those lines completely. It follows that syntactic
units of the makefile, such as rules, may safely be split across the
beginning or the end of the conditional.
make evaluates conditionals when it reads a makefile.
Consequently, you cannot use automatic variables in the tests of
conditionals because they are not defined until commands are run
(see section Automatic Variables).
To prevent intolerable confusion, it is not permitted to start a
conditional in one makefile and end it in another. However, you may
write an include directive within a conditional, provided you do
not attempt to terminate the conditional inside the included file.
You can write a conditional that tests make command flags such as
`-t' by using the variable MAKEFLAGS together with the
findstring function
(see section Functions for String Substitution and Analysis).
This is useful when touch is not enough to make a file appear up
to date.
The findstring function determines whether one string appears as a
substring of another. If you want to test for the `-t' flag,
use `t' as the first string and the value of MAKEFLAGS as
the other.
For example, here is how to arrange to use `ranlib -t' to finish marking an archive file up to date:
archive.a: ...
ifneq (,$(findstring t,$(MAKEFLAGS)))
+touch archive.a
+ranlib -t archive.a
else
ranlib archive.a
endif
The `+' prefix marks those command lines as "recursive" so
that they will be executed despite use of the `-t' flag.
See section Recursive Use of make.
Functions allow you to do text processing in the makefile to compute the files to operate on or the commands to use. You use a function in a function call, where you give the name of the function and some text (the arguments) for the function to operate on. The result of the function's processing is substituted into the makefile at the point of the call, just as a variable might be substituted.
A function call resembles a variable reference. It looks like this:
$(function arguments)
or like this:
${function arguments}
Here function is a function name; one of a short list of names that
are part of make. There is no provision for defining new functions.
The arguments are the arguments of the function. They are separated from the function name by one or more spaces or tabs, and if there is more than one argument, then they are separated by commas. Such whitespace and commas are not part of an argument's value. The delimiters which you use to surround the function call, whether parentheses or braces, can appear in an argument only in matching pairs; the other kind of delimiters may appear singly. If the arguments themselves contain other function calls or variable references, it is wisest to use the same kind of delimiters for all the references; write `$(subst a,b,$(x))', not `$(subst a,b,${x})'. This is because it is clearer, and because only one type of delimiter is matched to find the end of the reference.
The text written for each argument is processed by substitution of variables and function calls to produce the argument value, which is the text on which the function acts. The substitution is done in the order in which the arguments appear.
Commas and unmatched parentheses or braces cannot appear in the text of an
argument as written; leading spaces cannot appear in the text of the first
argument as written. These characters can be put into the argument value
by variable substitution. First define variables comma and
space whose values are isolated comma and space characters, then
substitute these variables where such characters are wanted, like this:
comma:= , empty:= space:= $(empty) $(empty) foo:= a b c bar:= $(subst $(space),$(comma),$(foo)) # bar is now `a,b,c'.
Here the subst function replaces each space with a comma, through
the value of foo, and substitutes the result.
Here are some functions that operate on strings:
$(subst from,to,text)
$(subst ee,EE,feet on the street)substitutes the string `fEEt on the strEEt'.
$(patsubst pattern,replacement,text)
patsubst function invocations can be
quoted with preceding backslashes (`\'). Backslashes that would
otherwise quote `%' characters can be quoted with more backslashes.
Backslashes that quote `%' characters or other backslashes are
removed from the pattern before it is compared file names or has a stem
substituted into it. Backslashes that are not in danger of quoting
`%' characters go unmolested. For example, the pattern
`the\%weird\\%pattern\\' has `the%weird\' preceding the
operative `%' character, and `pattern\\' following it. The
final two backslashes are left alone because they cannot affect any
`%' character.
Whitespace between words is folded into single space characters;
leading and trailing whitespace is discarded.
For example,
$(patsubst %.c,%.o,x.c.c bar.c)produces the value `x.c.o bar.o'. Substitution references (see section Substitution References) are a simpler way to get the effect of the
patsubst
function:
$(var:pattern=replacement)is equivalent to
$(patsubst pattern,replacement,$(var))The second shorthand simplifies one of the most common uses of
patsubst: replacing the suffix at the end of file names.
$(var:suffix=replacement)is equivalent to
$(patsubst %suffix,%replacement,$(var))For example, you might have a list of object files:
objects = foo.o bar.o baz.oTo get the list of corresponding source files, you could simply write:
$(objects:.o=.c)instead of using the general form:
$(patsubst %.o,%.c,$(objects))
$(strip string)
strip can be very useful when used in conjunction
with conditionals. When comparing something with the empty string
`' using ifeq or ifneq, you usually want a string of
just whitespace to match the empty string (see section Conditional Parts of Makefiles).
Thus, the following may fail to have the desired results:
.PHONY: all ifneq "$(needs_made)" "" all: $(needs_made) else all:;@echo 'Nothing to make!' endifReplacing the variable reference `$(needs_made)' with the function call `$(strip $(needs_made))' in the
ifneq
directive would make it more robust.
$(findstring find,in)
$(findstring a,a b c) $(findstring a,b c)produce the values `a' and `' (the empty string), respectively. See section Conditionals that Test Flags, for a practical application of
findstring.
$(filter pattern...,text)
patsubst function above.
The filter function can be used to separate out different types
of strings (such as file names) in a variable. For example:
sources := foo.c bar.c baz.s ugh.h
foo: $(sources)
cc $(filter %.c %.s,$(sources)) -o foo
says that `foo' depends of `foo.c', `bar.c',
`baz.s' and `ugh.h' but only `foo.c', `bar.c' and
`baz.s' should be specified in the command to the
compiler.
$(filter-out pattern...,text)
filter
function.
For example, given:
objects=main1.o foo.o main2.o bar.o mains=main1.o main2.othe following generates a list which contains all the object files not in `mains':
$(filter-out $(mains),$(objects))
$(sort list)
$(sort foo bar lose)returns the value `bar foo lose'. Incidentally, since
sort removes duplicate words, you can use
it for this purpose even if you don't care about the sort order.
Here is a realistic example of the use of subst and
patsubst. Suppose that a makefile uses the VPATH variable
to specify a list of directories that make should search for
dependency files
(see section VPATH: Search Path for All Dependencies).
This example shows how to
tell the C compiler to search for header files in the same list of
directories.
The value of VPATH is a list of directories separated by colons,
such as `src:../headers'. First, the subst function is used to
change the colons to spaces:
$(subst :, ,$(VPATH))
This produces `src ../headers'. Then patsubst is used to turn
each directory name into a `-I' flag. These can be added to the
value of the variable CFLAGS, which is passed automatically to the C
compiler, like this:
override CFLAGS += $(patsubst %,-I%,$(subst :, ,$(VPATH)))
The effect is to append the text `-Isrc -I../headers' to the
previously given value of CFLAGS. The override directive is
used so that the new value is assigned even if the previous value of
CFLAGS was specified with a command argument (see section The override Directive).
Several of the built-in expansion functions relate specifically to taking apart file names or lists of file names.
Each of the following functions performs a specific transformation on a file name. The argument of the function is regarded as a series of file names, separated by whitespace. (Leading and trailing whitespace is ignored.) Each file name in the series is transformed in the same way and the results are concatenated with single spaces between them.
$(dir names...)
$(dir src/foo.c hacks)produces the result `src/ ./'.
$(notdir names...)
$(notdir src/foo.c hacks)produces the result `foo.c hacks'.
$(suffix names...)
$(suffix src/foo.c src-1.0/bar.c hacks)produces the result `.c .c'.
$(basename names...)
$(basename src/foo.c src-1.0/bar hacks)produces the result `src/foo src-1.0/bar hacks'.
$(addsuffix suffix,names...)
$(addsuffix .c,foo bar)produces the result `foo.c bar.c'.
$(addprefix prefix,names...)
$(addprefix src/,foo bar)produces the result `src/foo src/bar'.
$(join list1,list2)
dir and
notdir functions, to produce the original list of files which
was given to those two functions.
$(word n,text)
$(word 2, foo bar baz)returns `bar'.
$(wordlist s,e,text)
make swaps them for you. For
example,
$(wordlist 2, 3, foo bar baz)returns `bar baz'.
$(words text)
$(word $(words text),text).
$(firstword names...)
$(firstword foo bar)produces the result `foo'. Although
$(firstword
text) is the same as $(word 1,text), the
firstword function is retained for its simplicity.
$(wildcard pattern)
wildcard is a space-separated list of the names of existing files
that match the pattern.
See section Using Wildcard Characters in File Names.
foreach Function
The foreach function is very different from other functions. It
causes one piece of text to be used repeatedly, each time with a different
substitution performed on it. It resembles the for command in the
shell sh and the foreach command in the C-shell csh.
The syntax of the foreach function is:
$(foreach var,list,text)
The first two arguments, var and list, are expanded before anything else is done; note that the last argument, text, is not expanded at the same time. Then for each word of the expanded value of list, the variable named by the expanded value of var is set to that word, and text is expanded. Presumably text contains references to that variable, so its expansion will be different each time.
The result is that text is expanded as many times as there are
whitespace-separated words in list. The multiple expansions of
text are concatenated, with spaces between them, to make the result
of foreach.
This simple example sets the variable `files' to the list of all files in the directories in the list `dirs':
dirs := a b c d files := $(foreach dir,$(dirs),$(wildcard $(dir)/*))
Here text is `$(wildcard $(dir)/*)'. The first repetition
finds the value `a' for dir, so it produces the same result
as `$(wildcard a/*)'; the second repetition produces the result
of `$(wildcard b/*)'; and the third, that of `$(wildcard c/*)'.
This example has the same result (except for setting `dirs') as the following example:
files := $(wildcard a/* b/* c/* d/*)
When text is complicated, you can improve readability by giving it a name, with an additional variable:
find_files = $(wildcard $(dir)/*) dirs := a b c d files := $(foreach dir,$(dirs),$(find_files))
Here we use the variable find_files this way. We use plain `='
to define a recursively-expanding variable, so that its value contains an
actual function call to be reexpanded under the control of foreach;
a simply-expanded variable would not do, since wildcard would be
called only once at the time of defining find_files.
The foreach function has no permanent effect on the variable
var; its value and flavor after the foreach function call are
the same as they were beforehand. The other values which are taken from
list are in effect only temporarily, during the execution of
foreach. The variable var is a simply-expanded variable
during the execution of foreach. If var was undefined
before the foreach function call, it is undefined after the call.
See section The Two Flavors of Variables.
You must take care when using complex variable expressions that result in variable names because many strange things are valid variable names, but are probably not what you intended. For example,
files := $(foreach Esta escrito en espanol!,b c ch,$(find_files))
might be useful if the value of find_files references the variable
whose name is `Esta escrito en espanol!' (es un nombre bastante largo,
no?), but it is more likely to be a mistake.
origin Function
The origin function is unlike most other functions in that it does
not operate on the values of variables; it tells you something about
a variable. Specifically, it tells you where it came from.
The syntax of the origin function is:
$(origin variable)
Note that variable is the name of a variable to inquire about; not a reference to that variable. Therefore you would not normally use a `$' or parentheses when writing it. (You can, however, use a variable reference in the name if you want the name not to be a constant.)
The result of this function is a string telling you how the variable variable was defined:
CC
and so on. See section Variables Used by Implicit Rules.
Note that if you have redefined a default variable, the origin
function will return the origin of the later definition.
override directive in a
makefile (see section The override Directive).
This information is primarily useful (other than for your curiosity) to
determine if you want to believe the value of a variable. For example,
suppose you have a makefile `foo' that includes another makefile
`bar'. You want a variable bletch to be defined in `bar'
if you run the command `make -f bar', even if the environment contains
a definition of bletch. However, if `foo' defined
bletch before including `bar', you do not want to override that
definition. This could be done by using an override directive in
`foo', giving that definition precedence over the later definition in
`bar'; unfortunately, the override directive would also
override any command line definitions. So, `bar' could
include:
ifdef bletch ifeq "$(origin bletch)" "environment" bletch = barf, gag, etc. endif endif
If bletch has been defined from the environment, this will redefine
it.
