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     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 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.

     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 {
                       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 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

     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.