man-pages/man7/unix.7

.\" This man page is Copyright (C) 1999 Andi Kleen <ak@muc.de>,
.\" Copyright (C) 2008-2014, Michael Kerrisk <mtk.manpages@gmail.com>,
.\" and Copyright (C) 2016, Heinrich Schuchardt <xypron.glpk@gmx.de>
.\"
.\" %%%LICENSE_START(VERBATIM_ONE_PARA)
.\" Permission is granted to distribute possibly modified copies
.\" of this page provided the header is included verbatim,
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.\" of the modification is added to the header.
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.\" Modified, 2003-12-02, Michael Kerrisk, <mtk.manpages@gmail.com>
.\" Modified, 2003-09-23, Adam Langley
.\" Modified, 2004-05-27, Michael Kerrisk, <mtk.manpages@gmail.com>
.\"	Added SOCK_SEQPACKET
.\" 2008-05-27, mtk, Provide a clear description of the three types of
.\"     address that can appear in the sockaddr_un structure: pathname,
.\"     unnamed, and abstract.
.\"
.TH UNIX  7 2021-03-22 "Linux" "Linux Programmer's Manual"
.SH NAME
unix \- sockets for local interprocess communication
.SH SYNOPSIS
.nf
.B #include <sys/socket.h>
.B #include <sys/un.h>
.PP
.IB unix_socket " = socket(AF_UNIX, type, 0);"
.IB error " = socketpair(AF_UNIX, type, 0, int *" sv ");"
.fi
.SH DESCRIPTION
The
.B AF_UNIX
(also known as
.BR AF_LOCAL )
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 filesystem pathname (marked as being of type socket).
Linux also supports an abstract namespace which is independent of the
filesystem.
.PP
Valid socket types in the UNIX domain are:
.BR SOCK_STREAM ,
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
sockets are always reliable and don't reorder datagrams);
and (since Linux 2.6.4)
.BR SOCK_SEQPACKET ,
for a sequenced-packet socket that is connection-oriented,
preserves message boundaries,
and delivers messages in the order that they were sent.
.PP
UNIX domain sockets support passing file descriptors or process credentials
to other processes using ancillary data.
.SS Address format
A UNIX domain socket address is represented in the following structure:
.PP
.in +4n
.EX
.\" #define UNIX_PATH_MAX    108
.\"
struct sockaddr_un {
    sa_family_t sun_family;               /* AF_UNIX */
    char        sun_path[108];            /* Pathname */
};
.EE
.in
.PP
The
.I sun_family
field always contains
.BR AF_UNIX .
On Linux,
.I sun_path
is 108 bytes in size; see also NOTES, below.
.PP
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.
.PP
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
.IP
    offsetof(struct sockaddr_un, sun_path) + strlen(sun_path) + 1
.IP
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
.BR bind (2)
has no name.
Likewise, the two sockets created by
.BR socketpair (2)
are unnamed.
When the address of an unnamed socket is returned,
its length is
.IR "sizeof(sa_family_t)" ,
and
.I sun_path
should not be inspected.
.\" There is quite some variation across implementations: FreeBSD
.\" says the length is 16 bytes, HP-UX 11 says it's zero bytes.
.IP *
.IR abstract :
an abstract socket address is distinguished (from a pathname socket)
by the fact that
.IR sun_path[0]
is a null byte (\(aq\e0\(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 filesystem pathnames.
When the address of an abstract socket is returned,
the returned
.I addrlen
is greater than
.IR "sizeof(sa_family_t)"
(i.e., greater than 2), and the name of the socket is contained in
the first
.IR "(addrlen \- sizeof(sa_family_t))"
bytes of
.IR sun_path .
.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:
.IP
.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 .
.PP
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
.PP
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 Pathname socket ownership and permissions
In the Linux implementation,
pathname sockets honor the permissions of the directory they are in.
Creation of a new socket fails if the process does not have write and
search (execute) permission on the directory in which the socket is created.
.PP
On Linux,
connecting to a stream socket object requires write permission on that socket;
sending a datagram to a datagram socket likewise
requires write permission on that socket.
POSIX does not make any statement about the effect of the permissions
on a socket file, and on some systems (e.g., older BSDs),
the socket permissions are ignored.
Portable programs should not rely on
this feature for security.
.PP
When creating a new socket, the owner and group of the socket file
are set according to the usual rules.
