.\" $NetBSD: unix.4,v 1.25.8.1 2019/09/02 16:39:20 martin Exp $ .\" .\" Copyright (c) 1991, 1993 .\" The Regents of the University of California. All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" 3. Neither the name of the University nor the names of its contributors .\" may be used to endorse or promote products derived from this software .\" without specific prior written permission. .\" .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .\" @(#)unix.4 8.1 (Berkeley) 6/9/93 .\" .Dd March 31, 2016 .Dt UNIX 4 .Os .Sh NAME .Nm unix .Nd UNIX-domain protocol family .Sh SYNOPSIS .In sys/types.h .In sys/un.h .Sh DESCRIPTION The .Tn UNIX Ns -domain protocol family is a collection of protocols that provides local (on-machine) interprocess communication through the normal .Xr socket 2 mechanisms. The .Tn UNIX Ns -domain family supports the .Dv SOCK_STREAM , .Dv SOCK_SEQPACKET , and .Dv SOCK_DGRAM socket types and uses filesystem pathnames for addressing. .Sh ADDRESSING .Tn UNIX Ns -domain addresses are variable-length filesystem pathnames of at most 104 characters. The include file .In sys/un.h defines this address: .Bd -literal -offset indent struct sockaddr_un { u_char sun_len; u_char sun_family; char sun_path[104]; }; .Ed .Pp Binding a name to a .Tn UNIX Ns -domain socket with .Xr bind 2 causes a socket file to be created in the filesystem. This file is .Em not removed when the socket is closed\(em\c .Xr unlink 2 must be used to remove the file. .Pp The length of .Tn UNIX Ns -domain address, required by .Xr bind 2 and .Xr connect 2 , can be calculated by the macro .Fn SUN_LEN defined in .In sys/un.h . The .Ar sun_path field must be terminated by a NUL character to be used with .Fn SUN_LEN , but the terminating NUL is .Em not part of the address. The .Nx kernel ignores any user-set value in the .Va sun_len member of the structure. .Pp The .Tn UNIX Ns -domain protocol family does not support broadcast addressing or any form of .Dq wildcard matching on incoming messages. All addresses are absolute- or relative-pathnames of other .Tn UNIX Ns -domain sockets. Normal filesystem access-control mechanisms are also applied when referencing pathnames; e.g., the destination of a .Xr connect 2 or .Xr sendto 2 must be writable. .Sh PROTOCOLS The .Tn UNIX Ns -domain protocol family comprises simple transport protocols that support the .Dv SOCK_STREAM , .Dv SOCK_SEQPACKET , and .Dv SOCK_DGRAM abstractions. .Dv SOCK_STREAM and .Dv SOCK_SEQPACKET sockets also support the communication of .Ux file descriptors through the use of the .Ar msg_control field in the .Ar msg argument to .Xr sendmsg 2 and .Xr recvmsg 2 . .Pp Any valid descriptor may be sent in a message. The file descriptor(s) to be passed are described using a .Ar struct cmsghdr that is defined in the include file .In sys/socket.h . The type of the message is .Dv 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. .Pp The received descriptor is a .Em duplicate of the sender's descriptor, as if it were created with a call to .Xr dup 2 . Per-process descriptor flags, set with .Xr fcntl 2 , are .Em 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. .Pp A UNIX-domain socket supports two .Tn socket-level options for use with .Xr setsockopt 2 and .Xr getsockopt 2 : .Pp The .Dv LOCAL_CREDS option may be enabled on a .Dv SOCK_DGRAM , .Dv SOCK_SEQPACKET , or a .Dv SOCK_STREAM socket. This option provides a mechanism for the receiver to receive the credentials of the process as a .Xr 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 .In sys/socket.h as follows: .Bd -literal struct sockcred { pid_t sc_pid; /* process id */ 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 */ }; .Ed .Pp The .Dv LOCAL_PEEREID option may be used with .Xr getsockopt 2 to get the PID and effective user and group IDs of a .Dv SOCK_STREAM or .Dv SOCK_SEQPACKET peer when it did .Xr connect 2 or .Xr bind 2 . The returned structure is .Bd -literal struct unpcbid { pid_t unp_pid; /* process id */ uid_t unp_euid; /* effective user id */ gid_t unp_egid; /* effective group id */ }; .Ed as defined in .In sys/un.h . .Pp The .Fn SOCKCREDSIZE macro computes the size of the sockcred structure for a specified number of groups. The cmsghdr fields have the following values: .Bd -literal cmsg_len = CMSG_LEN(SOCKCREDSIZE(ngroups)) cmsg_level = SOL_SOCKET cmsg_type = SCM_CREDS .Ed .Sh EXAMPLES The following code fragment shows how to bind a socket to pathname: .Bd -literal -offset indent const char *pathname = "/path/to/socket"; struct sockaddr_un addr; int ret; memset(&addr, 0, sizeof(addr)); addr.sun_family = AF_LOCAL; if (strlen(pathname) \*[Ge] sizeof(addr.sun_path)) goto too_long; strncpy(addr.sun_path, pathname, sizeof(addr.sun_path)); ret = bind(s, (const struct sockaddr *)&addr, SUN_LEN(&addr)); if (ret != 0) goto bind_failed; \&... .Ed .Sh COMPATIBILITY The .Ar sun_len field exists only in system derived from 4.4BSD. On systems which don't have the .Fn SUN_LEN macro, the following definition is recommended: .Bd -literal -offset indent #ifndef SUN_LEN #define SUN_LEN(su) sizeof(struct(sockaddr_un)) #endif .Ed .Sh SEE ALSO .Xr socket 2 , .Xr CMSG_DATA 3 , .Xr intro 4 .Rs .%T "An Introductory 4.4BSD Interprocess Communication Tutorial" .%A Stuart Sechrest .Re .Pq see Pa /usr/share/doc/psd/20.ipctut .Rs .%T "Advanced 4.4BSD IPC Tutorial" .%A Samuel J. Leffler .%A Robert S. Fabry .%A William N. Joy .%A Phil Lapsley .%A Steve Miller .%A Chris Torek .Re .Pq see Pa /usr/share/doc/psd/21.ipc .Sh HISTORY The .Ar sc_pid field was introduced in .Nx 8.0 .