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2009-03-05 19:26:54
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:
${function arguments}
Here function is a function name; one of a short list of names
that are part of make. You can also essentially create your own
functions by using the call builtin function.
The arguments are the arguments of the function. They are separated from the function name by one or more spaces or tabs, and if there is more than one argument, then they are separated by commas. Such whitespace and commas are not part of an argument's value. The delimiters which you use to surround the function call, whether parentheses or braces, can appear in an argument only in matching pairs; the other kind of delimiters may appear singly. If the arguments themselves contain other function calls or variable references, it is wisest to use the same kind of delimiters for all the references; write `$(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)`%' characters in 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 ) 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.o
To get the list of corresponding source files, you could simply write:
$(objects:.o=.c)
instead of using the general form:
$(patsubst %.o,%.c,$(objects))
$(strip string)The function strip can be very useful when used in conjunction
with conditionals. When comparing something with the empty string
`' using ifeq or ifneq, you usually want a string of
just whitespace to match the empty string (see ).
Thus, the following may fail to have the desired results:
.PHONY: all
ifneq "$(needs_made)" ""
all: $(needs_made)
else
all:;@echo 'Nothing to make!'
endif
Replacing the variable reference `$(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 , 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.o
the 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.
$(word n,text)$(word 2, foo bar baz)
returns `bar'.
$(wordlist s,e,text)$(wordlist 2, 3, foo bar baz)
returns `bar baz'.
$(words text)$(word $(words text),text).
$(firstword names...)For example,
$(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.
$(lastword names...)For example,
$(lastword foo bar)
produces the result `bar'. Although $(lastword
text) is the same as $(word $(words text),text),
the lastword function was added for its simplicity and better
performance.
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
prerequisite files
(see ).
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 ).
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...)A file name that ends with a slash becomes an empty string. This is unfortunate, because it means that the result does not always have the same number of whitespace-separated file names as the argument had; but we do not see any other valid alternative.
For example,
$(notdir src/foo.c hacks)
produces the result `foo.c hacks'.
$(suffix names...)For example,
$(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'.
$(join list1,list2)For example, `$(join a b,.c .o)' produces `a.c b.o'.
Whitespace between the words in the lists is not preserved; it is replaced with a single space.
This function can merge the results of the dir and
notdir functions, to produce the original list of files which
was given to those two functions.
$(wildcard pattern)wildcard is a space-separated list of the names of existing files
that match the pattern.
See .
$(realpath names...). or .. components,
nor any repeated path separators (/) or symlinks. In case of a
failure the empty string is returned. Consult the realpath(3)
documentation for a list of possible failure causes.
$(abspath names...). or .. components, nor any repeated path
separators (/). Note that, in contrast to realpath
function, abspath does not resolve symlinks and does not require
the file names to refer to an existing file or directory. Use the
wildcard function to test for existence.There are three functions that provide conditional expansion. A key aspect of these functions is that not all of the arguments are expanded initially. Only those arguments which need to be expanded, will be expanded.
$(if condition,then-part[,else-part])if function provides support for conditional expansion in a
functional context (as opposed to the GNU make makefile
conditionals such as ifeq (see ).
The first argument, condition, first has all preceding and trailing whitespace stripped, then is expanded. If it expands to any non-empty string, then the condition is considered to be true. If it expands to an empty string, the condition is considered to be false.
If the condition is true then the second argument, then-part, is
evaluated and this is used as the result of the evaluation of the entire
if function.
If the condition is false then the third argument, else-part, is
evaluated and this is the result of the if function. If there is
no third argument, the if function evaluates to nothing (the
empty string).
Note that only one of the then-part or the else-part will be
evaluated, never both. Thus, either can contain side-effects (such as
shell function calls, etc.)
$(or condition1[,condition2[,condition3...]])or function provides a “short-circuiting” OR operation.
Each argument is expanded, in order. If an argument expands to a
non-empty string the processing stops and the result of the expansion
is that string. If, after all arguments are expanded, all of them are
false (empty), then the result of the expansion is the empty string.
$(and condition1[,condition2[,condition3...]])and function provides a “short-circuiting” AND operation.
Each argument is expanded, in order. If an argument expands to an
empty string the processing stops and the result of the expansion is
the empty string. If all arguments expand to a non-empty string then
the result of the expansion is the expansion of the last argument.
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
call Function
The call function is unique in that it can be used to create new
parameterized functions. You can write a complex expression as the
value of a variable, then use call to expand it with different
values.
The syntax of the call function is:
When make expands this function, it assigns each param to
temporary variables $(1), $(2), etc. The variable
$(0) will contain variable. There is no maximum number of
parameter arguments. There is no minimum, either, but it doesn't make
sense to use call with no parameters.
Then variable is expanded as a make variable in the context
of these temporary assignments. Thus, any reference to $(1) in
the value of variable will resolve to the first param in the
invocation of call.
Note that variable is the name of a variable, not a reference to that variable. Therefore you would not normally use a `$' 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.)
If variable is the name of a builtin function, the builtin function
is always invoked (even if a make variable by that name also
exists).