The socket file has all permissions enabled,
other than those that are turned off by the process
.BR umask (2).
.PP
The owner, group, and permissions of a pathname socket can be changed (using
.BR chown (2)
and
.BR chmod (2)).
.\" However, fchown() and fchmod() do not seem to have an effect
.\"
.SS Abstract sockets
Socket permissions have no meaning for abstract sockets:
the process
.BR umask (2)
has no effect when binding an abstract socket,
and changing the ownership and permissions of the object (via
.BR fchown (2)
and
.BR fchmod (2))
has no effect on the accessibility of the socket.
.PP
Abstract sockets automatically disappear when all open references
to the socket are closed.
.PP
The abstract socket namespace is a nonportable Linux extension.
.\"
.SS Socket options
For historical reasons, these socket options are specified with a
.B SOL_SOCKET
type even though they are
.B AF_UNIX
specific.
They can be set with
.BR setsockopt (2)
and read with
.BR getsockopt (2)
by specifying
.B SOL_SOCKET
as the socket family.
.TP
.B SO_PASSCRED
Enabling this socket option causes receipt of the credentials of
the sending process in an
.B SCM_CREDENTIALS ancillary
message in each subsequently received message.
The returned credentials are those specified by the sender using
.BR SCM_CREDENTIALS ,
or a default that includes the sender's PID, real user ID, and real group ID,
if the sender did not specify
.B SCM_CREDENTIALS
ancillary data.
.IP
When this option is set and the socket is not yet connected,
a unique name in the abstract namespace will be generated automatically.
.IP
The value given as an argument to
.BR setsockopt (2)
and returned as the result of
.BR getsockopt (2)
is an integer boolean flag.
.TP
.B SO_PASSSEC
Enables receiving of the SELinux security label of the peer socket
in an ancillary message of type
.BR SCM_SECURITY
(see below).
.IP
The value given as an argument to
.BR setsockopt (2)
and returned as the result of
.BR getsockopt (2)
is an integer boolean flag.
.IP
The
.B SO_PASSSEC
option is supported for UNIX domain datagram sockets
.\" commit 877ce7c1b3afd69a9b1caeb1b9964c992641f52a
since Linux 2.6.18;
support for UNIX domain stream sockets was added
.\" commit 37a9a8df8ce9de6ea73349c9ac8bdf6ba4ec4f70
in Linux 4.2.
.TP
.BR SO_PEEK_OFF
See
.BR socket (7).
.TP
.B SO_PEERCRED
This read-only socket option returns the
credentials of the peer process connected to this socket.
The returned credentials are those that were in effect at the time
of the call to
.BR connect (2)
or
.BR socketpair (2).
.IP
The argument to
.BR getsockopt (2)
is a pointer to a
.I ucred
structure; define the
.B _GNU_SOURCE
feature test macro to obtain the definition of that structure from
.IR <sys/socket.h> .
.IP
The use of this option is possible only for connected
.B AF_UNIX
stream sockets and for
.B AF_UNIX
stream and datagram socket pairs created using
.BR socketpair (2).
.TP
.B SO_PEERSEC
This read-only socket option returns the
security context of the peer socket connected to this socket.
By default, this will be the same as the security context of
the process that created the peer socket unless overridden
by the policy or by a process with the required permissions.
.IP
The argument to
.BR getsockopt (2)
is a pointer to a buffer of the specified length in bytes
into which the security context string will be copied.
If the buffer length is less than the length of the security
context string, then
.BR getsockopt (2)
returns \-1, sets
.I errno
to
.BR ERANGE ,
and returns the required length via
.IR optlen .
The caller should allocate at least
.BR NAME_MAX
bytes for the buffer initially, although this is not guaranteed
to be sufficient.
Resizing the buffer to the returned length
and retrying may be necessary.
.IP
The security context string may include a terminating null character
in the returned length, but is not guaranteed to do so: a security
context "foo" might be represented as either {'f','o','o'} of length 3
or {'f','o','o','\\0'} of length 4, which are considered to be
interchangeable.
The string is printable, does not contain non-terminating null characters,
and is in an unspecified encoding (in particular, it
is not guaranteed to be ASCII or UTF-8).
