19.1 Conditionals
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:
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AM_CONDITIONAL(TRUE, true)
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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 configure:
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AM_CONDITIONAL(DEBUG, test "$enable_debug" = yes)
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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:
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if TRUE
## This is always used.
bin_PROGRAMS = foo
endif
if DEBUG
AM_CFLAGS = -g -DDEBUG
endif
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It's important to remember that Automake conditionals are
configure-time conditionals. They don't rely on any special
feature of 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:
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bin_PROGRAMS = echo
if FULL_ECHO
echo_SOURCES = echo.c extras.c getopt.c
else
echo_SOURCES = echo.c
endif
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In this case, the equivalent code without conditionals is more confusing
and correspondingly more difficult for the new Automake user to figure
out:
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bin_PROGRAMS = echo
echo_SOURCES = echo.c
echo_LDADD = @echo_extras@
EXTRA_echo_SOURCES = extras.c getopt.c
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Automake conditionals have some limitations. One known problem is that
conditionals don't interact properly with `+=' assignment. For
instance, consider this code:
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bin_PROGRAMS = z
z_SOURCES = z.c
if SOME_CONDITION
z_SOURCES += cond.c
endif
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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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