If you want to override a previous definition of bletch if it came
from the environment, even under `-e', you could instead write:
ifneq "$(findstring environment,$(origin bletch))" "" bletch = barf, gag, etc. endif
Here the redefinition takes place if `$(origin bletch)' returns either `environment' or `environment override'. See section Functions for String Substitution and Analysis.
shell Function
The shell function is unlike any other function except the
wildcard function
(see section The Function wildcard) in that it
communicates with the world outside of make.
The shell function performs the same function that backquotes
(``') perform in most shells: it does command expansion. This
means that it takes an argument that is a shell command and returns the
output of the command. The only processing make does on the result,
before substituting it into the surrounding text, is to convert each
newline or carriage-return / newline pair to a single space. It also
removes the trailing (carriage-return and) newline, if it's the last
thing in the result.
The commands run by calls to the shell function are run when the
function calls are expanded. In most cases, this is when the makefile is
read in. The exception is that function calls in the commands of the rules
are expanded when the commands are run, and this applies to shell
function calls like all others.
Here are some examples of the use of the shell function:
contents := $(shell cat foo)
sets contents to the contents of the file `foo', with a space
(rather than a newline) separating each line.
files := $(shell echo *.c)
sets files to the expansion of `*.c'. Unless make is
using a very strange shell, this has the same result as
`$(wildcard *.c)'.
make
A makefile that says how to recompile a program can be used in more
than one way. The simplest use is to recompile every file that is out
of date. Usually, makefiles are written so that if you run
make with no arguments, it does just that.
But you might want to update only some of the files; you might want to use a different compiler or different compiler options; you might want just to find out which files are out of date without changing them.
By giving arguments when you run make, you can do any of these
things and many others.
The exit status of make is always one of three values:
0
make is successful.
2
make encounters any errors.
It will print messages describing the particular errors.
1
make
determines that some target is not already up to date.
See section Instead of Executing the Commands.
The way to specify the name of the makefile is with the `-f' or `--file' option (`--makefile' also works). For example, `-f altmake' says to use the file `altmake' as the makefile.
If you use the `-f' flag several times and follow each `-f' with an argument, all the specified files are used jointly as makefiles.
If you do not use the `-f' or `--file' flag, the default is to try `GNUmakefile', `makefile', and `Makefile', in that order, and use the first of these three which exists or can be made (see section Writing Makefiles).
The goals are the targets that make should strive ultimately
to update. Other targets are updated as well if they appear as
dependencies of goals, or dependencies of dependencies of goals, etc.
By default, the goal is the first target in the makefile (not counting targets that start with a period). Therefore, makefiles are usually written so that the first target is for compiling the entire program or programs they describe. If the first rule in the makefile has several targets, only the first target in the rule becomes the default goal, not the whole list.
You can specify a different goal or goals with arguments to make.
Use the name of the goal as an argument. If you specify several goals,
make processes each of them in turn, in the order you name them.
Any target in the makefile may be specified as a goal (unless it
starts with `-' or contains an `=', in which case it will be
parsed as a switch or variable definition, respectively). Even
targets not in the makefile may be specified, if make can find
implicit rules that say how to make them.
Make will set the special variable MAKECMDGOALS to the
list of goals you specified on the command line. If no goals were given
on the command line, this variable is empty. Note that this variable
should be used only in special circumstances.
An example of appropriate use is to avoid including `.d' files
during clean rules (see section Generating Dependencies Automatically), so
make won't create them only to immediately remove them
again:
sources = foo.c bar.c ifneq ($(MAKECMDGOALS),clean) include $(sources:.c=.d) endif
One use of specifying a goal is if you want to compile only a part of the program, or only one of several programs. Specify as a goal each file that you wish to remake. For example, consider a directory containing several programs, with a makefile that starts like this:
.PHONY: all all: size nm ld ar as
If you are working on the program size, you might want to say
`make size' so that only the files of that program are recompiled.
Another use of specifying a goal is to make files that are not normally made. For example, there may be a file of debugging output, or a version of the program that is compiled specially for testing, which has a rule in the makefile but is not a dependency of the default goal.
Another use of specifying a goal is to run the commands associated with a phony target (see section Phony Targets) or empty target (see section Empty Target Files to Record Events). Many makefiles contain a phony target named `clean' which deletes everything except source files. Naturally, this is done only if you request it explicitly with `make clean'. Following is a list of typical phony and empty target names. See section Standard Targets for Users, for a detailed list of all the standard target names which GNU software packages use.
make.
The makefile tells make how to tell whether a target is up to date,
and how to update each target. But updating the targets is not always
what you want. Certain options specify other activities for make.
make pretends to compile
the targets but does not really change their contents.
make as being the present
time, although the actual modification times remain the same.
You can use the `-W' flag in conjunction with the `-n' flag
to see what would happen if you were to modify specific files.
With the `-n' flag, make prints the commands that it would
normally execute but does not execute them.
With the `-t' flag, make ignores the commands in the rules
and uses (in effect) the command touch for each target that needs to
be remade. The touch command is also printed, unless `-s' or
.SILENT is used. For speed, make does not actually invoke
the program touch. It does the work directly.
With the `-q' flag, make prints nothing and executes no
commands, but the exit status code it returns is zero if and only if the
targets to be considered are already up to date. If the exit status is
one, then some updating needs to be done. If make encounters an
error, the exit status is two, so you can distinguish an error from a
target that is not up to date.
It is an error to use more than one of these three flags in the same
invocation of make.
The `-n', `-t', and `-q' options do not affect command
lines that begin with `+' characters or contain the strings
`$(MAKE)' or `${MAKE}'. Note that only the line containing
the `+' character or the strings `$(MAKE)' or `${MAKE}'
is run regardless of these options. Other lines in the same rule are
not run unless they too begin with `+' or contain `$(MAKE)' or
`${MAKE}' (See section How the MAKE Variable Works.)
The `-W' flag provides two features:
make would do if you were to modify some files.
make is actually
executing commands, the `-W' flag can direct make to act
as if some files had been modified, without actually modifying the
files.
Note that the options `-p' and `-v' allow you to obtain other
information about make or about the makefiles in use
(see section Summary of Options).
Sometimes you may have changed a source file but you do not want to
recompile all the files that depend on it. For example, suppose you add a
macro or a declaration to a header file that many other files depend on.
Being conservative, make assumes that any change in the header file
requires recompilation of all dependent files, but you know that they do not
need to be recompiled and you would rather not waste the time waiting for
them to compile.
If you anticipate the problem before changing the header file, you can
use the `-t' flag. This flag tells make not to run the
commands in the rules, but rather to mark the target up to date by
changing its last-modification date. You would follow this procedure:
make, the changes in the
header files will not cause any recompilation.
If you have already changed the header file at a time when some files do need recompilation, it is too late to do this. Instead, you can use the `-o file' flag, which marks a specified file as "old" (see section Summary of Options). This means that the file itself will not be remade, and nothing else will be remade on its account. Follow this procedure:
An argument that contains `=' specifies the value of a variable: `v=x' sets the value of the variable v to x. If you specify a value in this way, all ordinary assignments of the same variable in the makefile are ignored; we say they have been overridden by the command line argument.
The most common way to use this facility is to pass extra flags to
compilers. For example, in a properly written makefile, the variable
CFLAGS is included in each command that runs the C compiler, so a
file `foo.c' would be compiled something like this:
cc -c $(CFLAGS) foo.c
Thus, whatever value you set for CFLAGS affects each compilation
that occurs. The makefile probably specifies the usual value for
CFLAGS, like this:
CFLAGS=-g
Each time you run make, you can override this value if you
wish. For example, if you say `make CFLAGS='-g -O'', each C
compilation will be done with `cc -c -g -O'. (This illustrates
how you can use quoting in the shell to enclose spaces and other
special characters in the value of a variable when you override it.)
The variable CFLAGS is only one of many standard variables that
exist just so that you can change them this way. See section Variables Used by Implicit Rules, for a complete list.
You can also program the makefile to look at additional variables of your own, giving the user the ability to control other aspects of how the makefile works by changing the variables.
When you override a variable with a command argument, you can define either a recursively-expanded variable or a simply-expanded variable. The examples shown above make a recursively-expanded variable; to make a simply-expanded variable, write `:=' instead of `='. But, unless you want to include a variable reference or function call in the value that you specify, it makes no difference which kind of variable you create.
There is one way that the makefile can change a variable that you have
overridden. This is to use the override directive, which is a line
that looks like this: `override variable = value'
(see section The override Directive).
Normally, when an error happens in executing a shell command, make
gives up immediately, returning a nonzero status. No further commands are
executed for any target. The error implies that the goal cannot be
correctly remade, and make reports this as soon as it knows.
When you are compiling a program that you have just changed, this is not
what you want. Instead, you would rather that make try compiling
every file that can be tried, to show you as many compilation errors
as possible.
On these occasions, you should use the `-k' or
`--keep-going' flag. This tells make to continue to
consider the other dependencies of the pending targets, remaking them
if necessary, before it gives up and returns nonzero status. For
example, after an error in compiling one object file, `make -k'
will continue compiling other object files even though it already
knows that linking them will be impossible. In addition to continuing
after failed shell commands, `make -k' will continue as much as
possible after discovering that it does not know how to make a target
or dependency file. This will always cause an error message, but
without `-k', it is a fatal error (see section Summary of Options).
The usual behavior of make assumes that your purpose is to get the
goals up to date; once make learns that this is impossible, it might
as well report the failure immediately. The `-k' flag says that the
real purpose is to test as much as possible of the changes made in the
program, perhaps to find several independent problems so that you can
correct them all before the next attempt to compile. This is why Emacs'
M-x compile command passes the `-k' flag by default.
Here is a table of all the options make understands:
make.
make
(see section Recursive Use of make).
make decides what to do.
make understands and then exit.
make runs as many jobs simultaneously as possible. If
there is more than one `-j' option, the last one is effective.
See section Parallel Execution,
for more information on how commands are run.
Note that this option is ignored on MS-DOS.
.SUFFIXES, and then define your own suffix rules. Note that only
rules are affected by the -r option; default variables
remain in effect (see section Variables Used by Implicit Rules).
make where `-k' might be inherited
from the top-level make via MAKEFLAGS
(see section Recursive Use of make)
or if you set `-k' in MAKEFLAGS in your environment.
make. See section Instead of Executing the Commands.
make program plus a copyright, a list
of authors, and a notice that there is no warranty; then exit.
make commands.
See section Recursive Use of make. (In practice, you
rarely need to specify this option since `make' does it for you;
see section The `--print-directory' Option.)
-w.
This option is useful when -w is turned on automatically,
but you do not want to see the extra messages.
See section The `--print-directory' Option.
touch command on the given file before running
make, except that the modification time is changed only in the
imagination of make.
See section Instead of Executing the Commands.
make sees a reference to an
undefined variable. This can be helpful when you are trying to debug
makefiles which use variables in complex ways.
Certain standard ways of remaking target files are used very often. For
example, one customary way to make an object file is from a C source file
using the C compiler, cc.
Implicit rules tell make how to use customary techniques so
that you do not have to specify them in detail when you want to use
them. For example, there is an implicit rule for C compilation. File
names determine which implicit rules are run. For example, C
compilation typically takes a `.c' file and makes a `.o' file.
So make applies the implicit rule for C compilation when it sees
this combination of file name endings.
A chain of implicit rules can apply in sequence; for example, make
will remake a `.o' file from a `.y' file by way of a `.c' file.
See section Chains of Implicit Rules.
The built-in implicit rules use several variables in their commands so
that, by changing the values of the variables, you can change the way the
implicit rule works. For example, the variable CFLAGS controls the
flags given to the C compiler by the implicit rule for C compilation.
See section Variables Used by Implicit Rules.
You can define your own implicit rules by writing pattern rules. See section Defining and Redefining Pattern Rules.
