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UNIX(7P) UNIX(7P)

NAME

UNIX - local communication domain protocol

SYNOPSIS

#include <sys/types.h>

#include <sys/un.h>

DESCRIPTION

The local communication domain protocol, commonly referred to in the industry as the Unix domain pro-

tocol, utilizes the path name address format and the

AF_UNIX address family. This protocol can be used

as an alternative to the Internet protocol family (TCP/IP or UDP/IP) for communication between processes

executing on the same node. It has a signiﬁcant throughput advantage when compared with local IP loop-

back, due primarily to its much lower code execution overhead. Data is looped back at the protocol layer

(OSI Level 4), rather than at the driver layer (OSI Level 2).

Only

SOCK_STREAM is supported in the AF_UNIX

address family.

The HP-UX implementation of the local communication domain protocol does not support the

MSG_OOB

ﬂag in recv() (see recv(2)) and send() (see send(2)).

Addressing

AF_UNIX socket addresses are path names. They are limited to 92 bytes in length, including a terminat-

ing null byte. Calls to bind() to an AF_UNIX socket utilize an addressing structure called

structsockaddr_un (see bind(2)). Pointers to this structure should be used in all

AF_UNIX socket sys-

tem calls wherever they require a pointer to a

struct sockaddr.

The include ﬁle <sys/un.h> deﬁnes this addressing structure. Within this structure are two notable

ﬁelds. The ﬁrst is sun_family, which must be set to AF_UNIX. The next is sun_path, which is the null-

terminated character string that speciﬁes the path name of the ﬁle associated with the socket (for example,

/tmp/mysocket).

Only the passive (listening) socket must bind to an address. The active socket connects to that address, but

it does not need an address of its own.

For additional information on using AF_UNIX sockets for interprocess communication, refer to the BSD

Sockets Interface Programmer’s Guide.

Socket Buffer Size

For stream and datagram sockets, the maximum send and receive buffer size is 262142 bytes. The default

buffer size is 32768 bytes. The send and receive buffer sizes can be altered by using the

SO_SNDBUF and

SO_RCVBUF options of the setsockopt() system call. Refer to getsockopt (2) for details.

AUTHOR

AF_UNIX was developed by the University of California, Berkeley.