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To avoid checking for the same features repeatedly in various
configure
scripts (or in repeated runs of one script),
configure
can optionally save the results of many checks in a
cache file (see section Cache Files). If a configure
script
runs with caching enabled and finds a cache file, it reads the results
of previous runs from the cache and avoids rerunning those checks. As a
result, configure
can then run much faster than if it had to
perform all of the checks every time.
configure
was not given the @option{--quiet} or
@option{--silent} option, print a message saying that the result was
cached; otherwise, run the shell commands commands-to-set-it. If
the shell commands are run to determine the value, the value will be
saved in the cache file just before configure
creates its output
files. See section Cache Variable Names, for how to choose the name of the
cache-id variable.
The commands-to-set-it must have no side effects except for setting the variable cache-id, see below.
AC_CACHE_VAL
that takes care of printing the
messages. This macro provides a convenient shorthand for the most
common way to use these macros. It calls AC_MSG_CHECKING
for
message, then AC_CACHE_VAL
with the cache-id and
commands arguments, and AC_MSG_RESULT
with cache-id.
The commands-to-set-it must have no side effects except for setting the variable cache-id, see below.
It is very common to find buggy macros using AC_CACHE_VAL
or
AC_CACHE_CHECK
, because people are tempted to call
AC_DEFINE
in the commands-to-set-it. Instead, the code that
follows the call to AC_CACHE_VAL
should call
AC_DEFINE
, by examining the value of the cache variable. For
instance, the following macro is broken:
AC_DEFUN([AC_SHELL_TRUE], [AC_CACHE_CHECK([whether true(1) works], [ac_cv_shell_true_works], [ac_cv_shell_true_works=no true && ac_cv_shell_true_works=yes if test $ac_cv_shell_true_works = yes; then AC_DEFINE([TRUE_WORKS], 1 [Define if `true(1)' works properly.]) fi]) ])
This fails if the cache is enabled: the second time this macro is run,
TRUE_WORKS
will not be defined. The proper implementation
is:
AC_DEFUN([AC_SHELL_TRUE], [AC_CACHE_CHECK([whether true(1) works], [ac_cv_shell_true_works], [ac_cv_shell_true_works=no true && ac_cv_shell_true_works=yes]) if test $ac_cv_shell_true_works = yes; then AC_DEFINE([TRUE_WORKS], 1 [Define if `true(1)' works properly.]) fi ])
Also, commands-to-set-it should not print any messages, for
example with AC_MSG_CHECKING
; do that before calling
AC_CACHE_VAL
, so the messages are printed regardless of whether
the results of the check are retrieved from the cache or determined by
running the shell commands.
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