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
"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 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
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.
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
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) $(libs)
The syntax of a simple conditional with no
else is as follows:
conditional-directive text-if-true endif
The text-if-true may be any lines of text, to be considered as part of the makefile if the condition is true. If the condition is false, no text is used instead.
The syntax of a complex conditional is as follows:
conditional-directive text-if-true else text-if-false endif
If the condition is true, text-if-true is used; otherwise, text-if-false is used instead. The text-if-false can be any number of lines of text.
The syntax of the conditional-directive is the same whether the conditional is simple or complex. There are four different directives that test different conditions. Here is a table of them:
ifeq (arg1, arg2)
ifeq 'arg1' 'arg2'
ifeq "arg1" "arg2"
ifeq "arg1" 'arg2'
ifeq 'arg1' "arg2"
stripfunction (see section Functions for String Substitution and Analysis) to avoid interpreting whitespace as a non-empty value. For example:
ifeq ($(strip $(foo)),) text-if-empty endifwill evaluate text-if-empty even if the expansion of
$(foo)contains whitespace characters.
ifneq (arg1, arg2)
ifneq 'arg1' 'arg2'
ifneq "arg1" "arg2"
ifneq "arg1" 'arg2'
ifneq 'arg1' "arg2"
ifdefonly tests whether a variable has a value. It does not expand the variable to see if that value is nonempty. Consequently, tests using
ifdefreturn true for all definitions except those like
foo =. To test for an empty value, use
ifeq ($(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'.
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
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
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
(see section Functions for String Substitution and Analysis).
This is useful when
touch is not enough to make a file appear up
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
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
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