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