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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 provides local (on-machine) interpro-cess interprocess
     cess 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 filesys-tem. filesystem.
     tem.  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 protocols that support the SOCK_STREAM
     and SOCK_DGRAM abstractions.  SOCK_STREAM sockets also support the communication of UNIX file descrip-tors descriptors
     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 descrip-tors descriptors
     tors to be passed.  The number of descriptors being passed is defined by the length field of the mes-sage; message;
     sage; the length field is the sum of the size of the header plus the size of the array of file descrip-tors. descriptors.
     tors.

     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) 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 credentials
     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