Suffix rules are a more limited way to define implicit rules. Pattern rules are more general and clearer, but suffix rules are retained for compatibility. See section Old-Fashioned Suffix Rules.
To allow make to find a customary method for updating a target file,
all you have to do is refrain from specifying commands yourself. Either
write a rule with no command lines, or don't write a rule at all. Then
make will figure out which implicit rule to use based on which
kind of source file exists or can be made.
For example, suppose the makefile looks like this:
foo : foo.o bar.o
cc -o foo foo.o bar.o $(CFLAGS) $(LDFLAGS)
Because you mention `foo.o' but do not give a rule for it, make
will automatically look for an implicit rule that tells how to update it.
This happens whether or not the file `foo.o' currently exists.
If an implicit rule is found, it can supply both commands and one or more dependencies (the source files). You would want to write a rule for `foo.o' with no command lines if you need to specify additional dependencies, such as header files, that the implicit rule cannot supply.
Each implicit rule has a target pattern and dependency patterns. There may
be many implicit rules with the same target pattern. For example, numerous
rules make `.o' files: one, from a `.c' file with the C compiler;
another, from a `.p' file with the Pascal compiler; and so on. The rule
that actually applies is the one whose dependencies exist or can be made.
So, if you have a file `foo.c', make will run the C compiler;
otherwise, if you have a file `foo.p', make will run the Pascal
compiler; and so on.
Of course, when you write the makefile, you know which implicit rule you
want make to use, and you know it will choose that one because you
know which possible dependency files are supposed to exist.
See section Catalogue of Implicit Rules,
for a catalogue of all the predefined implicit rules.
Above, we said an implicit rule applies if the required dependencies "exist or can be made". A file "can be made" if it is mentioned explicitly in the makefile as a target or a dependency, or if an implicit rule can be recursively found for how to make it. When an implicit dependency is the result of another implicit rule, we say that chaining is occurring. See section Chains of Implicit Rules.
In general, make searches for an implicit rule for each target, and
for each double-colon rule, that has no commands. A file that is mentioned
only as a dependency is considered a target whose rule specifies nothing,
so implicit rule search happens for it. See section Implicit Rule Search Algorithm, for the
details of how the search is done.
Note that explicit dependencies do not influence implicit rule search. For example, consider this explicit rule:
foo.o: foo.p
The dependency on `foo.p' does not necessarily mean that
make will remake `foo.o' according to the implicit rule to
make an object file, a `.o' file, from a Pascal source file, a
`.p' file. For example, if `foo.c' also exists, the implicit
rule to make an object file from a C source file is used instead,
because it appears before the Pascal rule in the list of predefined
implicit rules (see section Catalogue of Implicit Rules).
If you do not want an implicit rule to be used for a target that has no commands, you can give that target empty commands by writing a semicolon (see section Using Empty Commands).
Here is a catalogue of predefined implicit rules which are always available unless the makefile explicitly overrides or cancels them. See section Canceling Implicit Rules, for information on canceling or overriding an implicit rule. The `-r' or `--no-builtin-rules' option cancels all predefined rules.
Not all of these rules will always be defined, even when the `-r'
option is not given. Many of the predefined implicit rules are
implemented in make as suffix rules, so which ones will be
defined depends on the suffix list (the list of dependencies of
the special target .SUFFIXES). The default suffix list is:
.out, .a, .ln, .o, .c, .cc,
.C, .p, .f, .F, .r, .y,
.l, .s, .S, .mod, .sym, .def,
.h, .info, .dvi, .tex, .texinfo,
.texi, .txinfo, .w, .ch .web,
.sh, .elc, .el. All of the implicit rules
described below whose dependencies have one of these suffixes are
actually suffix rules. If you modify the suffix list, the only
predefined suffix rules in effect will be those named by one or two of
the suffixes that are on the list you specify; rules whose suffixes fail
to be on the list are disabled. See section Old-Fashioned Suffix Rules, for full details on suffix rules.
as. The precise command is
`$(AS) $(ASFLAGS)'.
`n.s' is made automatically from `n.S' by
running the C preprocessor, cpp. The precise command is
`$(CPP) $(CPPFLAGS)'.
ld) via the C compiler. The precise
command used is `$(CC) $(LDFLAGS) n.o $(LOADLIBES)'.
This rule does the right thing for a simple program with only one
source file. It will also do the right thing if there are multiple
object files (presumably coming from various other source files), one
of which has a name matching that of the executable file. Thus,
x: y.o z.owhen `x.c', `y.c' and `z.c' all exist will execute:
cc -c x.c -o x.o cc -c y.c -o y.o cc -c z.c -o z.o cc x.o y.o z.o -o x rm -f x.o rm -f y.o rm -f z.oIn more complicated cases, such as when there is no object file whose name derives from the executable file name, you must write an explicit command for linking. Each kind of file automatically made into `.o' object files will be automatically linked by using the compiler (`$(CC)', `$(FC)' or `$(PC)'; the C compiler `$(CC)' is used to assemble `.s' files) without the `-c' option. This could be done by using the `.o' object files as intermediates, but it is faster to do the compiling and linking in one step, so that's how it's done.
make to determine automatically which of the two
languages you are using in any particular case. If make is
called upon to remake an object file from a `.l' file, it must
guess which compiler to use. It will guess the C compiler, because
that is more common. If you are using Ratfor, make sure make
knows this by mentioning `n.r' in the makefile. Or, if you
are using Ratfor exclusively, with no C files, remove `.c' from
the list of implicit rule suffixes with:
.SUFFIXES: .SUFFIXES: .o .r .f .l ...
lint.
The precise command is `$(LINT) $(LINTFLAGS) $(CPPFLAGS) -i'.
The same command is used on the C code produced from
`n.y' or `n.l'.
Usually, you want to change only the variables listed in the table above, which are documented in the following section.
However, the commands in built-in implicit rules actually use
variables such as COMPILE.c, LINK.p, and
PREPROCESS.S, whose values contain the commands listed above.
make follows the convention that the rule to compile a
`.x' source file uses the variable COMPILE.x.
Similarly, the rule to produce an executable from a `.x'
file uses LINK.x; and the rule to preprocess a
`.x' file uses PREPROCESS.x.
Every rule that produces an object file uses the variable
OUTPUT_OPTION. make defines this variable either to
contain `-o $@', or to be empty, depending on a compile-time
option. You need the `-o' option to ensure that the output goes
into the right file when the source file is in a different directory,
as when using VPATH (see section Searching Directories for Dependencies). However,
compilers on some systems do not accept a `-o' switch for object
files. If you use such a system, and use VPATH, some
compilations will put their output in the wrong place.
A possible workaround for this problem is to give OUTPUT_OPTION
the value `; mv $*.o $@'.
The commands in built-in implicit rules make liberal use of certain
predefined variables. You can alter these variables in the makefile,
with arguments to make, or in the environment to alter how the
implicit rules work without redefining the rules themselves.
For example, the command used to compile a C source file actually says `$(CC) -c $(CFLAGS) $(CPPFLAGS)'. The default values of the variables used are `cc' and nothing, resulting in the command `cc -c'. By redefining `CC' to `ncc', you could cause `ncc' to be used for all C compilations performed by the implicit rule. By redefining `CFLAGS' to be `-g', you could pass the `-g' option to each compilation. All implicit rules that do C compilation use `$(CC)' to get the program name for the compiler and all include `$(CFLAGS)' among the arguments given to the compiler.
The variables used in implicit rules fall into two classes: those that are
names of programs (like CC) and those that contain arguments for the
programs (like CFLAGS). (The "name of a program" may also contain
some command arguments, but it must start with an actual executable program
name.) If a variable value contains more than one argument, separate them
with spaces.
Here is a table of variables used as names of programs in built-in rules:
AR
AS
CC
CXX
CO
CPP
FC
GET
LEX
PC
YACC
YACCR
MAKEINFO
TEX
TEXI2DVI
WEAVE
CWEAVE
TANGLE
CTANGLE
RM
Here is a table of variables whose values are additional arguments for the programs above. The default values for all of these is the empty string, unless otherwise noted.
ARFLAGS
ASFLAGS
CFLAGS
CXXFLAGS
COFLAGS
co program.
CPPFLAGS
FFLAGS
GFLAGS
get program.
LDFLAGS
LFLAGS
PFLAGS
RFLAGS
YFLAGS
Sometimes a file can be made by a sequence of implicit rules. For example,
a file `n.o' could be made from `n.y' by running
first Yacc and then cc. Such a sequence is called a chain.
If the file `n.c' exists, or is mentioned in the makefile, no
special searching is required: make finds that the object file can
be made by C compilation from `n.c'; later on, when considering
how to make `n.c', the rule for running Yacc is
used. Ultimately both `n.c' and `n.o' are
updated.
However, even if `n.c' does not exist and is not mentioned,
make knows how to envision it as the missing link between
`n.o' and `n.y'! In this case, `n.c' is
called an intermediate file. Once make has decided to use the
intermediate file, it is entered in the data base as if it had been
mentioned in the makefile, along with the implicit rule that says how to
create it.
Intermediate files are remade using their rules just like all other files. But intermediate files are treated differently in two ways.
The first difference is what happens if the intermediate file does not
exist. If an ordinary file b does not exist, and make
considers a target that depends on b, it invariably creates
b and then updates the target from b. But if b is an
intermediate file, then make can leave well enough alone. It
won't bother updating b, or the ultimate target, unless some
dependency of b is newer than that target or there is some other
reason to update that target.
The second difference is that if make does create b
in order to update something else, it deletes b later on after it
is no longer needed. Therefore, an intermediate file which did not
exist before make also does not exist after make.
make reports the deletion to you by printing a `rm -f'
command showing which file it is deleting.
Ordinarily, a file cannot be intermediate if it is mentioned in the
makefile as a target or dependency. However, you can explicitly mark a
file as intermediate by listing it as a dependency of the special target
.INTERMEDIATE. This takes effect even if the file is mentioned
explicitly in some other way.
You can prevent automatic deletion of an intermediate file by marking it
as a secondary file. To do this, list it as a dependency of the
special target .SECONDARY. When a file is secondary, make
will not create the file merely because it does not already exist, but
make does not automatically delete the file. Marking a file as
secondary also marks it as intermediate.
You can list the target pattern of an implicit rule (such as `%.o')
as a dependency of the special target .PRECIOUS to preserve
intermediate files made by implicit rules whose target patterns match
that file's name; see section Interrupting or Killing make.
A chain can involve more than two implicit rules. For example, it is
possible to make a file `foo' from `RCS/foo.y,v' by running RCS,
Yacc and cc. Then both `foo.y' and `foo.c' are
intermediate files that are deleted at the end.
No single implicit rule can appear more than once in a chain. This means
that make will not even consider such a ridiculous thing as making
`foo' from `foo.o.o' by running the linker twice. This
constraint has the added benefit of preventing any infinite loop in the
search for an implicit rule chain.
There are some special implicit rules to optimize certain cases that would
otherwise be handled by rule chains. For example, making `foo' from
`foo.c' could be handled by compiling and linking with separate
chained rules, using `foo.o' as an intermediate file. But what
actually happens is that a special rule for this case does the compilation
and linking with a single cc command. The optimized rule is used in
preference to the step-by-step chain because it comes earlier in the
ordering of rules.
You define an implicit rule by writing a pattern rule. A pattern rule looks like an ordinary rule, except that its target contains the character `%' (exactly one of them). The target is considered a pattern for matching file names; the `%' can match any nonempty substring, while other characters match only themselves. The dependencies likewise use `%' to show how their names relate to the target name.
Thus, a pattern rule `%.o : %.c' says how to make any file `stem.o' from another file `stem.c'.
Note that expansion using `%' in pattern rules occurs after any variable or function expansions, which take place when the makefile is read. See section How to Use Variables, and section Functions for Transforming Text.
