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.
Published under the terms of the GNU General Public License
