.\" This man page is Copyright (C) 1999 Andi Kleen <ak@muc.de>.
+.\" and Copyright (C) 2008-2014, Michael Kerrisk <mtk.manpages@gmail.com>
+.\"
+.\" %%%LICENSE_START(VERBATIM_ONE_PARA)
+.\" and Copyright (C) 2008, 2012 Michael Kerrisk <mtk.manpages@gmail.com>
.\" 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.
+.\" %%%LICENSE_END
.\"
.\" Modified, 2003-12-02, Michael Kerrisk, <mtk.manpages@gmail.com>
.\" Modified, 2003-09-23, Adam Langley
.\" address that can appear in the sockaddr_un structure: pathname,
.\" unnamed, and abstract.
.\"
-.TH UNIX 7 2010-10-10 "Linux" "Linux Programmer's Manual"
+.TH UNIX 7 2014-12-31 "Linux" "Linux Programmer's Manual"
.SH NAME
-unix, AF_UNIX, AF_LOCAL \- Sockets for local
-interprocess communication
+unix \- sockets for local interprocess communication
.SH SYNOPSIS
.B #include <sys/socket.h>
.br
socket family is used to communicate between processes on the same machine
efficiently.
Traditionally, UNIX domain sockets can be either unnamed,
-or bound to a file system pathname (marked as being of type socket).
+or bound to a filesystem pathname (marked as being of type socket).
Linux also supports an abstract namespace which is independent of the
-file system.
+filesystem.
-Valid types are:
+Valid socket types in the UNIX domain are:
.BR SOCK_STREAM ,
-for a stream-oriented socket and
+for a stream-oriented socket;
.BR SOCK_DGRAM ,
for a datagram-oriented socket that preserves message boundaries
(as on most UNIX implementations, UNIX domain datagram
UNIX domain sockets support passing file descriptors or process credentials
to other processes using ancillary data.
-.SS Address Format
+.SS Address format
A UNIX domain socket address is represented in the following structure:
.in +4n
.nf
.fi
.in
.PP
+The
.I sun_family
-always contains
+field always contains
.BR AF_UNIX .
-Three types of address are distinguished in this structure:
-.IP * 3
-.IR pathname :
-a UNIX domain socket can be bound to a null-terminated file
-system pathname using
-.BR bind (2).
-When the address of the socket is returned by
+Various systems calls (for example,
+.BR bind (2),
+.BR connect (2),
+and
+.BR sendto (2))
+take a
+.I sockaddr_un
+argument as input.
+Some other system calls (for example,
.BR getsockname (2),
.BR getpeername (2),
+.BR recvfrom (2),
and
-.BR accept (2),
+.BR accept (2))
+return an argument of this type.
+
+Three types of address are distinguished in the
+.I sockaddr_un
+structure:
+.IP * 3
+.IR pathname :
+a UNIX domain socket can be bound to a null-terminated
+filesystem pathname using
+.BR bind (2).
+When the address of a pathname socket is returned
+(by one of the system calls noted above),
its length is
-.IR "offsetof(struct sockaddr_un, sun_path) + strlen(sun_path) + 1" ,
+
+ offsetof(struct sockaddr_un, sun_path) + strlen(sun_path) + 1
+
and
.I sun_path
contains the null-terminated pathname.
+(On Linux, the above
+.BR offsetof ()
+expression equates to the same value as
+.IR sizeof(sa_family_t) ,
+but some other implementations include other fields before
+.IR sun_path ,
+so the
+.BR offsetof ()
+expression more portably describes the size of the address structure.)
+.IP
+For further details of pathname sockets, see below.
.IP *
.IR unnamed :
A stream socket that has not been bound to a pathname using
Likewise, the two sockets created by
.BR socketpair (2)
are unnamed.
-When the address of an unnamed socket is returned by
-.BR getsockname (2),
-.BR getpeername (2),
-and
-.BR accept (2),
+When the address of an unnamed socket is returned,
its length is
.IR "sizeof(sa_family_t)" ,
and
.\" says the length is 16 bytes, HP-UX 11 says it's zero bytes.
.IP *
.IR abstract :
-an abstract socket address is distinguished by the fact that
+an abstract socket address is distinguished (from a pathname socket)
+by the fact that
.IR sun_path[0]
-is a null byte ('\\0').
+is a null byte (\(aq\\0\(aq).
The socket's address in this namespace is given by the additional
bytes in
.IR sun_path
that are covered by the specified length of the address structure.
(Null bytes in the name have no special significance.)
-The name has no connection with file system pathnames.
-When the address of an abstract socket is returned by
-.BR getsockname (2),
-.BR getpeername (2),
-and
-.BR accept (2),
+The name has no connection with filesystem pathnames.
+When the address of an abstract socket is returned,
the returned
.I addrlen
is greater than
bytes of
.IR sun_path .
