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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.
8.1 Function Call Syntax How to write a function call. 8.2 Functions for String Substitution and Analysis General-purpose text manipulation functions. 8.3 Functions for File Names Functions for manipulating file names. 8.4 The foreach
FunctionRepeat some text with controlled variation. 8.5 The if
FunctionConditionally expand a value. 8.6 The call
FunctionExpand a user-defined function. 8.7 The origin
FunctionFind where a variable got its value. 8.8 The shell
FunctionSubstitute the output of a shell command. 8.9 Functions That Control Make Functions that control how make runs.
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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.
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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 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.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)) |
$(strip string)
The function strip
can be very useful when used in conjunction
with conditionals. When comparing something with the empty string
`' using ifeq
or ifneq
, you usually want a string of
just whitespace to match the empty string (see section 7. Conditional Parts of Makefiles).
Thus, the following may fail to have the desired results:
.PHONY: all ifneq "$(needs_made)" "" all: $(needs_made) else all:;@echo 'Nothing to make!' endif |
Replacing the variable reference `$(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 section 7.3 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.
Removes all whitespace-separated words in text that do
match the pattern words, returning only the words that do
not match. This is the exact opposite of the 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.
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 section VPATH
Search Path for All Prerequisites).
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).
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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.
$(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.
$(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.
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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.
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if
Function
The if
function provides support for conditional expansion in a
functional context (as opposed to the GNU make
makefile
conditionals such as ifeq
(see section Syntax of Conditionals).
An if
function call can contain either two or three arguments:
$(if condition,then-part[,else-part]) |
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.)
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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:
$(call variable,param,param,...) |
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
:
pathsearch = $(firstword $(wildcard $(addsufix /$(1),$(subst :, ,$(PATH))))) LS := $(call pathsearch,ls) |
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:
map = $(foreach a,$(2),$(call $(1),$(a))) |
Now you can map a function that normally takes only one argument,
such as origin
, to multiple values in one step:
o = $(call map,origin,o map MAKE) |
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
.
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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:
if variable was never defined.
if variable has a default definition, as is usual with 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.
if variable was defined as an environment variable and the `-e' option is not turned on (see section Summary of Options).
if variable was defined as an environment variable and the `-e' option is turned on (see section Summary of Options).
if variable was defined in a makefile.
if variable was defined on the command line.
if variable was defined with an override
directive in a
makefile (see section The override
Directive).
if variable is an automatic variable defined for the execution of the commands for each rule (see section Automatic Variables).
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.
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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)'.
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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.
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