man-pages/man7/unix.7

.\" This man page is Copyright (C) 1999 Andi Kleen <ak@muc.de>.
.\" Permission is granted to distribute possibly modified copies
.\" of this page provided the header is included verbatim,
.\" and in case of nontrivial modification author and date
.\" of the modification is added to the header.
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.\" Modified, 2003-12-02, Michael Kerrisk, <mtk-manpages@gmx.net>
.\" Modified, 2003-09-23, Adam Langley
.\" Modified, 2004-05-27, Michael Kerrisk, <mtk-manpages@gmx.net>
.\"	Added SOCK_SEQPACKET
.\"
.TH UNIX  7 2004-05-27 "Linux Man Page" "Linux Programmer's Manual" 
.SH NAME
unix, PF_UNIX, AF_UNIX, PF_LOCAL, AF_LOCAL \- Sockets for local interprocess communication
.SH SYNOPSIS
.B #include <sys/socket.h>
.br
.B #include <sys/un.h>

.IB unix_socket " = socket(PF_UNIX, type, 0);"
.br
.IB error " = socketpair(PF_UNIX, type, 0, int *" sv ");"

.SH DESCRIPTION
The
.B PF_UNIX
(also known as
.BR PF_LOCAL )
socket family is used to communicate between processes on the same machine
efficiently. Unix sockets can be either anonymous (created by 
.BR socketpair (2))
or associated with a file of type socket. 
Linux also supports an abstract namespace which is independent of the
file system.

Valid types are:
.BR SOCK_STREAM ,
for a stream-oriented socket and
.BR SOCK_DGRAM ,
for a datagram-oriented socket that preserves message boundaries
(as on most Unix implementations, Unix domain datagram
sockets are always reliable and don't reorder datagrams);
and (since kernel 2.6.4)
.BR SOCK_SEQPACKET ,
for a connection-oriented socket that preserves message boundaries
and delivers messages in the order that they were sent. 

Unix sockets support passing file descriptors or process credentials to other
processes using ancillary data.

.SH "ADDRESS FORMAT"
A Unix address is defined as a filename in the filesystem or 
as a unique string in the abstract namespace. Sockets created by 
.BR socketpair (2)
are anonymous. For non-anonymous sockets the target address can be set 
using
.BR connect (2). 
The local address can be set using
.BR bind (2). 
When a socket is connected and it doesn't already have a local address a
unique address in the abstract namespace will be generated automatically. 

.RS
.nf
#define UNIX_PATH_MAX	108

.ta 4n 17n 42n
struct sockaddr_un {
	sa_family_t	sun_family;	/* AF_UNIX */
	char	sun_path[UNIX_PATH_MAX];	/* pathname */
};
.fi
.RE 

.B sun_family 
always contains
.BR AF_UNIX .
.B sun_path
contains the zero-terminated pathname of the socket in the file system.
If 
.B sun_path
starts with a zero byte it refers to the abstract namespace maintained by
the Unix protocol module.
The socket's address in this namespace is given by the rest of the bytes in
.BR sun_path .
Note that names in the abstract namespace are not zero-terminated.

.SH "SOCKET OPTIONS"
For historical reasons these socket options are specified with a 
SOL_SOCKET type even though they are PF_UNIX specific.
They can be set with 
.BR setsockopt (2)
and read with 
.BR getsockopt (2)
by specifying SOL_SOCKET as the socket family.
.TP
.B SO_PASSCRED
Enables the receiving of the credentials of the sending process 
ancillary message. When this option is set and the socket is not yet connected
a unique name in the abstract namespace will be generated automatically.
Expects an integer boolean flag. 

.SH "ANCILLARY MESSAGES"
Ancillary data is sent and received using
.BR sendmsg (2)
and
.BR recvmsg (2).
For historical reasons the ancillary message types listed below
are specified with a SOL_SOCKET type even though they are PF_UNIX specific.
To send them set the
.B cmsg_level
field of the struct 
.B cmsghdr
to SOL_SOCKET and the 
.B cmsg_type 
field to the type. For more information see 
.BR cmsg (3). 

.TP
.B SCM_RIGHTS
Send or receive a set of open file descriptors from another process. 
The data portion contains an integer array of the file descriptors.
The passed file descriptors behave as though they have been created with
.BR dup (2).

