File: autoconf.info,  Node: System Services,  Next: C and Posix Variants,  Prev: Compilers and Preprocessors,  Up: Existing Tests

5.11 System Services
====================

The following macros check for operating system services or
capabilities.

 -- Macro: AC_PATH_X
     Try to locate the X Window System include files and libraries.  If
     the user gave the command line options ‘--x-includes=DIR’ and
     ‘--x-libraries=DIR’, use those directories.

     If either or both were not given, get the missing values by running
     ‘xmkmf’ (or an executable pointed to by the ‘XMKMF’ environment
     variable) on a trivial ‘Imakefile’ and examining the makefile that
     it produces.  Setting ‘XMKMF’ to ‘false’ disables this method.

     If this method fails to find the X Window System, ‘configure’ looks
     for the files in several directories where they often reside.  If
     either method is successful, set the shell variables ‘x_includes’
     and ‘x_libraries’ to their locations, unless they are in
     directories the compiler searches by default.

     If both methods fail, or the user gave the command line option
     ‘--without-x’, set the shell variable ‘no_x’ to ‘yes’; otherwise
     set it to the empty string.

 -- Macro: AC_PATH_XTRA
     An enhanced version of ‘AC_PATH_X’.  It adds the C compiler flags
     that X needs to output variable ‘X_CFLAGS’, and the X linker flags
     to ‘X_LIBS’.  Define ‘X_DISPLAY_MISSING’ if X is not available.

     This macro also checks for special libraries that some systems need
     in order to compile X programs.  It adds any that the system needs
     to output variable ‘X_EXTRA_LIBS’.  And it checks for special X11R6
     libraries that need to be linked with before ‘-lX11’, and adds any
     found to the output variable ‘X_PRE_LIBS’.

 -- Macro: AC_SYS_INTERPRETER
     Check whether the system supports starting scripts with a line of
     the form ‘#!/bin/sh’ to select the interpreter to use for the
     script.  After running this macro, shell code in ‘configure.ac’ can
     check the shell variable ‘interpval’; it is set to ‘yes’ if the
     system supports ‘#!’, ‘no’ if not.

 -- Macro: AC_SYS_LARGEFILE
     If the default ‘off_t’ type is a 32-bit integer, and therefore
     cannot be used with files 2 GiB or larger, make a wider ‘off_t’
     available if the system supports it.  Similarly, widen other types
     related to sizes of files and file systems if possible.  These
     types may include ‘blkcnt_t’, ‘dev_t’, ‘ino_t’, ‘fsblkcnt_t’,
     ‘fsfilcnt_t’, and ‘rlim_t’.

     Also, arrange for a ‘configure’ option ‘--enable-year2038’ to
     request widening the type ‘time_t’ as needed to represent file wand
     other timestamps after mid-January 2038.  This widening is possible
     only on 32-bit GNU/Linux x86 and ARM systems with glibc 2.34 or
     later.  If year-2038 support is requested but ‘configure’ fails to
     find a way to widen ‘time_t’ and inspection of the system suggests
     that this feature is available somehow, ‘configure’ will error out.
     If you want the default to be ‘--enable-year2038’, you can use
     ‘AC_SYS_YEAR2038’ or ‘AC_SYS_YEAR2038_RECOMMENDED’ instead of
     ‘AC_SYS_LARGEFILE’.  In other words, older packages that have long
     used ‘AC_SYS_LARGEFILE’ can have year-2038 support on 32-bit
     GNU/Linux x86 and ARM systems either by regenerating ‘configure’
     with current Autoconf and configuring with ‘--enable-year2038’, or
     by using ‘AC_SYS_YEAR2038’ or ‘AC_SYS_YEAR2038_RECOMMENDED’ and
     configuring without ‘--disable-year2038’.  A future version of
     Autoconf might change the ‘AC_SYS_LARGEFILE’ default to
     ‘--enable-year2038’; if and when that happens, ‘AC_SYS_LARGEFILE’
     and ‘AC_SYS_YEAR2038’ will become equivalent.  *Note
     AC_SYS_YEAR2038::.

