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Example of a Conditional
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Example of a Conditional
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, one else and one endif.
The
ifeq directive begins the conditional, specifying 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.
The
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.
The
endif directive ends the conditional. Every conditional must end with an endif. Unconditional makefile text follows.
As the following 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 previous example has this effect.
foo: $(objects)
$(CC) -o foo $(objects) $(libs_for_gcc)
When the variable, CC, has any other value
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, it takes the following effect.
foo: $(objects)
$(CC) -o foo $(objects) $(normal_libs)
Equivalent results can be obtained in another way by conditionalizing a
variable assignment and then using the variable unconditionally as in the following
example.
libs_for_gcc = -lgnu
normal_libs =
ifeq ($(CC),gcc)
libs=$(libs_for_gcc)
else
libs=$(normal_libs)
endif
foo: $(objects)
$(CC) -o foo $(objects) $(libs)
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