man/man4/unix.4

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.\"     @(#)unix.4	8.1 (Berkeley) 6/9/93
.\"
.Dd June 28, 2022
.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
.Fa 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
.Fa 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
.Fa msg_control
field in the
.Fa 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
.Vt 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
.Ux Ns -domain
socket supports several
.Dv SOL_LOCAL
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
.Fa msg_control
field in the
.Vt msghdr
structure points
to a buffer that contains a
.Vt cmsghdr
structure followed by a variable
length
.Vt sockcred
structure, defined in
.In sys/socket.h
as follows:
.Bd -literal -offset indent
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 -offset indent
struct unpcbid {
        pid_t unp_pid;          /* process id */
        uid_t unp_euid;         /* effective user id */
        gid_t unp_egid;         /* effective group id */
};
.Ed
.Pp
as defined in
.In sys/un.h .
.Pp
The
.Fn SOCKCREDSIZE
macro computes the size of the
.Vt sockcred
structure for a specified number of groups.
The
.Vt cmsghdr
fields have the following values:
.Bd -literal -offset indent
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) >= 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
.Fa 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/reference/ref3/sockets
.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/reference/ref3/sockets-advanced
.Sh HISTORY
The
.Fa sc_pid
field was introduced in
.Nx 8.0 .