automake-1.16: Usage of Conditionals
20.1 Usage of Conditionals
==========================
Before using a conditional, you must define it by using ‘AM_CONDITIONAL’
in the ‘configure.ac’ file (⇒Macros).
-- Macro: AM_CONDITIONAL (CONDITIONAL, CONDITION)
The conditional name, CONDITIONAL, should be a simple string
starting with a letter and containing only letters, digits, and
underscores. It must be different from ‘TRUE’ and ‘FALSE’, which
are reserved by Automake.
The shell CONDITION (suitable for use in a shell ‘if’ statement) is
evaluated when ‘configure’ is run. Note that you must arrange for
_every_ ‘AM_CONDITIONAL’ to be invoked every time ‘configure’ is
run. If ‘AM_CONDITIONAL’ is run conditionally (e.g., in a shell
‘if’ statement), then the result will confuse ‘automake’.
Conditionals typically depend upon options that the user provides to
the ‘configure’ script. Here is an example of how to write a
conditional that is true if the user uses the ‘--enable-debug’ option.
AC_ARG_ENABLE([debug],
[ --enable-debug Turn on debugging],
[case "${enableval}" in
yes) debug=true ;;
no) debug=false ;;
*) AC_MSG_ERROR([bad value ${enableval} for --enable-debug]) ;;
esac],[debug=false])
AM_CONDITIONAL([DEBUG], [test x$debug = xtrue])
Here is an example of how to use that conditional in ‘Makefile.am’:
if DEBUG
DBG = debug
else
DBG =
endif
noinst_PROGRAMS = $(DBG)
This trivial example could also be handled using ‘EXTRA_PROGRAMS’
(⇒Conditional Programs).
You may only test a single variable in an ‘if’ statement, possibly
negated using ‘!’. The ‘else’ statement may be omitted. Conditionals
may be nested to any depth. You may specify an argument to ‘else’ in
which case it must be the negation of the condition used for the current
‘if’. Similarly you may specify the condition that is closed on the
‘endif’ line:
if DEBUG
DBG = debug
else !DEBUG
DBG =
endif !DEBUG
Unbalanced conditions are errors. The ‘if’, ‘else’, and ‘endif’
statements should not be indented, i.e., start on column one.
The ‘else’ branch of the above two examples could be omitted, since
assigning the empty string to an otherwise undefined variable makes no
difference.
In order to allow access to the condition registered by
‘AM_CONDITIONAL’ inside ‘configure.ac’, and to allow conditional
‘AC_CONFIG_FILES’, ‘AM_COND_IF’ may be used:
-- Macro: AM_COND_IF (CONDITIONAL, [IF-TRUE], [IF-FALSE])
If CONDITIONAL is fulfilled, execute IF-TRUE, otherwise execute
IF-FALSE. If either branch contains ‘AC_CONFIG_FILES’, it will
cause ‘automake’ to output the rules for the respective files only
for the given condition.
‘AM_COND_IF’ macros may be nested when m4 quotation is used properly
(⇒(autoconf)M4 Quotation).
Here is an example of how to define a conditional config file:
AM_CONDITIONAL([SHELL_WRAPPER], [test "x$with_wrapper" = xtrue])
AM_COND_IF([SHELL_WRAPPER],
[AC_CONFIG_FILES([wrapper:wrapper.in])])