unistd.h

The unistd.h header file declares a number of implementation-specific functions:
There are also a large number of POSIX and UNIX functions declared, shown below with the minimum feature test macro needed to expose them:
POSIX1_SOURCE = 2
POSIX_C_SOURCE = 2

External Variables

_XOPEN_SOURCE
_XOPEN_SOURCE = 500

The unistd.h header file also defines many symbols to represent configuration variables and implementation features provided. Some of these are used at compile time, while others are used to interrogate the system at run time, using sysconf(), confstr(), pathconf(), or fpathconf().