A pattern rule contains the character `%' (exactly one of them) in the target; otherwise, it looks exactly like an ordinary rule. The target is a pattern for matching file names; the `%' matches any nonempty substring, while other characters match only themselves.
For example, `%.c' as a pattern matches any file name that ends in `.c'. `s.%.c' as a pattern matches any file name that starts with `s.', ends in `.c' and is at least five characters long. (There must be at least one character to match the `%'.) The substring that the `%' matches is called the stem.
`%' in a dependency of a pattern rule stands for the same stem that was matched by the `%' in the target. In order for the pattern rule to apply, its target pattern must match the file name under consideration, and its dependency patterns must name files that exist or can be made. These files become dependencies of the target.
Thus, a rule of the form
%.o : %.c ; command...
specifies how to make a file `n.o', with another file `n.c' as its dependency, provided that `n.c' exists or can be made.
There may also be dependencies that do not use `%'; such a dependency attaches to every file made by this pattern rule. These unvarying dependencies are useful occasionally.
A pattern rule need not have any dependencies that contain `%', or in fact any dependencies at all. Such a rule is effectively a general wildcard. It provides a way to make any file that matches the target pattern. See section Defining Last-Resort Default Rules.
Pattern rules may have more than one target. Unlike normal rules, this
does not act as many different rules with the same dependencies and
commands. If a pattern rule has multiple targets, make knows that
the rule's commands are responsible for making all of the targets. The
commands are executed only once to make all the targets. When searching
for a pattern rule to match a target, the target patterns of a rule other
than the one that matches the target in need of a rule are incidental:
make worries only about giving commands and dependencies to the file
presently in question. However, when this file's commands are run, the
other targets are marked as having been updated themselves.
The order in which pattern rules appear in the makefile is important since this is the order in which they are considered. Of equally applicable rules, only the first one found is used. The rules you write take precedence over those that are built in. Note however, that a rule whose dependencies actually exist or are mentioned always takes priority over a rule with dependencies that must be made by chaining other implicit rules.
Here are some examples of pattern rules actually predefined in
make. First, the rule that compiles `.c' files into `.o'
files:
%.o : %.c
$(CC) -c $(CFLAGS) $(CPPFLAGS) $< -o $@
defines a rule that can make any file `x.o' from `x.c'. The command uses the automatic variables `$@' and `$<' to substitute the names of the target file and the source file in each case where the rule applies (see section Automatic Variables).
Here is a second built-in rule:
% :: RCS/%,v
$(CO) $(COFLAGS) $<
defines a rule that can make any file `x' whatsoever from a corresponding file `x,v' in the subdirectory `RCS'. Since the target is `%', this rule will apply to any file whatever, provided the appropriate dependency file exists. The double colon makes the rule terminal, which means that its dependency may not be an intermediate file (see section Match-Anything Pattern Rules).
This pattern rule has two targets:
%.tab.c %.tab.h: %.y
bison -d $<
This tells make that the command `bison -d x.y' will
make both `x.tab.c' and `x.tab.h'. If the file
`foo' depends on the files `parse.tab.o' and `scan.o'
and the file `scan.o' depends on the file `parse.tab.h',
when `parse.y' is changed, the command `bison -d parse.y'
will be executed only once, and the dependencies of both
`parse.tab.o' and `scan.o' will be satisfied. (Presumably
the file `parse.tab.o' will be recompiled from `parse.tab.c'
and the file `scan.o' from `scan.c', while `foo' is
linked from `parse.tab.o', `scan.o', and its other
dependencies, and it will execute happily ever after.)
Suppose you are writing a pattern rule to compile a `.c' file into a `.o' file: how do you write the `cc' command so that it operates on the right source file name? You cannot write the name in the command, because the name is different each time the implicit rule is applied.
What you do is use a special feature of make, the automatic
variables. These variables have values computed afresh for each rule that
is executed, based on the target and dependencies of the rule. In this
example, you would use `$@' for the object file name and `$<'
for the source file name.
Here is a table of automatic variables:
$@
$%
make to Update Archive Files. For example, if the target is `foo.a(bar.o)' then
`$%' is `bar.o' and `$@' is `foo.a'. `$%' is
empty when the target is not an archive member.
$<
$?
make to Update Archive Files).
$^
make to Update Archive Files). A target has only one dependency on each other file
it depends on, no matter how many times each file is listed as a
dependency. So if you list a dependency more than once for a target,
the value of $^ contains just one copy of the name.
$+
$*
make does this bizarre thing only for compatibility
with other implementations of make. You should generally avoid
using `$*' except in implicit rules or static pattern rules.
If the target name in an explicit rule does not end with a recognized
suffix, `$*' is set to the empty string for that rule.
`$?' is useful even in explicit rules when you wish to operate on only the dependencies that have changed. For example, suppose that an archive named `lib' is supposed to contain copies of several object files. This rule copies just the changed object files into the archive:
lib: foo.o bar.o lose.o win.o
ar r lib $?
Of the variables listed above, four have values that are single file
names, and two have values that are lists of file names. These six have
variants that get just the file's directory name or just the file name
within the directory. The variant variables' names are formed by
appending `D' or `F', respectively. These variants are
semi-obsolete in GNU make since the functions dir and
notdir can be used to get a similar effect (see section Functions for File Names). Note, however, that the
`F' variants all omit the trailing slash which always appears in
the output of the dir function. Here is a table of the variants:
Note that we use a special stylistic convention when we talk about these
automatic variables; we write "the value of `$<'", rather than
"the variable <" as we would write for ordinary variables
such as objects and CFLAGS. We think this convention
looks more natural in this special case. Please do not assume it has a
deep significance; `$<' refers to the variable named < just
as `$(CFLAGS)' refers to the variable named CFLAGS.
You could just as well use `$(<)' in place of `$<'.
A target pattern is composed of a `%' between a prefix and a suffix, either or both of which may be empty. The pattern matches a file name only if the file name starts with the prefix and ends with the suffix, without overlap. The text between the prefix and the suffix is called the stem. Thus, when the pattern `%.o' matches the file name `test.o', the stem is `test'. The pattern rule dependencies are turned into actual file names by substituting the stem for the character `%'. Thus, if in the same example one of the dependencies is written as `%.c', it expands to `test.c'.
When the target pattern does not contain a slash (and it usually does not), directory names in the file names are removed from the file name before it is compared with the target prefix and suffix. After the comparison of the file name to the target pattern, the directory names, along with the slash that ends them, are added on to the dependency file names generated from the pattern rule's dependency patterns and the file name. The directories are ignored only for the purpose of finding an implicit rule to use, not in the application of that rule. Thus, `e%t' matches the file name `src/eat', with `src/a' as the stem. When dependencies are turned into file names, the directories from the stem are added at the front, while the rest of the stem is substituted for the `%'. The stem `src/a' with a dependency pattern `c%r' gives the file name `src/car'.
When a pattern rule's target is just `%', it matches any file name
whatever. We call these rules match-anything rules. They are very
useful, but it can take a lot of time for make to think about them,
because it must consider every such rule for each file name listed either
as a target or as a dependency.
Suppose the makefile mentions `foo.c'. For this target, make
would have to consider making it by linking an object file `foo.c.o',
or by C compilation-and-linking in one step from `foo.c.c', or by
Pascal compilation-and-linking from `foo.c.p', and many other
possibilities.
We know these possibilities are ridiculous since `foo.c' is a C source
file, not an executable. If make did consider these possibilities,
it would ultimately reject them, because files such as `foo.c.o' and
`foo.c.p' would not exist. But these possibilities are so
numerous that make would run very slowly if it had to consider
them.
To gain speed, we have put various constraints on the way make
considers match-anything rules. There are two different constraints that
can be applied, and each time you define a match-anything rule you must
choose one or the other for that rule.
One choice is to mark the match-anything rule as terminal by defining it with a double colon. When a rule is terminal, it does not apply unless its dependencies actually exist. Dependencies that could be made with other implicit rules are not good enough. In other words, no further chaining is allowed beyond a terminal rule.
For example, the built-in implicit rules for extracting sources from RCS
and SCCS files are terminal; as a result, if the file `foo.c,v' does
not exist, make will not even consider trying to make it as an
intermediate file from `foo.c,v.o' or from `RCS/SCCS/s.foo.c,v'.
RCS and SCCS files are generally ultimate source files, which should not be
remade from any other files; therefore, make can save time by not
looking for ways to remake them.
If you do not mark the match-anything rule as terminal, then it is nonterminal. A nonterminal match-anything rule cannot apply to a file name that indicates a specific type of data. A file name indicates a specific type of data if some non-match-anything implicit rule target matches it.
For example, the file name `foo.c' matches the target for the pattern
rule `%.c : %.y' (the rule to run Yacc). Regardless of whether this
rule is actually applicable (which happens only if there is a file
`foo.y'), the fact that its target matches is enough to prevent
consideration of any nonterminal match-anything rules for the file
`foo.c'. Thus, make will not even consider trying to make
`foo.c' as an executable file from `foo.c.o', `foo.c.c',
`foo.c.p', etc.
The motivation for this constraint is that nonterminal match-anything rules are used for making files containing specific types of data (such as executable files) and a file name with a recognized suffix indicates some other specific type of data (such as a C source file).
Special built-in dummy pattern rules are provided solely to recognize certain file names so that nonterminal match-anything rules will not be considered. These dummy rules have no dependencies and no commands, and they are ignored for all other purposes. For example, the built-in implicit rule
%.p :
exists to make sure that Pascal source files such as `foo.p' match a specific target pattern and thereby prevent time from being wasted looking for `foo.p.o' or `foo.p.c'.
Dummy pattern rules such as the one for `%.p' are made for every suffix listed as valid for use in suffix rules (see section Old-Fashioned Suffix Rules).
You can override a built-in implicit rule (or one you have defined yourself) by defining a new pattern rule with the same target and dependencies, but different commands. When the new rule is defined, the built-in one is replaced. The new rule's position in the sequence of implicit rules is determined by where you write the new rule.
You can cancel a built-in implicit rule by defining a pattern rule with the same target and dependencies, but no commands. For example, the following would cancel the rule that runs the assembler:
%.o : %.s
You can define a last-resort implicit rule by writing a terminal match-anything pattern rule with no dependencies (see section Match-Anything Pattern Rules). This is just like any other pattern rule; the only thing special about it is that it will match any target. So such a rule's commands are used for all targets and dependencies that have no commands of their own and for which no other implicit rule applies.
For example, when testing a makefile, you might not care if the source files contain real data, only that they exist. Then you might do this:
%::
touch $@
to cause all the source files needed (as dependencies) to be created automatically.
You can instead define commands to be used for targets for which there
are no rules at all, even ones which don't specify commands. You do
this by writing a rule for the target .DEFAULT. Such a rule's
commands are used for all dependencies which do not appear as targets in
any explicit rule, and for which no implicit rule applies. Naturally,
there is no .DEFAULT rule unless you write one.
If you use .DEFAULT with no commands or dependencies:
.DEFAULT:
the commands previously stored for .DEFAULT are cleared.
Then make acts as if you had never defined .DEFAULT at all.
If you do not want a target to get the commands from a match-anything
pattern rule or .DEFAULT, but you also do not want any commands
to be run for the target, you can give it empty commands (see section Using Empty Commands).
You can use a last-resort rule to override part of another makefile. See section Overriding Part of Another Makefile.
Suffix rules are the old-fashioned way of defining implicit rules for
make. Suffix rules are obsolete because pattern rules are more
general and clearer. They are supported in GNU make for
compatibility with old makefiles. They come in two kinds:
double-suffix and single-suffix.
A double-suffix rule is defined by a pair of suffixes: the target suffix and the source suffix. It matches any file whose name ends with the target suffix. The corresponding implicit dependency is made by replacing the target suffix with the source suffix in the file name. A two-suffix rule whose target and source suffixes are `.o' and `.c' is equivalent to the pattern rule `%.o : %.c'.
