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Automake conditionals are a way to omit or include different parts of
the ‘Makefile’ depending on what
configure discovers. A
conditional is introduced in ‘configure.in’ using the
‘AM_CONDITIONAL’ macro. This macro takes two arguments: the first
is the name of the condition, and the second is a shell expression which
returns true when the condition is true.
For instance, here is how to make a condition named ‘TRUE’ which is always true:
As another example, here is how to make a condition named
‘DEBUG’ which is true when the user has given the
‘--enable-debug’ option to
AM_CONDITIONAL(DEBUG, test "$enable_debug" = yes)
Once you’ve defined a condition in ‘configure.in’, you can refer to it in your ‘Makefile.am’ using the ‘if’ statement. Here is a part of a sample ‘Makefile.am’ that uses the conditions defined above:
if TRUE ## This is always used. bin_PROGRAMS = foo endif if DEBUG AM_CFLAGS = -g -DDEBUG endif
It’s important to remember that Automake conditionals are
configure-time conditionals. They don’t rely on any special
make, and there is no way for the user to affect the
conditionals from the
make command line. Automake conditionals
work by rewriting the ‘Makefile’ –
make is unaware that
these conditionals even exist.
Traditionally, Automake conditionals have been considered an advanced feature. However, practice has shown that they are often easier to use and understand than other approaches to solving the same problem. I now recommend the use of conditionals to everyone.
For instance, consider this example:
bin_PROGRAMS = echo if FULL_ECHO echo_SOURCES = echo.c extras.c getopt.c else echo_SOURCES = echo.c endif
In this case, the equivalent code without conditionals is more confusing and correspondingly more difficult for the new Automake user to figure out:
bin_PROGRAMS = echo echo_SOURCES = echo.c echo_LDADD = @echo_extras@ EXTRA_echo_SOURCES = extras.c getopt.c
Automake conditionals have some limitations. One known problem is that conditionals don’t interact properly with ‘+=’ assignment. For instance, consider this code:
bin_PROGRAMS = z z_SOURCES = z.c if SOME_CONDITION z_SOURCES += cond.c endif
This code appears to have an unambiguous meaning, but Automake 1.4 doesn’t implement this and will give an error. This bug will be fixed in the next major Automake release.
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