The abstract socket namespace is a nonportable Linux extension.
-.SS Socket Options
-For historical reasons these socket options are specified with a
+.SS Pathname sockets
+When binding a socket to a pathname, a few rules should be observed
+for maximum portability and ease of coding:
+.IP * 3
+The pathname in
+.I sun_path
+should be null-terminated.
+.IP *
+The length of the pathname, including the terminating null byte,
+should not exceed the size of
+.IR sun_path .
+.IP *
+The
+.I addrlen
+argument that describes the enclosing
+.I sockaddr_un
+structure should have a value of at least:
+
+.nf
+ offsetof(struct sockaddr_un, sun_path)+strlen(addr.sun_path)+1
+.fi
+.IP
+or, more simply,
+.I addrlen
+can be specified as
+.IR "sizeof(struct sockaddr_un)" .
+.PP
+There is some variation in how implementations handle UNIX domain
+socket addresses that do not follow the above rules.
+For example, some (but not all) implementations
+.\" Linux does this, including for the case where the supplied path
+.\" is 108 bytes
+append a null terminator if none is present in the supplied
+.IR sun_path .
+
+When coding portable applications,
+keep in mind that some implementations
+.\" HP-UX
+have
+.I sun_path
+as short as 92 bytes.
+.\" Modern BSDs generally have 104, Tru64 and AIX have 104,
+.\" Solaris and Irix have 108
+
+Various system calls
+.RB ( accept (2),
+.BR recvfrom (2),
+.BR getsockname (2),
+.BR getpeername (2))
+return socket address structures.
+When applied to UNIX domain sockets, the value-result
+.I addrlen
+argument supplied to the call should be initialized as above.
+Upon return, the argument is set to indicate the
+.I actual
+size of the address structure.
+The caller should check the value returned in this argument:
+if the output value exceeds the input value,
+then there is no guarantee that a null terminator is present in
+.IR sun_path .
+(See BUGS.)
+.SS Socket options
+For historical reasons, these socket options are specified with a
.B SOL_SOCKET
type even though they are
.B AF_UNIX
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.
-.SS Autobind Feature
+.SS Autobind feature
If a
-.BR bind ()
+.BR bind (2)
call specifies
.I addrlen
as
.IR sizeof(sa_family_t) ,
-.\" i.e. sizeof(short)
+.\" i.e., sizeof(short)
or the
.BR SO_PASSCRED
socket option was specified for a socket that was
The address consists of a null byte
followed by 5 bytes in the character set
.IR [0-9a-f] .
-(Thus, there is a limit of 2^20 autobind addresses.)
+Thus, there is a limit of 2^20 autobind addresses.
+(From Linux 2.1.15, when the autobind feature was added,
+8 bytes were used, and the limit was thus 2^32 autobind addresses.
+The change to 5 bytes came in Linux 2.3.15.)
.SS Sockets API
The following paragraphs describe domain-specific details and
unsupported features of the sockets API for UNIX domain sockets on Linux.
This limit is calculated as the doubled (see
.BR socket (7))
option value less 32 bytes used for overhead.
-.SS Ancillary Messages
+.SS Ancillary messages
Ancillary data is sent and received using
.BR sendmsg (2)
and
.RS
.nf
.BI int " value";
-.IB error " = ioctl(" tcp_socket ", " ioctl_type ", &" value ");"
+.IB error " = ioctl(" unix_socket ", " ioctl_type ", &" value ");"
.fi
.RE
.PP
.B SIOCINQ
is defined in
.IR <linux/sockios.h> .
-.\" FIXME http://sources.redhat.com/bugzilla/show_bug.cgi?id=12002,
+.\" FIXME . http://sources.redhat.com/bugzilla/show_bug.cgi?id=12002,
.\" filed 2010-09-10, may cause SIOCINQ to be defined in glibc headers
Alternatively,
you can use the synonymous
.SH ERRORS
.TP
.B EADDRINUSE
-Selected local address is already taken or file system socket
+The specified local address is already in use or the filesystem socket
object already exists.
.TP
.B ECONNREFUSED
+The remote address specified by
.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.
+was not a listening socket.
+This error can also occur if the target pathname is not a socket.
.TP
.B ECONNRESET
Remote socket was unexpectedly closed.
.TP
.B EINVAL
Invalid argument passed.
-A common cause is the missing setting of AF_UNIX
-in the
+A common cause is that the value
+.B AF_UNIX
+was not specified in the
.I sun_type
-field of passed addresses or the socket being in an
+field of passed addresses, or the socket was in an
invalid state for the applied operation.
.TP
.B EISCONN
called on an already connected socket or a target address was
specified on a connected socket.
.TP
+.B ENOENT
+The pathname in the remote address specified to
+.BR connect (2)
+did not exist.
+.TP
.B ENOMEM
Out of memory.
.TP
.BR recvmsg (2).