The call function expands the param arguments before
assigning them to temporary variables. This means that variable
values containing references to builtin functions that have special
expansion rules, like foreach or if, may not work as you
expect.
Some examples may make this clearer.
This macro simply reverses its arguments:
reverse = $(2) $(1) foo = $(call reverse,a,b)Here foo will contain `b a'.
This one is slightly more interesting: it defines a macro to search for
the first instance of a program in PATH:
Now the variable LS contains /bin/ls or similar.
The call function can be nested. Each recursive invocation gets
its own local values for $(1), etc. that mask the values of
higher-level call. For example, here is an implementation of a
map function:
Now you can map a function that normally takes only one argument,
such as origin, to multiple values in one step:
and end up with o containing something like `file file default'.
A final caution: be careful when adding whitespace to the arguments to
call. As with other functions, any whitespace contained in the
second and subsequent arguments is kept; this can cause strange
effects. It's generally safest to remove all extraneous whitespace when
providing parameters to call.
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 .
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.
value Function
The value function provides a way for you to use the value of a
variable without having it expanded. Please note that this
does not undo expansions which have already occurred; for example if
you create a simply expanded variable its value is expanded during the
definition; in that case the value function will return the
same result as using the variable directly.
The syntax of the value function is:
$(value variable)
Note that variable is the name of a variable; not a reference to that variable. Therefore you would not normally use a `$' 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 containing the value of variable, without any expansion occurring. For example, in this makefile:
FOO = $PATH
all:
@echo $(FOO)
@echo $(value FOO)
The first output line would be ATH, since the “$P” would be
expanded as a make variable, while the second output line would
be the current value of your $PATH environment variable, since
the value function avoided the expansion.
The value function is most often used in conjunction with the
eval function (see ).
eval Function
The eval function is very special: it allows you to define new
makefile constructs that are not constant; which are the result of
evaluating other variables and functions. The argument to the
eval function is expanded, then the results of that expansion
are parsed as makefile syntax. The expanded results can define new
make variables, targets, implicit or explicit rules, etc.
The result of the eval function is always the empty string;
thus, it can be placed virtually anywhere in a makefile without
causing syntax errors.
It's important to realize that the eval argument is expanded
twice; first by the eval function, then the results of
that expansion are expanded again when they are parsed as makefile
syntax. This means you may need to provide extra levels of escaping
for “$” characters when using eval. The value
function (see ) can sometimes be useful in these
situations, to circumvent unwanted expansions.
Here is an example of how eval can be used; this example
combines a number of concepts and other functions. Although it might
seem overly complex to use eval in this example, rather than
just writing out the rules, consider two things: first, the template
definition (in PROGRAM_template) could need to be much more
complex than it is here; and second, you might put the complex,
“generic” part of this example into another makefile, then include
it in all the individual makefiles. Now your individual makefiles are
quite straightforward.
PROGRAMS = server client
server_OBJS = server.o server_priv.o server_access.o
server_LIBS = priv protocol
client_OBJS = client.o client_api.o client_mem.o
client_LIBS = protocol
# Everything after this is generic
.PHONY: all
all: $(PROGRAMS)
define PROGRAM_template
$(1): $$($(1)_OBJS) $$($(1)_LIBS:%=-l%)
ALL_OBJS += $$($(1)_OBJS)
endef
$(foreach prog,$(PROGRAMS),$(eval $(call PROGRAM_template,$(prog))))
$(PROGRAMS):
$(LINK.o) $^ $(LDLIBS) -o $@
clean:
rm -f $(ALL_OBJS) $(PROGRAMS)
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 .
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 ).
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 .
flavor Function
The flavor function is unlike most other functions (and like
origin function) in that it does not operate on the values of
variables; it tells you something about a variable.
Specifically, it tells you the flavor of a variable (see ).
The syntax of the flavor function is:
$(flavor variable)
Note that variable is the name of a variable to inquire about; not a reference to that variable. Therefore you would not normally use a `$' 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 that identifies the flavor of the variable variable:
shell Function
The shell function is unlike any other function other than the
wildcard function
(see ) 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 as an argument a shell command and evaluates
to the output of the command. The only processing make does on
the result is to convert each newline (or carriage-return / newline
pair) to a single space. If there is a trailing (carriage-return
and) newline it will simply be removed.
The commands run by calls to the shell function are run when the
function calls are expanded (see ). Because this function involves
spawning a new shell, you should carefully consider the performance
implications of using the shell function within recursively
expanded variables vs. simply expanded variables (see ).
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)' (as long as at least one `.c' file
exists).
These functions control the way make runs. Generally, they are used to provide information to the user of the makefile or to cause make to stop if some sort of environmental error is detected.
$(error text...)For example,
ifdef ERROR1
$(error error is $(ERROR1))
endif
will generate a fatal error during the read of the makefile if the
make variable ERROR1 is defined. Or,
ERR = $(error found an error!)
.PHONY: err
err: ; $(ERR)
will generate a fatal error while make is running, if the
err target is invoked.
$(warning text...)error function, above,
except that make doesn't exit. Instead, text is expanded
and the resulting message is displayed, but processing of the makefile
continues.
The result of the expansion of this function is the empty string.
$(info text...)