The System V Interface Description (SVID) is a document describing the AT&T Unix System V operating system. It is to some extent a superset of the POSIX standard (see POSIX (The Portable Operating System Interface)).
The GNU C Library defines most of the facilities required by the SVID that are not also required by the ISO C or POSIX standards, for compatibility with System V Unix and other Unix systems (such as SunOS) which include these facilities. However, many of the more obscure and less generally useful facilities required by the SVID are not included. (In fact, Unix System V itself does not provide them all.)
The supported facilities from System V include the methods for
inter-process communication and shared memory, the hsearch and
drand48 families of functions, fmtmsg and several of the
mathematical functions.