These macros are used to find system header files not covered by the “particular” test macros. If you need to check the contents of a header as well as find out whether it is present, you have to write your own test for it (see Writing Tests).
If the system header file header-file is compilable, execute shell
commands action-if-found, otherwise execute
action-if-not-found. If you just want to define a symbol if the
header file is available, consider using AC_CHECK_HEADERS
instead.
includes should be the appropriate prerequisite code, i.e.
whatever might be required to appear above
‘#include <header-file>’ for it to compile without error.
This can be anything, but will normally be additional ‘#include’
directives. If includes is omitted or empty, configure will
use the contents of the macro AC_INCLUDES_DEFAULT.
See Default Includes.
This macro used to check only for the presence of a header, not
whether its contents were acceptable to the compiler. Some older
configure scripts rely on this behavior, so it is still
available by specifying ‘-’ as includes. This mechanism is
deprecated as of Autoconf 2.70; situations where a preprocessor-only
check is required should use AC_PREPROC_IFELSE.
See Running the Preprocessor.
This macro caches its result in the ac_cv_header_header-file
variable, with characters not suitable for a variable name mapped to
underscores.
For each given system header file header-file in the
blank-separated argument list that exists, define
HAVE_header-file (in all capitals). If action-if-found
is given, it is additional shell code to execute when one of the header
files is found. You can give it a value of ‘break’ to break out of
the loop on the first match. If action-if-not-found is given, it
is executed when one of the header files is not found.
includes is interpreted as in AC_CHECK_HEADER, in order to
choose the set of preprocessor directives supplied before the header
under test.
This macro caches its result in the ac_cv_header_header-file
variable, with characters not suitable for a variable name mapped to
underscores.
For each given system header file header-file in the
blank-separated argument list that exists, define
HAVE_header-file (in all capitals).
If you do not need the full power of AC_CHECK_HEADERS, this
variant generates smaller, faster configure files. All
headers passed to AC_CHECK_HEADERS_ONCE are checked for in one
pass, early during the configure run. The checks cannot be
conditionalized, you cannot specify an action-if-found or
action-if-not-found, and AC_INCLUDES_DEFAULT is always used
for the prerequisites.
In previous versions of Autoconf, these macros merely checked whether
the header was accepted by the preprocessor. This was changed because
the old test was inappropriate for typical uses. Headers are typically
used to compile, not merely to preprocess, and the old behavior
sometimes accepted headers that clashed at compile-time
(see Header Present But Cannot Be Compiled). If for some reason it is
inappropriate to check whether a header is compilable, you should use
AC_PREPROC_IFELSE (see Running the Preprocessor) instead of
these macros.
Requiring each header to compile improves the robustness of the test,
but it also requires you to make sure that the includes are
correct. Most system headers nowadays make sure to #include
whatever they require, or else have their dependencies satisfied by
AC_INCLUDES_DEFAULT (see Default Includes), but
see Portability of Headers, for known exceptions. In general, if you
are looking for bar.h, which requires that foo.h be
included first if it exists, you should do something like this:
AC_CHECK_HEADERS([foo.h]) AC_CHECK_HEADERS([bar.h], [], [], [#ifdef HAVE_FOO_H # include <foo.h> #endif ])