Address format

A UNIX domain socket address is represented in the following structure:

The sun_family field always contains AF_UNIX. On Linux sun_path is 108 bytes in size; see also NOTES, below.

Various systems calls (for example, bind(2), connect(2), and sendto(2)) take a sockaddr_un argument as input. Some other system calls (for example, getsockname(2), getpeername(2), recvfrom(2), and accept(2)) return an argument of this type.

Three types of address are distinguished in the sockaddr_un structure:

Pathname sockets

When binding a socket to a pathname, a few rules should be observed for maximum portability and ease of coding:

There is some variation in how implementations handle UNIX domain socket addresses that do not follow the above rules. For example, some (but not all) implementations append a null terminator if none is present in the supplied sun_path.

When coding portable applications, keep in mind that some implementations have sun_path as short as 92 bytes.

Various system calls (accept(2), recvfrom(2), getsockname(2), getpeername(2)) return socket address structures. When applied to UNIX domain sockets, the value-result addrlen argument supplied to the call should be initialized as above. Upon return, the argument is set to indicate the actual size of the address structure. The caller should check the value returned in this argument: if the output value exceeds the input value, then there is no guarantee that a null terminator is present in sun_path. (See BUGS.)

Pathname socket ownership and permissions

In the Linux implementation, pathname sockets honor the permissions of the directory they are in. Creation of a new socket will fail if the process does not have write and search (execute) permission on the directory in which the socket is created.

On Linux, connecting to a stream socket object requires write permission on that socket; sending a datagram to a datagram socket likewise requires write permission on that socket. POSIX does not make any statement about the effect of the permissions on a socket file, and on some systems (e.g., older BSDs), the socket permissions are ignored. Portable programs should not rely on this feature for security.

When creating a new socket, the owner and group of the socket file are set according to the usual rules. The socket file has all permissions enabled, other than those that are turned off by the process umask(2).

The owner, group, and permissions of a pathname socket can be changed (using chown(2) and chmod(2)).

Abstract sockets

Socket permissions have no meaning for abstract sockets: the process umask(2) has no effect when binding an abstract socket, and changing the ownership and permissions of the object (via fchown(2) and fchmod(2)) has no effect on the accessibility of the socket.

Abstract sockets automatically disappear when all open references to the socket are closed.

The abstract socket namespace is a nonportable Linux extension.

Socket options

For historical reasons, these socket options are specified with a SOL_SOCKET type even though they are AF_UNIX specific. They can be set with setsockopt(2) and read with getsockopt(2) by specifying SOL_SOCKET as the socket family.

Autobind feature

If a bind(2) call specifies addrlen as sizeof(sa_family_t), or the SO_PASSCRED socket option was specified for a socket that was not explicitly bound to an address, then the socket is autobound to an abstract address. The address consists of a null byte followed by 5 bytes in the character set [0-9a-f]. Thus, there is a limit of 2^20 autobind addresses. (From Linux 2.1.15, when the autobind feature was added, 8 bytes were used, and the limit was thus 2^32 autobind addresses. The change to 5 bytes came in Linux 2.3.15.)

Sockets API

The following paragraphs describe domain-specific details and unsupported features of the sockets API for UNIX domain sockets on Linux.

UNIX domain sockets do not support the transmission of out-of-band data (the MSG_OOB flag for send(2) and recv(2)).

The send(2) MSG_MORE flag is not supported by UNIX domain sockets.

The use of MSG_TRUNC in the flags argument of recv(2) is not supported by UNIX domain sockets.

The SO_SNDBUF socket option does have an effect for UNIX domain sockets, but the SO_RCVBUF option does not. For datagram sockets, the SO_SNDBUF value imposes an upper limit on the size of outgoing datagrams. This limit is calculated as the doubled (see socket(7)) option value less 32 bytes used for overhead.

Ancillary messages

Ancillary data is sent and received using sendmsg(2) and recvmsg(2). For historical reasons the ancillary message types listed below are specified with a SOL_SOCKET type even though they are AF_UNIX specific. To send them set the cmsg_level field of the struct cmsghdr to SOL_SOCKET and the cmsg_type field to the type. For more information see cmsg(3).

