15.3 socketpair Function

The socketpair function creates two sockets that are then connected together. This function applies only to Unix domain sockets.

#include <sys/socket.h>
int socketpair(int family, int type, int protocol, int sockfd[2]);
Returns: nonzero if OK, -1 on error

The family must be AF_LOCAL and the protocol must be 0. The type, however, can be either SOCK_STREAM or SOCK_DGRAM. The two socket descriptors that are created are returned as sockfd[0] and sockfd[1].

This function is similar to the Unix pipe function: Two descriptors are returned, and each descriptor is connected to the other. Indeed, Berkeley-derived implementations employ pipe by performing the same internal operations as socketpair (pp. 253–254 of TCPv3).

The two created sockets are unnamed; that is, there is no implicit bind involved.

The result of socketpair with a type of SOCK_STREAM is called a stream pipe. It is similar to a regular Unix pipe (created by the pipe function), but a stream pipe is full-duplex; that is, both descriptors can be read and written. We show a picture of a stream pipe created by socketpair in Figure 15.7.

POSIX does not require full-duplex pipes. On SVR4, pipe returns two full-duplex descriptors, while Berkeley-derived kernels traditionally return two half-duplex descriptors (Figure 17.31 of TCPv3).