     Set the shell variable ‘ac_have_largefile’ to ‘yes’ or ‘no’
     depending on whether a wide ‘off_t’ is available, regardless of
     whether arrangements were necessary.  Similarly, set the shell
     variable ‘ac_have_year2038’ to ‘yes’ or ‘no’ depending on whether a
     wide-enough ‘time_t’ is available.

     Define preprocessor macros if necessary to make types wider; for
     example, on GNU/Linux systems the macros ‘_FILE_OFFSET_BITS’ and
     ‘_TIME_BITS’ can be defined.  Some of these macros work only if
     defined before the first system header is included; therefore, when
     using this macro in concert with ‘AC_CONFIG_HEADERS’, make sure
     that ‘config.h’ is included before any system headers.

     On obsolete IRIX systems, also change the output variable ‘CC’ to
     add compiler options needed for wide ‘off_t’.

     Large-file support can be disabled by configuring with the
     ‘--disable-largefile’ option, and year-2038 support can be enabled
     and disabled via the ‘--enable-year2038’ and ‘--disable-year2038’
     options.  These options have no effect on systems where types are
     wide enough by default.  Large-file support is required for
     year-2038 support: if you configure with ‘--disable-largefile’ on a
     platform with 32-bit ‘time_t’, then year-2038 support is not
     available.

     Disabling large-file or year-2038 support can have surprising
     effects, such as causing functions like ‘readdir’ and ‘stat’ to
     fail even on a small file because its inode number or timestamp is
     out of range.

     Regardless of whether you use this macro, portable programs should
     not assume that any of the types listed above fit into a ‘long
     int’.  For example, it is not portable to print an arbitrary
     ‘off_t’ or ‘time_t’ value ‘X’ with ‘printf ("%ld", (long int) X)’.

     The standard C library functions ‘fseek’ and ‘ftell’ do not use
     ‘off_t’.  If you need to use either of these functions, you should
     use ‘AC_FUNC_FSEEKO’ as well as ‘AC_SYS_LARGEFILE’, and then use
     their Posix replacements ‘fseeko’ and ‘ftello’.  *Note
     AC_FUNC_FSEEKO::.

     When using ‘AC_SYS_LARGEFILE’ in different packages that are linked
     together and that have interfaces that depend on the width of
     ‘off_t’, ‘time_t’ or related types, the simplest thing is to
     configure all components the same way.  For example, if an
     application uses ‘AC_SYS_LARGEFILE’ and is configured with
     ‘--enable-year2038’, libraries it links to with an ‘off_t’- or
     ‘time_t’-dependent interface should be configured equivalently.
     Alternatively, you can modify libraries to support both 32- and
     64-bit interfaces though this is more work and few libraries other
     than the C library itself are modified in this way.

     Applications and libraries should be configured compatibly.  If
     ‘off_t’, ‘time_t’ or related types appear in a library's public
     interface, enabling or disabling the library's large-file or
     year-2038 support may break binary compatibility with applications
     or with other libraries.  Similarly, if an application links to a
     such a library, enabling or disabling the application's large-file
     support may break binary compatibility with that library.

 -- Macro: AC_SYS_LONG_FILE_NAMES
     If the system supports file names longer than 14 characters, define
     ‘HAVE_LONG_FILE_NAMES’.

 -- Macro: AC_SYS_POSIX_TERMIOS
     Check to see if the Posix termios headers and functions are
     available on the system.  If so, set the shell variable
     ‘ac_cv_sys_posix_termios’ to ‘yes’.  If not, set the variable to
     ‘no’.

 -- Macro: AC_SYS_YEAR2038
     This is like ‘AC_SYS_LARGEFILE’ except it defaults to enabling
     instead of disabling year-2038 support.  Year-2038 support for
     applications and libraries should be configured compatibly.  *Note
     AC_SYS_LARGEFILE::.

 -- Macro: AC_SYS_YEAR2038_RECOMMENDED
     This macro has the same effect as ‘AC_SYS_YEAR2038’, but also
     declares that the program being configured should support
     timestamps after mid-January 2038.  If a large ‘time_t’ is
     unavailable, ‘configure’ will error out unless the
     ‘--disable-year2038’ option is specified.

     Year-2038 support for applications and libraries should be
     configured compatibly.  *Note AC_SYS_YEAR2038::.