UNIX(4)                 NetBSD 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 pro-
     vides 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 filesystem pathnames for 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

     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

     The UNIX-domain protocol family comprises 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 await-
     ing delivery, or that are purposely not received, are automatically
     closed by the system when the destination socket is closed.

     There are two socket-level setsockopt(2) /getsockopt(2) option available
     in the unix domain:

     The LOCAL_CREDS 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_con-
     trol 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 {
             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 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 val-

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

     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;

     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 recom-

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

     socket(2), intro(4)

     Stuart Sechrest, An Introductory 4.4BSD Interprocess Communication
     Tutorial.  (see /usr/share/doc/psd/20.ipctut)

     Samuel J. Leffler, Robert S. Fabry, William N. Joy, Phil Lapsley, Steve
     Miller, and Chris Torek, Advanced 4.4BSD IPC Tutorial.  (see

NetBSD 5.0.1                   October 30, 2006                   NetBSD 5.0.1

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