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)