A single-suffix rule is defined by a single suffix, which is the source suffix. It matches any file name, and the corresponding implicit dependency name is made by appending the source suffix. A single-suffix rule whose source suffix is `.c' is equivalent to the pattern rule `% : %.c'.
Suffix rule definitions are recognized by comparing each rule's target
against a defined list of known suffixes. When make sees a rule
whose target is a known suffix, this rule is considered a single-suffix
rule. When make sees a rule whose target is two known suffixes
concatenated, this rule is taken as a double-suffix rule.
For example, `.c' and `.o' are both on the default list of
known suffixes. Therefore, if you define a rule whose target is
`.c.o', make takes it to be a double-suffix rule with source
suffix `.c' and target suffix `.o'. Here is the old-fashioned
way to define the rule for compiling a C source file:
.c.o:
$(CC) -c $(CFLAGS) $(CPPFLAGS) -o $@ $<
Suffix rules cannot have any dependencies of their own. If they have any, they are treated as normal files with funny names, not as suffix rules. Thus, the rule:
.c.o: foo.h
$(CC) -c $(CFLAGS) $(CPPFLAGS) -o $@ $<
tells how to make the file `.c.o' from the dependency file `foo.h', and is not at all like the pattern rule:
%.o: %.c foo.h
$(CC) -c $(CFLAGS) $(CPPFLAGS) -o $@ $<
which tells how to make `.o' files from `.c' files, and makes all `.o' files using this pattern rule also depend on `foo.h'.
Suffix rules with no commands are also meaningless. They do not remove previous rules as do pattern rules with no commands (see section Canceling Implicit Rules). They simply enter the suffix or pair of suffixes concatenated as a target in the data base.
The known suffixes are simply the names of the dependencies of the special
target .SUFFIXES. You can add your own suffixes by writing a rule
for .SUFFIXES that adds more dependencies, as in:
.SUFFIXES: .hack .win
which adds `.hack' and `.win' to the end of the list of suffixes.
If you wish to eliminate the default known suffixes instead of just adding
to them, write a rule for .SUFFIXES with no dependencies. By
special dispensation, this eliminates all existing dependencies of
.SUFFIXES. You can then write another rule to add the suffixes you
want. For example,
.SUFFIXES: # Delete the default suffixes .SUFFIXES: .c .o .h # Define our suffix list
The `-r' or `--no-builtin-rules' flag causes the default list of suffixes to be empty.
The variable SUFFIXES is defined to the default list of suffixes
before make reads any makefiles. You can change the list of suffixes
with a rule for the special target .SUFFIXES, but that does not alter
this variable.
Here is the procedure make uses for searching for an implicit rule
for a target t. This procedure is followed for each double-colon
rule with no commands, for each target of ordinary rules none of which have
commands, and for each dependency that is not the target of any rule. It
is also followed recursively for dependencies that come from implicit
rules, in the search for a chain of rules.
Suffix rules are not mentioned in this algorithm because suffix rules are converted to equivalent pattern rules once the makefiles have been read in.
For an archive member target of the form `archive(member)', the following algorithm is run twice, first using the entire target name t, and second using `(member)' as the target t if the first run found no rule.
.DEFAULT, if any,
applies. In that case, give t the same commands that
.DEFAULT has. Otherwise, there are no commands for t.
Once a rule that applies has been found, for each target pattern of the rule other than the one that matched t or n, the `%' in the pattern is replaced with s and the resultant file name is stored until the commands to remake the target file t are executed. After these commands are executed, each of these stored file names are entered into the data base and marked as having been updated and having the same update status as the file t.
When the commands of a pattern rule are executed for t, the automatic variables are set corresponding to the target and dependencies. See section Automatic Variables.
make to Update Archive Files
Archive files are files containing named subfiles called
members; they are maintained with the program ar and their
main use is as subroutine libraries for linking.
An individual member of an archive file can be used as a target or
dependency in make. You specify the member named member in
archive file archive as follows:
archive(member)
This construct is available only in targets and dependencies, not in
commands! Most programs that you might use in commands do not support this
syntax and cannot act directly on archive members. Only ar and
other programs specifically designed to operate on archives can do so.
Therefore, valid commands to update an archive member target probably must
use ar. For example, this rule says to create a member
`hack.o' in archive `foolib' by copying the file `hack.o':
foolib(hack.o) : hack.o
ar cr foolib hack.o
In fact, nearly all archive member targets are updated in just this way
and there is an implicit rule to do it for you. Note: The
`c' flag to ar is required if the archive file does not
already exist.
To specify several members in the same archive, you can write all the member names together between the parentheses. For example:
foolib(hack.o kludge.o)
is equivalent to:
foolib(hack.o) foolib(kludge.o)
You can also use shell-style wildcards in an archive member reference. See section Using Wildcard Characters in File Names. For example, `foolib(*.o)' expands to all existing members of the `foolib' archive whose names end in `.o'; perhaps `foolib(hack.o) foolib(kludge.o)'.
Recall that a target that looks like `a(m)' stands for the member named m in the archive file a.
When make looks for an implicit rule for such a target, as a special
feature it considers implicit rules that match `(m)', as well as
those that match the actual target `a(m)'.
This causes one special rule whose target is `(%)' to match. This rule updates the target `a(m)' by copying the file m into the archive. For example, it will update the archive member target `foo.a(bar.o)' by copying the file `bar.o' into the archive `foo.a' as a member named `bar.o'.
When this rule is chained with others, the result is very powerful. Thus, `make "foo.a(bar.o)"' (the quotes are needed to protect the `(' and `)' from being interpreted specially by the shell) in the presence of a file `bar.c' is enough to cause the following commands to be run, even without a makefile:
cc -c bar.c -o bar.o ar r foo.a bar.o rm -f bar.o
Here make has envisioned the file `bar.o' as an intermediate
file. See section Chains of Implicit Rules.
Implicit rules such as this one are written using the automatic variable `$%'. See section Automatic Variables.
An archive member name in an archive cannot contain a directory name, but
it may be useful in a makefile to pretend that it does. If you write an
archive member target `foo.a(dir/file.o)', make will perform
automatic updating with this command:
ar r foo.a dir/file.o
which has the effect of copying the file `dir/file.o' into a member
named `file.o'. In connection with such usage, the automatic variables
%D and %F may be useful.
An archive file that is used as a library usually contains a special member
named `__.SYMDEF' that contains a directory of the external symbol
names defined by all the other members. After you update any other
members, you need to update `__.SYMDEF' so that it will summarize the
other members properly. This is done by running the ranlib program:
ranlib archivefile
Normally you would put this command in the rule for the archive file, and make all the members of the archive file dependencies of that rule. For example,
libfoo.a: libfoo.a(x.o) libfoo.a(y.o) ...
ranlib libfoo.a
The effect of this is to update archive members `x.o', `y.o',
etc., and then update the symbol directory member `__.SYMDEF' by
running ranlib. The rules for updating the members are not shown
here; most likely you can omit them and use the implicit rule which copies
files into the archive, as described in the preceding section.
This is not necessary when using the GNU ar program, which
updates the `__.SYMDEF' member automatically.
It is important to be careful when using parallel execution (the
-j switch; see section Parallel Execution) and archives.
If multiple ar commands run at the same time on the same archive
file, they will not know about each other and can corrupt the file.
Possibly a future version of make will provide a mechanism to
circumvent this problem by serializing all commands that operate on the
same archive file. But for the time being, you must either write your
makefiles to avoid this problem in some other way, or not use -j.
You can write a special kind of suffix rule for dealing with archive
files. See section Old-Fashioned Suffix Rules, for a full explanation of suffix rules.
Archive suffix rules are obsolete in GNU make, because pattern
rules for archives are a more general mechanism (see section Implicit Rule for Archive Member Targets). But they are retained for compatibility with other
makes.
To write a suffix rule for archives, you simply write a suffix rule using the target suffix `.a' (the usual suffix for archive files). For example, here is the old-fashioned suffix rule to update a library archive from C source files:
.c.a:
$(CC) $(CFLAGS) $(CPPFLAGS) -c $< -o $*.o
$(AR) r $@ $*.o
$(RM) $*.o
This works just as if you had written the pattern rule:
(%.o): %.c
$(CC) $(CFLAGS) $(CPPFLAGS) -c $< -o $*.o
$(AR) r $@ $*.o
$(RM) $*.o
In fact, this is just what make does when it sees a suffix rule
with `.a' as the target suffix. Any double-suffix rule
`.x.a' is converted to a pattern rule with the target
pattern `(%.o)' and a dependency pattern of `%.x'.
Since you might want to use `.a' as the suffix for some other kind
of file, make also converts archive suffix rules to pattern rules
in the normal way (see section Old-Fashioned Suffix Rules). Thus a double-suffix rule
`.x.a' produces two pattern rules: `(%.o):
%.x' and `%.a: %.x'.
make
Here is a summary of the features of GNU make, for comparison
with and credit to other versions of make. We consider the
features of make in 4.2 BSD systems as a baseline. If you are
concerned with writing portable makefiles, you should use only the
features of make not listed here or in section Incompatibilities and Missing Features.
Many features come from the version of make in System V.
VPATH variable and its special meaning.
See section Searching Directories for Dependencies.
This feature exists in System V make, but is undocumented.
It is documented in 4.3 BSD make (which says it mimics System V's
VPATH feature).
MAKEFLAGS to recursive
invocations of make.
See section Communicating Options to a Sub-make.
$% is set to the member name
in an archive reference. See section Automatic Variables.
$@, $*, $<, $%,
and $? have corresponding forms like $(@F) and
$(@D). We have generalized this to $^ as an obvious
extension. See section Automatic Variables.
make, these options actually do something.
make via the variable
MAKE even if `-n', `-q' or `-t' is specified.
See section Recursive Use of make.
make, because the
general feature of rule chaining (see section Chains of Implicit Rules) allows one pattern rule for installing members in an
archive (see section Implicit Rule for Archive Member Targets) to be sufficient.
The following features were inspired by various other versions of
make. In some cases it is unclear exactly which versions inspired
which others.
make.
We're not sure who invented it first, but it's been spread around a bit.
See section Defining and Redefining Pattern Rules.
make
for AT&T Eighth Edition Research Unix, and later by Andrew Hume of
AT&T Bell Labs in his mk program (where he terms it
"transitive closure"). We do not really know if
we got this from either of them or thought it up ourselves at the
same time. See section Chains of Implicit Rules.
$^ containing a list of all dependencies
of the current target. We did not invent this, but we have no idea who
did. See section Automatic Variables. The automatic variable
$+ is a simple extension of $^.
make) was (as far as we know)
invented by Andrew Hume in mk.
See section Instead of Executing the Commands.
make and similar programs, though not in the
System V or BSD implementations. See section Command Execution.
make by the
patsubst function before the alternate syntax was implemented
for compatibility with SunOS 4. It is not altogether clear who
inspired whom, since GNU make had patsubst before SunOS
4 was released.
make. See section Appending More Text to Variables.
make.
See section Archive Members as Targets.
-include directive to include makefiles with no error for a
nonexistent file comes from SunOS 4 make. (But note that SunOS 4
make does not allow multiple makefiles to be specified in one
-include directive.) The same feature appears with the name
sinclude in SGI make and perhaps others.
The remaining features are inventions new in GNU make:
make.
MAKE to recursive make invocations.
See section Recursive Use of make.
define.
See section Defining Variables Verbatim.
.PHONY.
Andrew Hume of AT&T Bell Labs implemented a similar feature with a
different syntax in his mk program. This seems to be a case of
parallel discovery. See section Phony Targets.
make; it seems a natural extension derived from the features
of the C preprocessor and similar macro languages and is not a
revolutionary concept. See section Conditional Parts of Makefiles.
MAKEFILES.
make, they must begin with
`.' and not contain any `/' characters.
make recursion using the
variable MAKELEVEL. See section Recursive Use of make.
MAKECMDGOALS. See section Arguments to Specify the Goals.
vpath search.
See section Searching Directories for Dependencies.
make has a very, very limited form of this
functionality in that it will check out SCCS files for makefiles.
make.
