UNIX(4)                   BSD Kernel Interfaces Manual                   UNIX(4)

     unix — UNIX-domain protocol family

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

     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 and SOCK_DGRAM
     socket types and uses file system pathnames for addressing.

     UNIX-domain addresses are variable-length file system 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 file system.  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 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 file
     system access-control mechanisms are also applied when referencing
     pathnames; e.g., the destination of a connect(2) or sendto(2) must be

     The UNIX-domain protocol family is comprised of simple transport protocols
     that support the SOCK_STREAM and SOCK_DGRAM abstractions.  SOCK_STREAM
     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

     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.

     The effective credentials (i.e., the user ID and group list) of a peer on a
     SOCK_STREAM socket may be obtained using the LOCAL_PEERCRED socket option.
     This may be used by a server to obtain and verify the credentials of its
     client, and vice versa by the client to verify the credentials of the
     server.  These will arrive in the form of a filled in struct xucred
     (defined in <sys/ucred.h>).  The credentials presented to the server (the
     listen(2) caller) are those of the client when it called connect(2); the
     credentials presented to the client (the connect(2) caller) are those of
     the server when it called listen(2).  This mechanism is reliable; there is
     no way for either party to influence the credentials presented to its peer
     except by calling the appropriate system call (e.g., connect(2) or
     listen(2)) under different effective credentials.

     UNIX domain sockets support a number of socket options which can be set
     with setsockopt(2) and tested with getsockopt(2):

     LOCAL_CREDS     This option may be enabled on a SOCK_DGRAM 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 {
                       id_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 SOCKCREDSIZE() macro computes the size of the sockcred
                     structure for a specified number of groups.  The cmsghdr
                     fields have the following values:

                     cmsg_len = sizeof(struct cmsghdr) + SOCKCREDSIZE(ngroups)
                     cmsg_level = SOL_SOCKET
                     cmsg_type = SCM_CREDS

     LOCAL_CONNWAIT  Used with SOCK_STREAM sockets, this option causes the
                     connect(2) function to block until accept(2) has been
                     called on the listening socket.

     socket(2), intro(4)

     “An Introductory 4.3 BSD Interprocess Communication Tutorial”, PS1, 7.

     “An Advanced 4.3 BSD Interprocess Communication Tutorial”, PS1, 8.

BSD                               July 15, 2001                              BSD