|GNU Make Manual||www.imodulo.com · 2003-04-05|
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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
else and 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 ignored.
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 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)
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