The make programs in various other systems support a few features
that are not implemented in GNU make. The POSIX.2 standard
(IEEE Standard 1003.2-1992) which specifies make does not
require any of these features.
make because of the
nonmodularity of putting knowledge into make of the internal
format of archive file symbol tables.
See section Updating Archive Symbol Directories.
make;
they refer to the SCCS file that corresponds
to the file one would get without the `~'. For example, the
suffix rule `.c~.o' would make the file `n.o' from
the SCCS file `s.n.c'. For complete coverage, a whole
series of such suffix rules is required.
See section Old-Fashioned Suffix Rules.
In GNU make, this entire series of cases is handled by two
pattern rules for extraction from SCCS, in combination with the
general feature of rule chaining.
See section Chains of Implicit Rules.
make, the string `$$@' has the strange meaning
that, in the dependencies of a rule with multiple targets, it stands
for the particular target that is being processed.
This is not defined in GNU make because `$$' should always
stand for an ordinary `$'.
It is possible to get this functionality through the use of static pattern
rules (see section Static Pattern Rules).
The System V make rule:
$(targets): $$@.o lib.acan be replaced with the GNU
make static pattern rule:
$(targets): %: %.o lib.a
make, files found by VPATH search
(see section Searching Directories for Dependencies) have their names changed inside command
strings. We feel it is much cleaner to always use automatic variables
and thus make this feature obsolete.
makes, the automatic variable $* appearing in
the dependencies of a rule has the amazingly strange "feature" of
expanding to the full name of the target of that rule. We cannot
imagine what went on in the minds of Unix make developers to do
this; it is utterly inconsistent with the normal definition of $*.
makes, implicit rule search
(see section Using Implicit Rules) is apparently done for
all targets, not just those without commands. This means you can
do:
foo.o:
cc -c foo.c
and Unix make will intuit that `foo.o' depends on
`foo.c'.
We feel that such usage is broken. The dependency properties of
make are well-defined (for GNU make, at least),
and doing such a thing simply does not fit the model.
make does not include any built-in implicit rules for
compiling or preprocessing EFL programs. If we hear of anyone who is
using EFL, we will gladly add them.
make, a suffix rule can be specified with
no commands, and it is treated as if it had empty commands
(see section Using Empty Commands). For example:
.c.a:will override the built-in `.c.a' suffix rule. We feel that it is cleaner for a rule without commands to always simply add to the dependency list for the target. The above example can be easily rewritten to get the desired behavior in GNU
make:
.c.a: ;
make invoke the shell with the `-e' flag,
except under `-k' (see section Testing the Compilation of a Program). The `-e' flag tells the shell to exit as soon as any
program it runs returns a nonzero status. We feel it is cleaner to
write each shell command line to stand on its own and not require this
special treatment.
This chapter describes conventions for writing the Makefiles for GNU programs.
Every Makefile should contain this line:
SHELL = /bin/sh
to avoid trouble on systems where the SHELL variable might be
inherited from the environment. (This is never a problem with GNU
make.)
Different make programs have incompatible suffix lists and
implicit rules, and this sometimes creates confusion or misbehavior. So
it is a good idea to set the suffix list explicitly using only the
suffixes you need in the particular Makefile, like this:
.SUFFIXES: .SUFFIXES: .c .o
The first line clears out the suffix list, the second introduces all suffixes which may be subject to implicit rules in this Makefile.
Don't assume that `.' is in the path for command execution. When you need to run programs that are a part of your package during the make, please make sure that it uses `./' if the program is built as part of the make or `$(srcdir)/' if the file is an unchanging part of the source code. Without one of these prefixes, the current search path is used.
The distinction between `./' (the build directory) and `$(srcdir)/' (the source directory) is important because users can build in a separate directory using the `--srcdir' option to `configure'. A rule of the form:
foo.1 : foo.man sedscript
sed -e sedscript foo.man > foo.1
will fail when the build directory is not the source directory, because `foo.man' and `sedscript' are in the the source directory.
When using GNU make, relying on `VPATH' to find the source
file will work in the case where there is a single dependency file,
since the make automatic variable `$<' will represent the
source file wherever it is. (Many versions of make set `$<'
only in implicit rules.) A Makefile target like
foo.o : bar.c
$(CC) -I. -I$(srcdir) $(CFLAGS) -c bar.c -o foo.o
should instead be written as
foo.o : bar.c
$(CC) -I. -I$(srcdir) $(CFLAGS) -c $< -o $@
in order to allow `VPATH' to work correctly. When the target has multiple dependencies, using an explicit `$(srcdir)' is the easiest way to make the rule work well. For example, the target above for `foo.1' is best written as:
foo.1 : foo.man sedscript
sed -e $(srcdir)/sedscript $(srcdir)/foo.man > $@
GNU distributions usually contain some files which are not source files--for example, Info files, and the output from Autoconf, Automake, Bison or Flex. Since these files normally appear in the source directory, they should always appear in the source directory, not in the build directory. So Makefile rules to update them should put the updated files in the source directory.
However, if a file does not appear in the distribution, then the Makefile should not put it in the source directory, because building a program in ordinary circumstances should not modify the source directory in any way.
Try to make the build and installation targets, at least (and all their
subtargets) work correctly with a parallel make.
Write the Makefile commands (and any shell scripts, such as
configure) to run in sh, not in csh. Don't use any
special features of ksh or bash.
The configure script and the Makefile rules for building and
installation should not use any utilities directly except these:
cat cmp cp diff echo egrep expr false grep install-info ln ls mkdir mv pwd rm rmdir sed sleep sort tar test touch true
The compression program gzip can be used in the dist rule.
Stick to the generally supported options for these programs. For example, don't use `mkdir -p', convenient as it may be, because most systems don't support it.
It is a good idea to avoid creating symbolic links in makefiles, since a few systems don't support them.
The Makefile rules for building and installation can also use compilers
and related programs, but should do so via make variables so that the
user can substitute alternatives. Here are some of the programs we
mean:
ar bison cc flex install ld ldconfig lex make makeinfo ranlib texi2dvi yacc
Use the following make variables to run those programs:
$(AR) $(BISON) $(CC) $(FLEX) $(INSTALL) $(LD) $(LDCONFIG) $(LEX) $(MAKE) $(MAKEINFO) $(RANLIB) $(TEXI2DVI) $(YACC)
When you use ranlib or ldconfig, you should make sure
nothing bad happens if the system does not have the program in question.
Arrange to ignore an error from that command, and print a message before
the command to tell the user that failure of this command does not mean
a problem. (The Autoconf `AC_PROG_RANLIB' macro can help with
this.)
If you use symbolic links, you should implement a fallback for systems that don't have symbolic links.
Additional utilities that can be used via Make variables are:
chgrp chmod chown mknod
It is ok to use other utilities in Makefile portions (or scripts) intended only for particular systems where you know those utilities exist.
Makefiles should provide variables for overriding certain commands, options, and so on.
In particular, you should run most utility programs via variables.
Thus, if you use Bison, have a variable named BISON whose default
value is set with `BISON = bison', and refer to it with
$(BISON) whenever you need to use Bison.
File management utilities such as ln, rm, mv, and
so on, need not be referred to through variables in this way, since users
don't need to replace them with other programs.
Each program-name variable should come with an options variable that is
used to supply options to the program. Append `FLAGS' to the
program-name variable name to get the options variable name--for
example, BISONFLAGS. (The names CFLAGS for the C
compiler, YFLAGS for yacc, and LFLAGS for lex, are
exceptions to this rule, but we keep them because they are standard.)
Use CPPFLAGS in any compilation command that runs the
preprocessor, and use LDFLAGS in any compilation command that
does linking as well as in any direct use of ld.
If there are C compiler options that must be used for proper
compilation of certain files, do not include them in CFLAGS.
Users expect to be able to specify CFLAGS freely themselves.
Instead, arrange to pass the necessary options to the C compiler
independently of CFLAGS, by writing them explicitly in the
compilation commands or by defining an implicit rule, like this:
CFLAGS = -g
ALL_CFLAGS = -I. $(CFLAGS)
.c.o:
$(CC) -c $(CPPFLAGS) $(ALL_CFLAGS) $<
Do include the `-g' option in CFLAGS, because that is not
required for proper compilation. You can consider it a default
that is only recommended. If the package is set up so that it is
compiled with GCC by default, then you might as well include `-O'
in the default value of CFLAGS as well.
Put CFLAGS last in the compilation command, after other variables
containing compiler options, so the user can use CFLAGS to
override the others.
CFLAGS should be used in every invocation of the C compiler,
both those which do compilation and those which do linking.
Every Makefile should define the variable INSTALL, which is the
basic command for installing a file into the system.
Every Makefile should also define the variables INSTALL_PROGRAM
and INSTALL_DATA. (The default for each of these should be
$(INSTALL).) Then it should use those variables as the commands
for actual installation, for executables and nonexecutables
respectively. Use these variables as follows:
$(INSTALL_PROGRAM) foo $(bindir)/foo $(INSTALL_DATA) libfoo.a $(libdir)/libfoo.a
Always use a file name, not a directory name, as the second argument of the installation commands. Use a separate command for each file to be installed.
Installation directories should always be named by variables, so it is easy to install in a nonstandard place. The standard names for these variables are described below. They are based on a standard filesystem layout; variants of it are used in SVR4, 4.4BSD, Linux, Ultrix v4, and other modern operating systems.
These two variables set the root for the installation. All the other installation directories should be subdirectories of one of these two, and nothing should be directly installed into these two directories.
prefix should be `/usr/local'.
When building the complete GNU system, the prefix will be empty and
`/usr' will be a symbolic link to `/'.
(If you are using Autoconf, write it as `@prefix@'.)
exec_prefix should
be $(prefix).
(If you are using Autoconf, write it as `@exec_prefix@'.)
Generally, $(exec_prefix) is used for directories that contain
machine-specific files (such as executables and subroutine libraries),
while $(prefix) is used directly for other directories.
Executable programs are installed in one of the following directories.
Data files used by the program during its execution are divided into categories in two ways.
This makes for six different possibilities. However, we want to discourage the use of architecture-dependent files, aside from object files and libraries. It is much cleaner to make other data files architecture-independent, and it is generally not hard.
Therefore, here are the variables Makefiles should use to specify directories:
libdir should normally be
`/usr/local/lib', but write it as `$(exec_prefix)/lib'.
(If you are using Autoconf, write it as `@libdir@'.)
lispdir='${datadir}/emacs/site-lisp'
AC_SUBST(lispdir)
includedir and one
specified by oldincludedir.
oldincludedir is empty. If it is, they should not try to use
it; they should cancel the second installation of the header files.
A package should not replace an existing header in this directory unless
the header came from the same package. Thus, if your Foo package
provides a header file `foo.h', then it should install the header
file in the oldincludedir directory if either (1) there is no
`foo.h' there or (2) the `foo.h' that exists came from the Foo
package.
To tell whether `foo.h' came from the Foo package, put a magic
string in the file--part of a comment--and grep for that string.
Unix-style man pages are installed in one of the following:
And finally, you should set the following variable:
configure shell script.
(If you are using Autconf, use `srcdir = @srcdir@'.)
For example:
# Common prefix for installation directories. # NOTE: This directory must exist when you start the install. prefix = /usr/local exec_prefix = $(prefix) # Where to put the executable for the command `gcc'. bindir = $(exec_prefix)/bin # Where to put the directories used by the compiler. libexecdir = $(exec_prefix)/libexec # Where to put the Info files. infodir = $(prefix)/info
If your program installs a large number of files into one of the
standard user-specified directories, it might be useful to group them
into a subdirectory particular to that program. If you do this, you
should write the install rule to create these subdirectories.
Do not expect the user to include the subdirectory name in the value of any of the variables listed above. The idea of having a uniform set of variable names for installation directories is to enable the user to specify the exact same values for several different GNU packages. In order for this to be useful, all the packages must be designed so that they will work sensibly when the user does so.