Ioctls

The following ioctl(2) calls return information in value. The correct syntax is:

ioctl_type can be:

Example output

Program source

/*
 * File connection.h
 */

#define SOCKET_NAME "/tmp/9Lq7BNBnBycd6nxy.socket"
#define BUFFER_SIZE 12

/*
 * File server.c
 */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <unistd.h>
#include "connection.h"

int
main(int argc, char *argv[])
{
    struct sockaddr_un name;
    int down_flag = 0;
    int ret;
    int connection_socket;
    int data_socket;
    int result;
    char buffer[BUFFER_SIZE];

    /*
     * In case the program exited inadvertently on the last run,
     * remove the socket.
     */

    unlink(SOCKET_NAME);

    /* Create local socket. */

    connection_socket = socket(AF_UNIX, SOCK_SEQPACKET, 0);
    if (connection_socket == −1) {
        perror("socket");
        exit(EXIT_FAILURE);
    }

    /*
     * For portability clear the whole structure, since some
     * implementations have additional (nonstandard) fields in
     * the structure.
     */

    memset(&name, 0, sizeof(struct sockaddr_un));

    /* Bind socket to socket name. */

    name.sun_family = AF_UNIX;
    strncpy(name.sun_path, SOCKET_NAME, sizeof(name.sun_path) − 1);

    ret = bind(connection_socket, (const struct sockaddr *) &name,
               sizeof(struct sockaddr_un));
    if (ret == −1) {
        perror("bind");
        exit(EXIT_FAILURE);
    }

    /*
     * Prepare for accepting connections. The backlog size is set
     * to 20. So while one request is being processed other requests
     * can be waiting.
     */

    ret = listen(connection_socket, 20);
    if (ret == −1) {
        perror("listen");
        exit(EXIT_FAILURE);
    }

    /* This is the main loop for handling connections. */

    for (;;) {

        /* Wait for incoming connection. */

        data_socket = accept(connection_socket, NULL, NULL);
        if (data_socket == −1) {
            perror("accept");
            exit(EXIT_FAILURE);
        }

        result = 0;
        for(;;) {

            /* Wait for next data packet. */

            ret = read(data_socket, buffer, BUFFER_SIZE);
            if (ret == −1) {
                perror("read");
                exit(EXIT_FAILURE);
            }

            /* Ensure buffer is 0−terminated. */

            buffer[BUFFER_SIZE − 1] = 0;

            /* Handle commands. */

            if (!strncmp(buffer, "DOWN", BUFFER_SIZE)) {
                down_flag = 1;
                break;
            }

            if (!strncmp(buffer, "END", BUFFER_SIZE)) {
                break;
            }

            /* Add received summand. */

            result += atoi(buffer);
        }

        /* Send result. */

        sprintf(buffer, "%d", result);
        ret = write(data_socket, buffer, BUFFER_SIZE);

        if (ret == −1) {
            perror("write");
            exit(EXIT_FAILURE);
        }

        /* Close socket. */

        close(data_socket);

        /* Quit on DOWN command. */

        if (down_flag) {
            break;
        }
    }

    close(connection_socket);

    /* Unlink the socket. */

    unlink(SOCKET_NAME);

    exit(EXIT_SUCCESS);
}

/*
 * File client.c
 */

#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <unistd.h>
#include "connection.h"

int
main(int argc, char *argv[])
{
    struct sockaddr_un addr;
    int i;
    int ret;
    int data_socket;
    char buffer[BUFFER_SIZE];

    /* Create local socket. */

    data_socket = socket(AF_UNIX, SOCK_SEQPACKET, 0);
    if (data_socket == −1) {
        perror("socket");
        exit(EXIT_FAILURE);
    }

    /*
     * For portability clear the whole structure, since some
     * implementations have additional (nonstandard) fields in
     * the structure.
     */

    memset(&addr, 0, sizeof(struct sockaddr_un));

    /* Connect socket to socket address */

    addr.sun_family = AF_UNIX;
    strncpy(addr.sun_path, SOCKET_NAME, sizeof(addr.sun_path) − 1);

    ret = connect (data_socket, (const struct sockaddr *) &addr,
                   sizeof(struct sockaddr_un));
    if (ret == −1) {
        fprintf(stderr, "The server is down.\n");
        exit(EXIT_FAILURE);
    }

    /* Send arguments. */

    for (i = 1; i < argc; ++i) {
        ret = write(data_socket, argv[i], strlen(argv[i]) + 1);
        if (ret == −1) {
            perror("write");
            break;
        }
    }

    /* Request result. */

    strcpy (buffer, "END");
    ret = write(data_socket, buffer, strlen(buffer) + 1);
    if (ret == −1) {
        perror("write");
        exit(EXIT_FAILURE);
    }

    /* Receive result. */

    ret = read(data_socket, buffer, BUFFER_SIZE);
    if (ret == −1) {
        perror("read");
        exit(EXIT_FAILURE);
    }

    /* Ensure buffer is 0−terminated. */

    buffer[BUFFER_SIZE − 1] = 0;

    printf("Result = %s\n", buffer);

    /* Close socket. */

    close(data_socket);

    exit(EXIT_SUCCESS);
}

For an example of the use of SCM_RIGHTS see cmsg(3).