.TP
.B SCM_CREDENTIALS
Send or receive Unix credentials.  This can be used for authentication.
The credentials are passed as a 
.B struct ucred
ancillary message.

.RS
.nf
.ta 4n 11n 17n
struct ucred {
	pid_t	pid;	/* process id of the sending process */  
	uid_t	uid;	/* user id of the sending process */ 
	gid_t	gid;	/* group id of the sending process */ 
};
.fi
.RE 
 
The credentials which the sender specifies are checked by the kernel.
A process with effective user ID 0 is allowed to specify values that do 
not match its own. 
The sender must specify its own process ID (unless it has the capability
.BR CAP_SYS_ADMIN ),
its user ID, effective user ID or set user ID (unless it has
.BR CAP_SETUID ),
and its group id, effective group ID or set group ID (unless it has
.BR CAP_SETGID ).
To receive a
.B struct ucred
message the
.B SO_PASSCRED 
option must be enabled on the socket.

.SH VERSIONS
.B SCM_CREDENTIALS 
and the abstract namespace were introduced with Linux 2.2 and should not
be used in portable programs.
(Some BSD-derived systems also support credential passing,
but the implementation details differ.)

.SH NOTES
In the Linux implementation, sockets which are visible in the
filesystem honour the permissions of the directory they are in. Their
owner, group and their permissions can be changed.
Creation of a new socket will fail if the process does not have write and
search (execute) permission on the directory the socket is created in.
Connecting to the socket object requires read/write permission.
This behavior differs from many BSD-derived systems which
ignore permissions for Unix sockets. Portable programs should not rely on
this feature for security.

Binding to a socket with a filename creates a socket
in the file system that must be deleted by the caller when it is no
longer needed (using
.BR unlink (2)).
The usual Unix close-behind semantics apply; the socket can be unlinked
at any time and will be finally removed from the file system when the last 
reference to it is closed.

To pass file descriptors or credentials over a SOCK_STREAM, you need
to send/recv at least one byte of non-ancillary data in the same
send/recv_msg call.

Unix domain stream sockets do not support the notion of out-of-band data.
.SH ERRORS
.TP
.B ENOMEM
Out of memory.
.TP
.B ECONNREFUSED
.BR connect (2)
called with a socket object that isn't listening. This can happen when
the remote socket does not exist or the filename is not a socket.
.TP
.B EINVAL
Invalid argument passed. A common cause is the missing setting of AF_UNIX
in the sun_type field of passed addresses or the socket being in an invalid
state for the applied operation.
.TP
.B EOPNOTSUPP
Stream operation called on non-stream oriented socket or tried to 
use the out-of-band data option.
.TP
.B EPROTONOSUPPORT
Passed protocol is not PF_UNIX.
.TP
.B ESOCKTNOSUPPORT
Unknown socket type.
.TP 
.B EPROTOTYPE
Remote socket does not match the local socket type (SOCK_DGRAM vs.
SOCK_STREAM)
.TP
.B EADDRINUSE
Selected local address is already taken or filesystem socket object already
exists. 
.TP
.B EISCONN
.BR connect (2)
called on an already connected socket or a target address was
specified on a connected socket.
.TP
.B ENOTCONN
Socket operation needs a target address, but the socket is not connected.
.TP
.B ECONNRESET
Remote socket was unexpectedly closed.
.TP
.B EPIPE
Remote socket was closed on a stream socket. If enabled, a 
.B SIGPIPE 
is sent as well. This can be avoided by passing the 
.B MSG_NOSIGNAL
flag to
.BR sendmsg (2)
or
.BR recvmsg (2).
.TP
.B EFAULT
User memory address was not valid.
.TP
.B EPERM
The sender passed invalid credentials in the
.BR "struct ucred" .
.PP
Other errors can be generated by the generic socket layer or 
by the filesystem while generating a filesystem socket object. See
the appropriate manual pages for more information. 
.SH "SEE ALSO"
.BR recvmsg (2),
.BR sendmsg (2),
.BR socket (2),
.BR socketpair (2),
.BR cmsg (3),
.BR capabilities (7),
.BR socket (7)