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         #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 file system pathnames for address-


         UNIX-domain addresses are variable-length file system pathnames of at
         most 104 characters.  The include file #include <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 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 proto-
         cols that support the SOCK_STREAM and SOCK_DGRAM abstractions.
         SOCK_STREAM sockets also support the communication of UNIX file descrip-
         tors 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 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
         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 #include <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.

    BSD July 15, 2001 BSD


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