All GNU programs should have the following targets in their Makefiles:
install-strip target to do that.
If possible, write the install target rule so that it does not
modify anything in the directory where the program was built, provided
`make all' has just been done. This is convenient for building the
program under one user name and installing it under another.
The commands should create all the directories in which files are to be
installed, if they don't already exist. This includes the directories
specified as the values of the variables prefix and
exec_prefix, as well as all subdirectories that are needed.
One way to do this is by means of an installdirs target
as described below.
Use `-' before any command for installing a man page, so that
make will ignore any errors. This is in case there are systems
that don't have the Unix man page documentation system installed.
The way to install Info files is to copy them into `$(infodir)'
with $(INSTALL_DATA) (see section Variables for Specifying Commands), and then run
the install-info program if it is present. install-info
is a program that edits the Info `dir' file to add or update the
menu entry for the given Info file; it is part of the Texinfo package.
Here is a sample rule to install an Info file:
$(infodir)/foo.info: foo.info
$(POST_INSTALL)
# There may be a newer info file in . than in srcdir.
-if test -f foo.info; then d=.; \
else d=$(srcdir); fi; \
$(INSTALL_DATA) $$d/foo.info $@; \
# Run install-info only if it exists.
# Use `if' instead of just prepending `-' to the
# line so we notice real errors from install-info.
# We use `$(SHELL) -c' because some shells do not
# fail gracefully when there is an unknown command.
if $(SHELL) -c 'install-info --version' \
>/dev/null 2>&1; then \
install-info --dir-file=$(infodir)/dir \
$(infodir)/foo.info; \
else true; fi
When writing the install target, you must classify all the
commands into three categories: normal ones, pre-installation
commands and post-installation commands. See section Install Command Categories.
install, but strip the executable files while installing
them. In many cases, the definition of this target can be very simple:
install-strip:
$(MAKE) INSTALL_PROGRAM='$(INSTALL_PROGRAM) -s' \
install
Normally we do not recommend stripping an executable unless you are sure
the program has no bugs. However, it can be reasonable to install a
stripped executable for actual execution while saving the unstripped
executable elsewhere in case there is a bug.
distclean, plus more: C source files produced by
Bison, tags tables, Info files, and so on.
The reason we say "almost everything" is that running the command
`make maintainer-clean' should not delete `configure' even if
`configure' can be remade using a rule in the Makefile. More generally,
`make maintainer-clean' should not delete anything that needs to
exist in order to run `configure' and then begin to build the
program. This is the only exception; maintainer-clean should
delete everything else that can be rebuilt.
The `maintainer-clean' target is intended to be used by a maintainer of
the package, not by ordinary users. You may need special tools to
reconstruct some of the files that `make maintainer-clean' deletes.
Since these files are normally included in the distribution, we don't
take care to make them easy to reconstruct. If you find you need to
unpack the full distribution again, don't blame us.
To help make users aware of this, the commands for the special
maintainer-clean target should start with these two:
@echo 'This command is intended for maintainers to use; it' @echo 'deletes files that may need special tools to rebuild.'
info: foo.info
foo.info: foo.texi chap1.texi chap2.texi
$(MAKEINFO) $(srcdir)/foo.texi
You must define the variable MAKEINFO in the Makefile. It should
run the makeinfo program, which is part of the Texinfo
distribution.
Normally a GNU distribution comes with Info files, and that means the
Info files are present in the source directory. Therefore, the Make
rule for an info file should update it in the source directory. When
users build the package, ordinarily Make will not update the Info files
because they will already be up to date.
dvi: foo.dvi
foo.dvi: foo.texi chap1.texi chap2.texi
$(TEXI2DVI) $(srcdir)/foo.texi
You must define the variable TEXI2DVI in the Makefile. It should
run the program texi2dvi, which is part of the Texinfo
distribution.(3) Alternatively,
write just the dependencies, and allow GNU make to provide the command.
ln or cp to install the proper files in it, and
then tar that subdirectory.
Compress the tar file file with gzip. For example, the actual
distribution file for GCC version 1.40 is called `gcc-1.40.tar.gz'.
The dist target should explicitly depend on all non-source files
that are in the distribution, to make sure they are up to date in the
distribution.
See section `Making Releases' in GNU Coding Standards.
The following targets are suggested as conventional names, for programs in which they are useful.
installcheck
installdirs
# Make sure all installation directories (e.g. $(bindir))
# actually exist by making them if necessary.
installdirs: mkinstalldirs
$(srcdir)/mkinstalldirs $(bindir) $(datadir) \
$(libdir) $(infodir) \
$(mandir)
This rule should not modify the directories where compilation is done.
It should do nothing but create installation directories.
When writing the install target, you must classify all the
commands into three categories: normal ones, pre-installation
commands and post-installation commands.
Normal commands move files into their proper places, and set their modes. They may not alter any files except the ones that come entirely from the package they belong to.
Pre-installation and post-installation commands may alter other files; in particular, they can edit global configuration files or data bases.
Pre-installation commands are typically executed before the normal commands, and post-installation commands are typically run after the normal commands.
The most common use for a post-installation command is to run
install-info. This cannot be done with a normal command, since
it alters a file (the Info directory) which does not come entirely and
solely from the package being installed. It is a post-installation
command because it needs to be done after the normal command which
installs the package's Info files.
Most programs don't need any pre-installation commands, but we have the feature just in case it is needed.
To classify the commands in the install rule into these three
categories, insert category lines among them. A category line
specifies the category for the commands that follow.
A category line consists of a tab and a reference to a special Make variable, plus an optional comment at the end. There are three variables you can use, one for each category; the variable name specifies the category. Category lines are no-ops in ordinary execution because these three Make variables are normally undefined (and you should not define them in the makefile).
Here are the three possible category lines, each with a comment that explains what it means:
$(PRE_INSTALL) # Pre-install commands follow.
$(POST_INSTALL) # Post-install commands follow.
$(NORMAL_INSTALL) # Normal commands follow.
If you don't use a category line at the beginning of the install
rule, all the commands are classified as normal until the first category
line. If you don't use any category lines, all the commands are
classified as normal.
These are the category lines for uninstall:
$(PRE_UNINSTALL) # Pre-uninstall commands follow.
$(POST_UNINSTALL) # Post-uninstall commands follow.
$(NORMAL_UNINSTALL) # Normal commands follow.
Typically, a pre-uninstall command would be used for deleting entries from the Info directory.
If the install or uninstall target has any dependencies
which act as subroutines of installation, then you should start
each dependency's commands with a category line, and start the
main target's commands with a category line also. This way, you can
ensure that each command is placed in the right category regardless of
which of the dependencies actually run.
Pre-installation and post-installation commands should not run any programs except for these:
[ basename bash cat chgrp chmod chown cmp cp dd diff echo egrep expand expr false fgrep find getopt grep gunzip gzip hostname install install-info kill ldconfig ln ls md5sum mkdir mkfifo mknod mv printenv pwd rm rmdir sed sort tee test touch true uname xargs yes
The reason for distinguishing the commands in this way is for the sake of making binary packages. Typically a binary package contains all the executables and other files that need to be installed, and has its own method of installing them--so it does not need to run the normal installation commands. But installing the binary package does need to execute the pre-installation and post-installation commands.
Programs to build binary packages work by extracting the pre-installation and post-installation commands. Here is one way of extracting the pre-installation commands:
make -n install -o all \
PRE_INSTALL=pre-install \
POST_INSTALL=post-install \
NORMAL_INSTALL=normal-install \
| gawk -f pre-install.awk
where the file `pre-install.awk' could contain this:
$0 ~ /^\t[ \t]*(normal_install|post_install)[ \t]*$/ {on = 0}
on {print $0}
$0 ~ /^\t[ \t]*pre_install[ \t]*$/ {on = 1}
The resulting file of pre-installation commands is executed as a shell script as part of installing the binary package.
This appendix summarizes the directives, text manipulation functions,
and special variables which GNU make understands.
See section Special Built-in Target Names, section Catalogue of Implicit Rules,
and section Summary of Options,
for other summaries.
Here is a summary of the directives GNU make recognizes:
define variable
endef
ifdef variable
ifndef variable
ifeq (a,b)
ifeq "a" "b"
ifeq 'a' 'b'
ifneq (a,b)
ifneq "a" "b"
ifneq 'a' 'b'
else
endif
include file
override variable = value
override variable := value
override variable += value
override define variable
endef
override Directive.
export
make to export all variables to child processes by default.make.
export variable
export variable = value
export variable := value
export variable += value
unexport variable
make whether or not to export a particular variable to child
processes.make.
vpath pattern path
vpath Directive.
vpath pattern
vpath
vpath
directive.
Here is a summary of the text manipulation functions (see section Functions for Transforming Text):
$(subst from,to,text)
$(patsubst pattern,replacement,text)
$(strip string)
$(findstring find,text)
$(filter pattern...,text)
$(filter-out pattern...,text)
$(sort list)
$(dir names...)
$(notdir names...)
$(suffix names...)
$(basename names...)
$(addsuffix suffix,names...)
$(addprefix prefix,names...)
$(join list1,list2)
$(word n,text)
$(words text)
$(firstword names...)
$(wildcard pattern...)
wildcard.
$(shell command)
shell Function.
$(origin variable)
make variable variable was
defined.origin Function.
$(foreach var,words,text)
foreach Function.
Here is a summary of the automatic variables. See section Automatic Variables, for full information.
$@
$%
$<
$?
make to Update Archive Files).
$^
$+
make to Update Archive Files). The value of $^ omits duplicate
dependencies, while $+ retains them and preserves their order.
$*
$(@D)
$(@F)
$@.
$(*D)
$(*F)
$*.
$(%D)
$(%F)
$%.
$(<D)
$(<F)
$<.
$(^D)
$(^F)
$^.
$(+D)
$(+F)
$+.
$(?D)
$(?F)
$?.
These variables are used specially by GNU make:
MAKEFILES
make.MAKEFILES.
VPATH
VPATH: Search Path for All Dependencies.
SHELL
SHELL in the makefile to change the shell used to run
commands. See section Command Execution.
MAKESHELL
make. This value takes precedence over the value of
SHELL. See section Command Execution.
MAKE
make was invoked.
Using this variable in commands has special meaning.
See section How the MAKE Variable Works.
MAKELEVEL
makes).make.
MAKEFLAGS
make. You can set this in the environment or
a makefile to set flags.make.
MAKECMDGOALS
make on the command line. Setting this
variable has no effect on the operation of make.CURDIR
-C options are processed, if any). Setting this variable has no
effect on the operation of make.make.
SUFFIXES
make reads any makefiles.
Here is a list of the most common errors you might see generated by
make, and some information about what they mean and how to fix
them.
Sometimes make errors are not fatal, especially in the presence
of a - prefix on a command script line, or the -k command
line option. Errors that are fatal are prefixed with the string
***.
Error messages are all either prefixed with the name of the program (usually `make'), or, if the error is found in a makefile, the name of the file and linenumber containing the problem.
In the table below, these common prefixes are left off.
make errors at all. They mean that a
program that make invoked as part of a command script returned a
non-0 error code (`Error NN'), which make interprets
as failure, or it exited in some other abnormal fashion (with a
signal of some type).
If no *** is attached to the message, then the subprocess failed
but the rule in the makefile was prefixed with the - special
character, so make ignored the error.
make's generic "Huh?" error message. It means that
make was completely unsuccessful at parsing this line of your
makefile. It basically means "syntax error".
One of the most common reasons for this message is that you (or perhaps
your oh-so-helpful editor, as is the case with many MS-Windows editors)
have attempted to indent your command scripts with spaces instead of a
TAB character. Remember that every line in the command script must
begin with a TAB character. Eight spaces do not count.
make command (such as a variable assignment). Command scripts
must always be associated with a target.
The second form is generated if the line has a semicolon as the first
non-whitespace character; make interprets this to mean you left
out the "target: dependency" section of a rule.
make decided it needed to build a target, but
then couldn't find any instructions in the makefile on how to do that,
either explicit or implicit (including in the default rules database).
