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7.4 Caching Results
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 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.
— Macro: AC_CACHE_VAL ( cache-id, commands-to-set-it)
Ensure that the results of the check identified by cache-id are
available. If the results of the check were in the cache file that was
read, and configure was not given the --quiet or
--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 is
saved in the cache file just before configure creates its output
files. See 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.
— Macro: AC_CACHE_CHECK ( message, cache-id, commands-to-set-it)
A wrapper for 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 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) 2>/dev/null && ac_cv_shell_true_works=yes
if test "$ac_cv_shell_true_works" = yes; then
AC_DEFINE([TRUE_WORKS], [1],
[Define if `true(1)' works properly.])
fi])
])
This fails if the cache is enabled: the second time this macro is run,
TRUE_WORKS will not be defined. The proper implementation
is:
AC_DEFUN([AC_SHELL_TRUE],
[AC_CACHE_CHECK([whether true(1) works], [ac_cv_shell_true_works],
[ac_cv_shell_true_works=no
(true) 2>/dev/null && ac_cv_shell_true_works=yes])
if test "$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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