###UNIX Domain Sockets
UNIX domain sockets are used to communicate with processes running on the same machine. Although Internet domain sockets can be used for this same purpose, UNIX domain sockets are more efficient. UNIX domain sockets only copy data; they have no protocol processing to perform, no network headers to add or remove, no checksums to calculate, no sequence numbers to generate, and no acknowledgements to send.
UNIX domain sockets provide both stream and datagram interfaces. The UNIX domain datagram service is reliable, however. Messages are neither lost nor delivered out of order. UNIX domain sockets are like a cross between sockets and pipes. You can use the network-oriented socket interfaces with them, or you can use the socketpair function to create a pair of unnamed, connected, UNIX domain sockets.
#include <sys/socket.h> int socketpair(int domain, int type, int protocol, int sockfd); Returns: 0 if OK, −1 on error
A pair of connected UNIX domain sockets acts like a full-duplex pipe: both ends are open for reading and writing . We’ll refer to these as ‘‘fd-pipes’’ to distinguish them from normal, half-duplex pipes.
###Naming UNIX Domain Sockets Although the socketpair function creates sockets that are connected to each other, the individual sockets don’t have names. This means that they can’t be addressed by unrelated processes.
###Unique Connections A server can arrange for unique UNIX domain connections to clients using the standard bind, listen, and accept functions. Clients use connect to contact the server; after the connect request is accepted by the server, a unique connection exists between the client and the server.
###Passing File Descriptors Passing an open file descriptor between processes is a powerful technique. It can lead to different ways of designing client–server applications. It allows one process (typically a server) to do everything that is required to open a file
What normally happens when a descriptor is passed from one process to another is that the sending process, after passing the descriptor, then closes the descriptor. Closing the descriptor by the sender doesn’t really close the file or device, since the descriptor is still considered open by the receiving process (even if the receiver hasn’t specifically received the descriptor yet).
#include <sys/socket.h> unsigned char *CMSG_DATA(struct cmsghdr *cp); Returns: pointer to data associated with cmsghdr structure struct cmsghdr *CMSG_FIRSTHDR(struct msghdr *mp); Returns: pointer to first cmsghdr structure associated with the msghdr structure, or NULL if none exists struct cmsghdr *CMSG_NXTHDR(struct msghdr *mp, struct cmsghdr *cp); Returns: pointer to next cmsghdr structure associated with the msghdr structure given the current cmsghdr structure, or NULL if we’re at the last one unsigned int CMSG_LEN(unsigned int nbytes); Returns: size to allocate for data object nbytes large