15.3 socketpair Function
The socketpair function creates two
sockets that are then connected together. This function applies
only to Unix domain sockets.
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#include <sys/socket.h>
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int socketpair(int family, int type, int protocol, int sockfd[2]);
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Returns: nonzero if OK, -1 on error
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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鈥?54 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).
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