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UNIX(4)               DragonFly 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) interprocess communication through the normal
     socket(2) mechanisms.  The UNIX-domain family supports the SOCK_STREAM,
     SOCK_DGRAM and SOCK_SEQPACKET 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;
           sa_family_t     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
     protocols that support the SOCK_STREAM, SOCK_DGRAM and SOCK_SEQPACKET
     abstractions.  SOCK_STREAM, SOCK_DGRAM 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 via dup(fd) or fcntl(fd, F_DUPFD_CLOEXEC, 0) depending on
     whether MSG_CMSG_CLOEXEC is passed in the recvmsg(2) call.  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.


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.

DragonFly 5.9-DEVELOPMENT      November 1, 2015      DragonFly 5.9-DEVELOPMENT

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