SYNOPSIS

#include <sys/socket.h>
#include <sys/un.h>

unix_socket = socket(AF_UNIX, type, 0);
error = socketpair(AF_UNIX, type, 0, int *sv);

DESCRIPTION

The AF_UNIX (also known as AF_LOCAL) socket family is used to communicate between processes on the same machine efficiently. Traditionally, Unix sockets can be either unnamed, or bound to a file system pathname (marked as being of type socket). Linux also supports an abstract namespace which is independent of the file system.

Valid types are: SOCK_STREAM, for a stream-oriented socket and SOCK_DGRAM, for a datagram-oriented socket that preserves message boundaries (as on most Unix implementations, Unix domain datagram sockets are always reliable and don't reorder datagrams); and (since Linux 2.6.4) SOCK_SEQPACKET, for a connection-oriented socket that preserves message boundaries and delivers messages in the order that they were sent.

Unix sockets support passing file descriptors or process credentials to other processes using ancillary data.

Address Format

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

#define UNIX_PATH_MAX    108
struct sockaddr_un {
    sa_family_t sun_family;               /* AF_UNIX */
    char        sun_path[UNIX_PATH_MAX];  /* pathname */
};

sun_family always contains AF_UNIX.

Three types of address are distinguished in this structure:

*

pathname: a Unix domain socket can be bound to a null-terminated file system pathname using bind(2). When the address of the socket is returned by getsockname(2), getpeername(2), and accept(2), its length is sizeof(sa_family_t) + strlen(sun_path) + 1, and sun_path contains the null-terminated pathname.

*

unnamed: A stream socket that has not been bound to a pathname using bind(2) has no name. Likewise, the two sockets created by socketpair(2) are unnamed. When the address of an unnamed socket is returned by getsockname(2), getpeername(2), and accept(2), its length is sizeof(sa_family_t), and sun_path should not be inspected.

*

abstract: an abstract socket address is distinguished by the fact that sun_path[0] is a null byte ('\0'). All of the remaining bytes in sun_path define the "name" of the socket. (Null bytes in the name have no special significance.) The name has no connection with file system pathnames. The socket's address in this namespace is given by the rest of the bytes in sun_path. When the address of an abstract socket is returned by getsockname(2), getpeername(2), and accept(2), its length is sizeof(struct sockaddr_un), and sun_path contains the abstract name. 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.

SO_PASSCRED

Enables the receiving of the credentials of the sending process ancillary message. When this option is set and the socket is not yet connected a unique name in the abstract namespace will be generated automatically. Expects an integer boolean flag.

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).

SCM_RIGHTS

Send or receive a set of open file descriptors from another process. The data portion contains an integer array of the file descriptors. The passed file descriptors behave as though they have been created with dup(2).

SCM_CREDENTIALS

Send or receive Unix credentials. This can be used for authentication. The credentials are passed as a struct ucred ancillary message. Thus structure is defined in <sys/socket.h> as follows:

struct ucred {
    pid_t pid;    /* process ID of the sending process */
    uid_t uid;    /* user ID of the sending process */
    gid_t gid;    /* group ID of the sending process */
};

Since glibc 2.8, the _GNU_SOURCE feature test macro must be defined (before including any header files) in order to obtain the definition of this structure.

The credentials which the sender specifies are checked by the kernel. A process with effective user ID 0 is allowed to specify values that do not match its own. The sender must specify its own process ID (unless it has the capability CAP_SYS_ADMIN), its user ID, effective user ID, or saved set-user-ID (unless it has CAP_SETUID), and its group ID, effective group ID, or saved set-group-ID (unless it has CAP_SETGID). To receive a struct ucred message the SO_PASSCRED option must be enabled on the socket.

Ioctls

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

int value;
error = ioctl(tcp_socket, ioctl_type, &value);

ioctl_type can be:

SIOCINQ

Returns the amount of queued unread data in the receive buffer. The socket must not be in LISTEN state, otherwise an error (EINVAL) is returned. SIOCINQ is defined in <linux/sockios.h>. Alternatively, you can use the synonymous FIONREAD, defined in <sys/ioctl.h>.

VERSIONS

SCM_CREDENTIALS and the abstract namespace were introduced with Linux 2.2 and should not be used in portable programs. (Some BSD-derived systems also support credential passing, but the implementation details differ.)

NOTES

In the Linux implementation, sockets which are visible in the file system honor the permissions of the directory they are in. Their owner, group and their permissions can be changed. Creation of a new socket will fail if the process does not have write and search (execute) permission on the directory the socket is created in. Connecting to the socket object requires read/write permission. This behavior differs from many BSD-derived systems which ignore permissions for Unix sockets. Portable programs should not rely on this feature for security.

Binding to a socket with a filename creates a socket in the file system that must be deleted by the caller when it is no longer needed (using unlink(2)). The usual Unix close-behind semantics apply; the socket can be unlinked at any time and will be finally removed from the file system when the last reference to it is closed.

To pass file descriptors or credentials over a SOCK_STREAM, you need to send or receive at least one byte of nonancillary data in the same sendmsg(2) or recvmsg(2) call.

Unix domain stream sockets do not support the notion of out-of-band data.

EXAMPLE

See bind(2).

COLOPHON

This page is part of release 3.27 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.