The GNU C Library - System Options

Overall System Options

POSIX defines certain system-specific options that not all POSIX systems support. Since these options are provided in the kernel, not in the library, simply using the GNU C library does not guarantee any of these features is supported; it depends on the system you are using.

You can test for the availability of a given option using the macros in this section, together with the function sysconf . The macros are defined only if you include `unistd.h'.

For the following macros, if the macro is defined in `unistd.h', then the option is supported. Otherwise, the option may or may not be supported; use sysconf to find out. See Sysconf.

Macro int _POSIX_JOB_CONTROL

If this symbol is defined, it indicates that the system supports job control. Otherwise, the implementation behaves as if all processes within a session belong to a single process group. See Job Control.

Macro int _POSIX_SAVED_IDS

If this symbol is defined, it indicates that the system remembers the effective user and group IDs of a process before it executes an executable file with the set-user-ID or set-group-ID bits set, and that explicitly changing the effective user or group IDs back to these values is permitted. If this option is not defined, then if a nonprivileged process changes its effective user or group ID to the real user or group ID of the process, it can't change it back again. See Enable/Disable Setuid.

For the following macros, if the macro is defined in `unistd.h', then its value indicates whether the option is supported. A value of -1 means no, and any other value means yes. If the macro is not defined, then the option may or may not be supported; use sysconf to find out. See Sysconf.

Macro int _POSIX2_C_DEV

If this symbol is defined, it indicates that the system has the POSIX.2 C compiler command, c89 . The GNU C library always defines this as 1 , on the assumption that you would not have installed it if you didn't have a C compiler.

Macro int _POSIX2_FORT_DEV

If this symbol is defined, it indicates that the system has the POSIX.2 Fortran compiler command, fort77 . The GNU C library never defines this, because we don't know what the system has.

Macro int _POSIX2_FORT_RUN

If this symbol is defined, it indicates that the system has the POSIX.2 asa command to interpret Fortran carriage control. The GNU C library never defines this, because we don't know what the system has.

Macro int _POSIX2_LOCALEDEF

If this symbol is defined, it indicates that the system has the POSIX.2 localedef command. The GNU C library never defines this, because we don't know what the system has.

Macro int _POSIX2_SW_DEV

If this symbol is defined, it indicates that the system has the POSIX.2 commands ar , make , and strip . The GNU C library always defines this as 1 , on the assumption that you had to have ar and make to install the library, and it's unlikely that strip would be absent when those are present.