The socketpair() call creates an unnamed pair of connected sockets in the specified domain, of the specified type, and using the optionally specified protocol. For further details of these arguments, see socket(2).
The file descriptors used in referencing the new sockets are returned in sv and sv. The two sockets are indistinguishable.
On success, zero is returned. On error, -1 is returned, and errno is set appropriately.
On Linux (and other systems), socketpair() does not modify sv on failure. A requirement standardizing this behavior was added in POSIX.1-2016.
On Linux, the only supported domains for this call are AF_UNIX (or synonymously, AF_LOCAL) and AF_TIPC (since Linux 4.12).
Since Linux 2.6.27, socketpair() supports the SOCK_NONBLOCK and SOCK_CLOEXEC flags in the type argument, as described in socket(2).
POSIX.1 does not require the inclusion of <sys/types.h>, and this header file is not required on Linux. However, some historical (BSD) implementations required this header file, and portable applications are probably wise to include it.