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UNIX(4)                      Device Drivers 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,
     SOCK_SEQPACKET, 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, 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

           cmsg_len = CMSG_LEN(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

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

     socket(2), CMSG_DATA(3), intro(4)

     Stuart Sechrest, An Introductory 4.4BSD Interprocess Communication
     Tutorial.  (see /usr/share/doc/reference/ref3/sockets)

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

     The sc_pid field was introduced in NetBSD 8.0.

NetBSD 10.99                     June 28, 2022                    NetBSD 10.99