.\" 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
.\" address that can appear in the sockaddr_un structure: pathname,
.\" unnamed, and abstract.
.\"
-.TH UNIX 7 2012-05-10 "Linux" "Linux Programmer's Manual"
+.TH UNIX 7 2014-12-31 "Linux" "Linux Programmer's Manual"
.SH NAME
unix \- sockets for local interprocess communication
.SH SYNOPSIS
Linux also supports an abstract namespace which is independent of the
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
.fi
.in
.PP
+The
.I sun_family
-always contains
+field always contains
.BR AF_UNIX .
-Three types of address are distinguished in this structure:
+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))
+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 the socket is returned by
-.BR getsockname (2),
-.BR getpeername (2),
-and
-.BR accept (2),
+When the address of a pathname socket is returned
+(by one of the system calls noted above),
its length is
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 (\(aq\\0\(aq).
The socket's address in this namespace is given by the additional
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 filesystem pathnames.
-When the address of an abstract socket is returned by
-.BR getsockname (2),
-.BR getpeername (2),
-and
-.BR accept (2),
+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 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
+For historical reasons, these socket options are specified with a
.B SOL_SOCKET
type even though they are
.B AF_UNIX
.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
The remote address specified by
.BR connect (2)
was not a listening socket.
-This error can also occur if the target filename is not a socket.
+This error can also occur if the target pathname is not a socket.
.TP
.B ECONNRESET
Remote socket was unexpectedly closed.
.SH NOTES
In the Linux implementation, sockets which are visible in the
filesystem honor the permissions of the directory they are in.
-Their owner, group and their permissions can be changed.
+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.
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).
.BR credentials (7),
.BR socket (7)
.SH COLOPHON
-This page is part of release 3.67 of the Linux
+This page is part of release 3.79 of the Linux
.I man-pages
project.
A description of the project,
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