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(make) Conditional Example

Info Catalog (make) Conditionals (make) Conditionals (make) Conditional Syntax
 
 7.1 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, 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.
 
    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 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)
 
Info Catalog (make) Conditionals (make) Conditionals (make) Conditional Syntax
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