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

NAME
     unix -- UNIX-domain protocol family

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

DESCRIPTION
     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.

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 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
     writable.

PROTOCOLS
     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 <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.

     The effective credentials (i.e., the user ID and group list) the 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 creden-
     tials presented to its peer except by calling the appropriate system call
     (e.g., connect(2) or listen(2)) under different effective credentials.

SEE ALSO
     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                              June 9, 1993                              BSD