If you want that file to be built, you will need to add a rule to your
makefile describing how that target can be built. Other possible
sources of this problem are typos in the makefile (if that filename is
wrong) or a corrupted source tree (if that file is not supposed to be
built, but rather only a dependency).
make couldn't find any makefiles to read in.
The latter means that some makefile was found, but it didn't contain any
default target and none was given on the command line. GNU make
has nothing to do in these situations.
make allows commands to be specified only once per target
(except for double-colon rules). If you give commands for a target
which already has been defined to have commands, this warning is issued
and the second set of commands will overwrite the first set.
make detected a loop in the dependency graph:
after tracing the dependency yyy of target xxx, and its
dependencies, etc., one of them depended on xxx again.
make variable
xxx that, when its expanded, will refer to itself (xxx).
This is not allowed; either use simply-expanded variables (:=) or
use the append operator (+=).
%).
Here is the makefile for the GNU tar program. This is a
moderately complex makefile.
Because it is the first target, the default goal is `all'. An
interesting feature of this makefile is that `testpad.h' is a
source file automatically created by the testpad program,
itself compiled from `testpad.c'.
If you type `make' or `make all', then make creates
the `tar' executable, the `rmt' daemon that provides
remote tape access, and the `tar.info' Info file.
If you type `make install', then make not only creates
`tar', `rmt', and `tar.info', but also installs
them.
If you type `make clean', then make removes the `.o'
files, and the `tar', `rmt', `testpad',
`testpad.h', and `core' files.
If you type `make distclean', then make not only removes
the same files as does `make clean' but also the
`TAGS', `Makefile', and `config.status' files.
(Although it is not evident, this makefile (and
`config.status') is generated by the user with the
configure program, which is provided in the tar
distribution, but is not shown here.)
If you type `make realclean', then make removes the same
files as does `make distclean' and also removes the Info files
generated from `tar.texinfo'.
In addition, there are targets shar and dist that create
distribution kits.
# Generated automatically from Makefile.in by configure.
# Un*x Makefile for GNU tar program.
# Copyright (C) 1991 Free Software Foundation, Inc.
# This program is free software; you can redistribute
# it and/or modify it under the terms of the GNU
# General Public License ...
...
...
SHELL = /bin/sh
#### Start of system configuration section. ####
srcdir = .
# If you use gcc, you should either run the
# fixincludes script that comes with it or else use
# gcc with the -traditional option. Otherwise ioctl
# calls will be compiled incorrectly on some systems.
CC = gcc -O
YACC = bison -y
INSTALL = /usr/local/bin/install -c
INSTALLDATA = /usr/local/bin/install -c -m 644
# Things you might add to DEFS:
# -DSTDC_HEADERS If you have ANSI C headers and
# libraries.
# -DPOSIX If you have POSIX.1 headers and
# libraries.
# -DBSD42 If you have sys/dir.h (unless
# you use -DPOSIX), sys/file.h,
# and st_blocks in `struct stat'.
# -DUSG If you have System V/ANSI C
# string and memory functions
# and headers, sys/sysmacros.h,
# fcntl.h, getcwd, no valloc,
# and ndir.h (unless
# you use -DDIRENT).
# -DNO_MEMORY_H If USG or STDC_HEADERS but do not
# include memory.h.
# -DDIRENT If USG and you have dirent.h
# instead of ndir.h.
# -DSIGTYPE=int If your signal handlers
# return int, not void.
# -DNO_MTIO If you lack sys/mtio.h
# (magtape ioctls).
# -DNO_REMOTE If you do not have a remote shell
# or rexec.
# -DUSE_REXEC To use rexec for remote tape
# operations instead of
# forking rsh or remsh.
# -DVPRINTF_MISSING If you lack vprintf function
# (but have _doprnt).
# -DDOPRNT_MISSING If you lack _doprnt function.
# Also need to define
# -DVPRINTF_MISSING.
# -DFTIME_MISSING If you lack ftime system call.
# -DSTRSTR_MISSING If you lack strstr function.
# -DVALLOC_MISSING If you lack valloc function.
# -DMKDIR_MISSING If you lack mkdir and
# rmdir system calls.
# -DRENAME_MISSING If you lack rename system call.
# -DFTRUNCATE_MISSING If you lack ftruncate
# system call.
# -DV7 On Version 7 Unix (not
# tested in a long time).
# -DEMUL_OPEN3 If you lack a 3-argument version
# of open, and want to emulate it
# with system calls you do have.
# -DNO_OPEN3 If you lack the 3-argument open
# and want to disable the tar -k
# option instead of emulating open.
# -DXENIX If you have sys/inode.h
# and need it 94 to be included.
DEFS = -DSIGTYPE=int -DDIRENT -DSTRSTR_MISSING \
-DVPRINTF_MISSING -DBSD42
# Set this to rtapelib.o unless you defined NO_REMOTE,
# in which case make it empty.
RTAPELIB = rtapelib.o
LIBS =
DEF_AR_FILE = /dev/rmt8
DEFBLOCKING = 20
CDEBUG = -g
CFLAGS = $(CDEBUG) -I. -I$(srcdir) $(DEFS) \
-DDEF_AR_FILE=\"$(DEF_AR_FILE)\" \
-DDEFBLOCKING=$(DEFBLOCKING)
LDFLAGS = -g
prefix = /usr/local
# Prefix for each installed program,
# normally empty or `g'.
binprefix =
# The directory to install tar in.
bindir = $(prefix)/bin
# The directory to install the info files in.
infodir = $(prefix)/info
#### End of system configuration section. ####
SRC1 = tar.c create.c extract.c buffer.c \
getoldopt.c update.c gnu.c mangle.c
SRC2 = version.c list.c names.c diffarch.c \
port.c wildmat.c getopt.c
SRC3 = getopt1.c regex.c getdate.y
SRCS = $(SRC1) $(SRC2) $(SRC3)
OBJ1 = tar.o create.o extract.o buffer.o \
getoldopt.o update.o gnu.o mangle.o
OBJ2 = version.o list.o names.o diffarch.o \
port.o wildmat.o getopt.o
OBJ3 = getopt1.o regex.o getdate.o $(RTAPELIB)
OBJS = $(OBJ1) $(OBJ2) $(OBJ3)
AUX = README COPYING ChangeLog Makefile.in \
makefile.pc configure configure.in \
tar.texinfo tar.info* texinfo.tex \
tar.h port.h open3.h getopt.h regex.h \
rmt.h rmt.c rtapelib.c alloca.c \
msd_dir.h msd_dir.c tcexparg.c \
level-0 level-1 backup-specs testpad.c
all: tar rmt tar.info
tar: $(OBJS)
$(CC) $(LDFLAGS) -o $@ $(OBJS) $(LIBS)
rmt: rmt.c
$(CC) $(CFLAGS) $(LDFLAGS) -o $@ rmt.c
tar.info: tar.texinfo
makeinfo tar.texinfo
install: all
$(INSTALL) tar $(bindir)/$(binprefix)tar
-test ! -f rmt || $(INSTALL) rmt /etc/rmt
$(INSTALLDATA) $(srcdir)/tar.info* $(infodir)
$(OBJS): tar.h port.h testpad.h
regex.o buffer.o tar.o: regex.h
# getdate.y has 8 shift/reduce conflicts.
testpad.h: testpad
./testpad
testpad: testpad.o
$(CC) -o $@ testpad.o
TAGS: $(SRCS)
etags $(SRCS)
clean:
rm -f *.o tar rmt testpad testpad.h core
distclean: clean
rm -f TAGS Makefile config.status
realclean: distclean
rm -f tar.info*
shar: $(SRCS) $(AUX)
shar $(SRCS) $(AUX) | compress \
> tar-`sed -e '/version_string/!d' \
-e 's/[^0-9.]*\([0-9.]*\).*/\1/' \
-e q
version.c`.shar.Z
dist: $(SRCS) $(AUX)
echo tar-`sed \
-e '/version_string/!d' \
-e 's/[^0-9.]*\([0-9.]*\).*/\1/' \
-e q
version.c` > .fname
-rm -rf `cat .fname`
mkdir `cat .fname`
ln $(SRCS) $(AUX) `cat .fname`
-rm -rf `cat .fname` .fname
tar chZf `cat .fname`.tar.Z `cat .fname`
tar.zoo: $(SRCS) $(AUX)
-rm -rf tmp.dir
-mkdir tmp.dir
-rm tar.zoo
for X in $(SRCS) $(AUX) ; do \
echo $$X ; \
sed 's/$$/^M/' $$X \
> tmp.dir/$$X ; done
cd tmp.dir ; zoo aM ../tar.zoo *
-rm -rf tmp.dir
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# (comments), in makefile
#include
$, in function call
$, in rules
$, in variable name
$, in variable reference
%, in pattern rules
%, quoting in static pattern
%, quoting in patsubst
%, quoting in vpath
%, quoting with \ (backslash), %, quoting with \ (backslash), %, quoting with \ (backslash)
* (wildcard character)
define
- (in commands)
define
--assume-new, --assume-new
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--assume-old, and recursion
--debug
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-C, and recursion
-C, and -w
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-e (shell flag)
-f, -f, -f
-f, and recursion
-h
-I, -I
-i, -i
-j, -j
-j, and archive update
-j, and recursion
-k, -k, -k
-l
-l (library search)
-l (load average)
-m
-M (to compiler)
-MM (to GNU compiler)
-n, -n, -n
-o, -o
-o, and recursion
-p
-q, -q
-r
-S
-s, -s
-t, -t
-t, and recursion
-v
-W, -W
-w
-W, and recursion
-w, and recursion
-w, disabling
-w, and -C
.a (archives)
.d
.PRECIOUS intermediate files
:: rules (double-colon)
? (wildcard character)
@ (in commands)
define
[...] (wildcard characters)
\ (backslash), for continuation lines
\ (backslash), in commands
\ (backslash), to quote %, \ (backslash), to quote %, \ (backslash), to quote %
__.SYMDEF
all (standard target)
-j
\), for continuation lines
\), in commands
\), to quote %, backslash (\), to quote %, backslash (\), to quote %
cd (shell command), cd (shell command)
check (standard target)
clean (standard target)
clean target, clean target
clobber (standard target)
\) in
VPATH)
VPATH), and implicit rules
VPATH), and link libraries
VPATH), and shell commands
dist (standard target)
distclean (standard target)
$), in function call
$), in rules
$), in variable name
$), in variable reference
M-x compile)
SHELL in
make
FORCE
VPATH
MAKEFILES variable)
install (standard target)
lint, rule to run
lpr (shell command), lpr (shell command)
make depend
MAKECMDGOALS
MAKEFILES variable
make processes
mostlyclean (standard target)
::)
OBJ
obj
OBJECTS
objects
OBJS
objs
override
.PRECIOUS, preserving with .PRECIOUS
.SECONDARY
print (standard target)
print target, print target
%, in static pattern
%, in patsubst
%, in vpath
README
realclean (standard target)
-C
-f
-j
-o
-t
-W
-w
MAKE variable
MAKEFILES variable
rm (shell command), rm (shell command), rm (shell command), rm (shell command)
$
::)
VPATH
VPATH)
VPATH), and implicit rules
VPATH), and link libraries
sed (shell command)
shar (standard target)
include)
include)
SHELL, MS-DOS specifics
make
TAGS (standard target)
tar (standard target)
test (standard target)
~)
touch (shell command), touch (shell command)
VPATH, and implicit rules
VPATH, and link libraries
include
yacc
~ (tilde)
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GNU Make compiled for MS-DOS and MS-Windows behaves as if prefix has been defined to be the root of the DJGPP tree hierarchy.
On MS-DOS, the value of current working directory is global, so changing it will affect the following command lines on those systems.
texi2dvi uses TeX to do the real work
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This document was generated on 7 November 1998 using the texi2html translator version 1.52.