Appendix A UNIX Domain Sockets

Introduction

UNIX domain sockets are named with UNIX paths. For example, a socket might be named /tmp/foo. UNIX domain sockets communicate only between processes on a single host. Sockets in the UNIX domain are not considered part of the network protocols because they can only be used to communicate between processes on a single host.

Socket types define the communication properties visible to a user. The Internet domain sockets provide access to the TCP/IP transport protocols. The Internet domain is identified by the value AF_INET. Sockets exchange data only with sockets in the same domain.

Socket Creation

The socket(3SOCKET) call creates a socket in the specified family and of the specified type.

s = socket(family, type, protocol);

If the protocol is unspecified (a value of 0), the system selects a protocol that supports the requested socket type. The socket handle (a file descriptor) is returned.

The family is specified by one of the constants defined in sys/socket.h. Constants named AF_suite specify the address format to use in interpreting names as shown in Table 2-1.

The following creates a datagram socket for intramachine use:

s = socket(AF_UNIX, SOCK_DGRAM, 0);

Use the default protocol (the protocol argument is 0) in most situations.

Binding Local Names

A socket is created with no name. A remote process has no way to refer to a socket until an address is bound to it. Communicating processes are connected through addresses. In the UNIX family, a connection is composed of (usually) one or two path names. UNIX family sockets need not always be bound to a name, but, when bound, there can never be duplicate ordered sets such as: local pathname or foreign pathname. The path names cannot refer to existing files.

The bind(3SOCKET) call allows a process to specify the local address of the socket. This provides local pathname, while connect(3SOCKET) and accept(3SOCKET) complete a socket's association by fixing the remote half of the address. bind(3SOCKET) is used as follows:

bind (s, name, namelen);

The socket handle is s. The bound name is a byte string that is interpreted by the supporting protocol(s). UNIX family names contain a path name and a family. The example shows binding the name /tmp/foo to a UNIX family socket.

#include <sys/un.h>
 ...
struct sockaddr_un addr;
 ...
strcpy(addr.sun_path, "/tmp/foo");
addr.sun_family = AF_UNIX;
bind (s, (struct sockaddr *) &addr,
		strlen(addr.sun_path) + sizeof (addr.sun_family));

When determining the size of an AF_UNIX socket address, null bytes are not counted, which is why strlen(3C) use is fine.

The file name referred to in addr.sun_path is created as a socket in the system file name space. The caller must have write permission in the directory where addr.sun_path is created. The file should be deleted by the caller when it is no longer needed. AF_UNIX sockets can be deleted with unlink(1M).

Connection Establishment

Connection establishment is usually asymmetric. One process acts as the client and the other as the server. The server binds a socket to a well-known address associated with the service and blocks on its socket for a connect request. An unrelated process can then connect to the server. The client requests services from the server by initiating a connection to the server's socket. On the client side, the connect(3SOCKET) call initiates a connection. In the UNIX family, this might appear as:

struct sockaddr_un server;
		server.sun.family = AF_UNIX;
		 ...
		connect(s, (struct sockaddr *)&server, strlen(server.sun_path) 
         + sizeof (server.sun_family));

See "Connection Errors" for information on connection errors. "Data Transfer" tells you how to transfer data. "Closing Sockets" tells you how to close a socket.