.TP
.B EPROTONOSUPPORT
-Passed protocol is not AF_UNIX.
+Passed protocol is not
+.BR AF_UNIX .
.TP
.B EPROTOTYPE
Remote socket does not match the local socket type
.RB ( SOCK_DGRAM
-vs.
+versus
.BR SOCK_STREAM )
.TP
.B ESOCKTNOSUPPORT
Unknown socket type.
.PP
Other errors can be generated by the generic socket layer or
-by the file system while generating a file system socket object.
+by the filesystem while generating a filesystem socket object.
See the appropriate manual pages for more information.
.SH VERSIONS
.B SCM_CREDENTIALS
but the implementation details differ.)
.SH NOTES
In the Linux implementation, sockets which are visible in the
-file system honor the permissions of the directory they are in.
-Their owner, group and their permissions can be changed.
+filesystem honor the permissions of the directory they are in.
+Their owner, group, and 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 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
+in the filesystem 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
+at any time and will be finally removed from the filesystem when the last
reference to it is closed.
To pass file descriptors or credentials over a
call.
UNIX domain stream sockets do not support the notion of out-of-band data.
+.\"
+.SH BUGS
+When binding a socket to an address,
+Linux is one of the implementations that appends a null terminator
+if none is supplied in
+.IR sun_path .
+In most cases this is unproblematic:
+when the socket address is retrieved,
+it will be one byte longer than that supplied when the socket was bound.
+However, there is one case where confusing behavior can result:
+if 108 non-null bytes are supplied when a socket is bound,
+then the addition of the null terminator takes the length of
+the pathname beyond
+.IR sizeof(sun_path) .
+Consequently, when retrieving the socket address
+(for example, via
+.BR accept (2)),
+.\" The behavior on Solaris is quite similar.
+if the input
+.I addrlen
+argument for the retrieving call is specified as
+.IR "sizeof(struct sockaddr_un)" ,
+then the returned address structure
+.I won't
+have a null terminator in
+.IR sun_path .
+
+In addition, some implementations
+.\" i.e., traditional BSD
+don't require a null terminator when binding a socket (the
+.I addrlen
+argument is used to determine the length of
+.IR sun_path )
+and when the socket address is retrieved on these implementations,
+there is no null terminator in
+.IR sun_path .
+
+Applications that retrieve socket addresses can (portably) code
+to handle the possibility that there is no null terminator in
+.IR sun_path
+by respecting the fact that the number of valid bytes in the pathname is:
+
+ strnlen(addr.sun_path, addrlen \- offsetof(sockaddr_un, sun_path))
+.\" The following patch to amend kernel behavior was rejected:
+.\" http://thread.gmane.org/gmane.linux.kernel.api/2437
+.\" Subject: [patch] Fix handling of overlength pathname in AF_UNIX sun_path
+.\" 2012-04-17
+.\" And there was a related discussion in the Austin list:
+.\" http://thread.gmane.org/gmane.comp.standards.posix.austin.general/5735
+.\" Subject: Having a sun_path with no null terminator
+.\" 2012-04-18
+.\"
+.\" FIXME . Track http://austingroupbugs.net/view.php?id=561
+
+Alternatively, an application can retrieve
+the socket address by allocating a buffer of size
+.I "sizeof(struct sockaddr_un)+1"
+that is zeroed out before the retrieval.
+The retrieving call can specify
+.I addrlen
+as
+.IR "sizeof(struct sockaddr_un)" ,
+and the extra zero byte ensures that there will be
+a null terminator for the string returned in
+.IR sun_path :
+
+.nf
+.in +3
+void *addrp;
+
+addrlen = sizeof(struct sockaddr_un);
+addrp = malloc(addrlen + 1);
+if (addrp == NULL)
+ /* Handle error */ ;
+memset(addrp, 0, addrlen + 1);
+
+if (getsockname(sfd, (struct sockaddr *) addrp, &addrlen)) == \-1)
+ /* handle error */ ;
+
+printf("sun_path = %s\\n", ((struct sockaddr_un *) addrp)\->sun_path);
+.in
+.fi
+
+This sort of messiness can be avoided if it is guaranteed
+that the applications that
+.I create
+pathname sockets follow the rules outlined above under
+.IR "Pathname sockets" .
.SH EXAMPLE
See
.BR bind (2).
-.SH "SEE ALSO"
+
+For an example of the use of
+.BR SCM_RIGHTS
+see
+.BR cmsg (3).
+.SH SEE ALSO
.BR recvmsg (2),
.BR sendmsg (2),
.BR socket (2),
.BR capabilities (7),
.BR credentials (7),
.BR socket (7)
+.SH COLOPHON
+This page is part of release 3.79 of the Linux
+.I man-pages
+project.
+A description of the project,
+information about reporting bugs,
+and the latest version of this page,
+can be found at
+\%http://www.kernel.org/doc/man\-pages/.
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