xref: /netbsd/src/share/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;
.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.
.Pp
.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 .
Supported file descriptors may be sent in control messages of type
.Dv SCM_RIGHTS
holding an array of
.Vt int
file descriptor objects; see
.Xr cmsg 3
for details on assembling and examining control messages.
.Pp
A file descriptor received this way 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 the following socket options for use with
.Xr setsockopt 2
and
.Xr getsockopt 2
at the level
.Dv SOL_LOCAL :
.Bl -tag -width 6n
.It Dv LOCAL_CONNWAIT Pq Vt int
May be enabled with
.Xr setsockopt 2
on a
.Dv SOCK_SEQPACKET
or
.Dv SOCK_STREAM
socket.
If enabled,
.Xr connect 2
will block until a peer is waiting in
.Xr accept 2 .
.It Dv LOCAL_CREDS Pq Vt int
May be enabled with
.Xr setsockopt 2
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
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
.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
.It Dv LOCAL_PEEREID Pq Vt "struct unpcbid"
May be queried 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
.Pp
as defined in
.In sys/un.h .
.El
.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
.Bx 4.4 .
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 .