UNIX(4)

NAME
unix - UNIX-domain protocol family

SYNOPSIS
#include <sys/types.h>
#include <sys/un.h>

DESCRIPTION
The UNIX-domain protocol family is a collection of protocols that provides local (on-machine) interprocess communication through the normal socket(2) mechanisms. The UNIX-domain family supports the SOCK_STREAM, SOCK_SEQPACKET, and SOCK_DGRAM socket types and uses filesystem pathnames for addressing.

ADDRESSING
UNIX-domain addresses are variable-length filesystem pathnames of at most 104 characters. The include file <sys/un.h> defines this address:

    struct sockaddr_un {
            u_char  sun_len;
            u_char  sun_family;
            char    sun_path[104];
    };

Binding a name to a UNIX-domain socket with bind(2) causes a socket file to be created in the filesystem. This file is not removed when the socket is closed—unlink(2) must be used to remove the file.

The length of UNIX-domain address, required by bind(2) and connect(2), can be calculated by the macro SUN_LEN() defined in <sys/un.h>. The sun_path field must be terminated by a NUL character to be used with SUN_LEN(), but the terminating NUL is not part of the address. The NetBSD kernel ignores any user-set value in the sun_len member of the structure.

The UNIX-domain protocol family does not support broadcast addressing or any form of “wildcard” matching on incoming messages. All addresses are absolute- or relative-pathnames of other UNIX-domain sockets. Normal filesystem access-control mechanisms are also applied when referencing pathnames; e.g., the destination of a connect(2) or sendto(2) must be writable.

PROTOCOLS
The UNIX-domain protocol family comprises simple transport protocols that support the SOCK_STREAM, SOCK_SEQPACKET, and SOCK_DGRAM abstractions. SOCK_STREAM and SOCK_SEQPACKET sockets also support the communication of UNIX file descriptors through the use of the msg_control field in the msg argument to sendmsg(2) and recvmsg(2).

Any valid descriptor may be sent in a message. The file descriptor(s) to be passed are described using a struct cmsghdr that is defined in the include file <sys/socket.h>. The type of the message is SCM_RIGHTS, and the data portion of the messages is an array of integers representing the file descriptors to be passed. The number of descriptors being passed is defined by the length field of the message; the length field is the sum of the size of the header plus the size of the array of file descriptors.

The received descriptor is a duplicate of the sender's descriptor, as if it were created with a call to dup(2). Per-process descriptor flags, set with fcntl(2), are not passed to a receiver. Descriptors that are awaiting delivery, or that are purposely not received, are automatically closed by the system when the destination socket is closed.

A UNIX-domain socket supports several SOL_LOCAL level options for use with setsockopt(2) and getsockopt(2):

The LOCAL_CREDS option may be enabled on a SOCK_DGRAM, SOCK_SEQPACKET, or a SOCK_STREAM socket. This option provides a mechanism for the receiver to receive the credentials of the process as a recvmsg(2) control message. The msg_control field in the msghdr structure points to a buffer that contains a cmsghdr structure followed by a variable length sockcred structure, defined in <sys/socket.h> as follows:

    struct sockcred {
            pid_t   sc_pid;         /* process id */
            uid_t   sc_uid;         /* real user id */
            uid_t   sc_euid;        /* effective user id */
            gid_t   sc_gid;         /* real group id */
            gid_t   sc_egid;        /* effective group id */
            int     sc_ngroups;     /* number of supplemental groups */
            gid_t   sc_groups[1];   /* variable length */
    };

The LOCAL_PEEREID option may be used with getsockopt(2) to get the PID and effective user and group IDs of a SOCK_STREAM or SOCK_SEQPACKET peer when it did connect(2) or bind(2). The returned structure is

    struct unpcbid {
            pid_t unp_pid;          /* process id */
            uid_t unp_euid;         /* effective user id */
            gid_t unp_egid;         /* effective group id */
    };

as defined in <sys/un.h>.

The SOCKCREDSIZE() macro computes the size of the sockcred structure for a specified number of groups. The cmsghdr fields have the following values:

    cmsg_len = CMSG_LEN(SOCKCREDSIZE(ngroups))
    cmsg_level = SOL_SOCKET
    cmsg_type = SCM_CREDS

EXAMPLES
The following code fragment shows how to bind a socket to pathname:

    const char *pathname = "/path/to/socket";
    struct sockaddr_un addr;
    int ret;
    memset(&addr, 0, sizeof(addr));
    addr.sun_family = AF_LOCAL;
    if (strlen(pathname) >= sizeof(addr.sun_path))
            goto too_long;
    strncpy(addr.sun_path, pathname, sizeof(addr.sun_path));
    ret = bind(s, (const struct sockaddr *)&addr, SUN_LEN(&addr));
    if (ret != 0)
            goto bind_failed;
    ...

COMPATIBILITY
The sun_len field exists only in system derived from 4.4BSD. On systems which don't have the SUN_LEN() macro, the following definition is recommended:

    #ifndef SUN_LEN
    #define SUN_LEN(su)	sizeof(struct(sockaddr_un))
    #endif

SEE ALSO
socket(2), CMSG_DATA(3), intro(4) An Introductory 4.4BSD Interprocess Communication Tutorial, Stuart Sechrest. (see /usr/share/doc/reference/ref3/sockets) Advanced 4.4BSD IPC Tutorial, Samuel J. Leffler, Robert S. Fabry, William N. Joy, Phil Lapsley, Steve Miller, Chris Torek. (see /usr/share/doc/reference/ref3/sockets-advanced)

HISTORY
The sc_pid field was introduced in NetBSD 8.0.