.IP
The use of this option for sockets in the
.B AF_UNIX
address family is supported since Linux 2.6.2 for connected stream sockets,
and since Linux 4.18
.\" commit 0b811db2cb2aabc910e53d34ebb95a15997c33e7
also for stream and datagram socket pairs created using
.BR socketpair (2).
.\"
.SS Autobind feature
If a
.BR bind (2)
call specifies
.I addrlen
as
.IR sizeof(sa_family_t) ,
.\" i.e., sizeof(short)
or the
.BR SO_PASSCRED
socket option was specified for a socket that was
not explicitly bound to an address,
then the socket is autobound to an abstract address.
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.
(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.
.PP
UNIX domain sockets do not support the transmission of
out-of-band data (the
.B MSG_OOB
flag for
.BR send (2)
and
.BR recv (2)).
.PP
The
.BR send (2)
.B MSG_MORE
flag is not supported by UNIX domain sockets.
.PP
Before Linux 3.4,
.\" commit 9f6f9af7694ede6314bed281eec74d588ba9474f
the use of
.B MSG_TRUNC
in the
.I flags
argument of
.BR recv (2)
was not supported by UNIX domain sockets.
.PP
The
.B SO_SNDBUF
socket option does have an effect for UNIX domain sockets, but the
.B SO_RCVBUF
option does not.
For datagram sockets, the
.B SO_SNDBUF
value imposes an upper limit on the size of outgoing datagrams.
This limit is calculated as the doubled (see
.BR socket (7))
option value less 32 bytes used for overhead.
.SS 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
.B SOL_SOCKET
type even though they are
.B AF_UNIX
specific.
To send them, set the
.I cmsg_level
field of the struct
.I cmsghdr
to
.B SOL_SOCKET
and the
.I 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.
.IP
Commonly, this operation is referred to as "passing a file descriptor"
to another process.
However, more accurately,
what is being passed is a reference to an open file description (see
.BR open (2)),
and in the receiving process it is likely that a different
file descriptor number will be used.
Semantically, this operation is equivalent to duplicating
.RB ( dup (2))
a file descriptor into the file descriptor table of another process.
.IP
If the buffer used to receive the ancillary data containing
file descriptors is too small (or is absent),
then the ancillary data is truncated (or discarded)
and the excess file descriptors are automatically closed
in the receiving process.
.IP
If the number of file descriptors received in the ancillary data would
cause the process to exceed its
.B RLIMIT_NOFILE
resource limit (see
.BR getrlimit (2)),
the excess file descriptors are automatically closed
in the receiving process.
.IP
The kernel constant
.BR SCM_MAX_FD
defines a limit on the number of file descriptors in the array.
Attempting to send an array larger than this limit causes
.BR sendmsg (2)
to fail with the error
.BR EINVAL .
.BR SCM_MAX_FD
has the value 253
(or 255 in kernels
.\" commit bba14de98753cb6599a2dae0e520714b2153522d
before 2.6.38).
.TP
.B SCM_CREDENTIALS
Send or receive UNIX credentials.
This can be used for authentication.
The credentials are passed as a
.I struct ucred
ancillary message.
This structure is defined in
.I <sys/socket.h>
as follows:
.IP
.in +4n
.EX
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 */
};
.EE
.in
.IP
Since glibc 2.8, the
.B _GNU_SOURCE
feature test macro must be defined (before including
.I any
header files) in order to obtain the definition
of this structure.
.IP
The credentials which the sender specifies are checked by the kernel.
A privileged process 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 ,
in which case the PID of any existing process may be specified),
its real user ID, effective user ID, or saved set-user-ID (unless it has
.BR CAP_SETUID ),
and its real group ID, effective group ID, or saved set-group-ID
(unless it has
.BR CAP_SETGID ).
.IP
To receive a
.I struct ucred
message, the
.B SO_PASSCRED
option must be enabled on the socket.
.TP
.B SCM_SECURITY
Receive the SELinux security context (the security label)
of the peer socket.
The received ancillary data is a null-terminated string containing
the security context.
The receiver should allocate at least
.BR NAME_MAX
bytes in the data portion of the ancillary message for this data.
.IP
To receive the security context, the
.B SO_PASSSEC
option must be enabled on the socket (see above).
.PP
When sending ancillary data with
.BR sendmsg (2),
only one item of each of the above types may be included in the sent message.
.PP
At least one byte of real data should be sent when sending ancillary data.
On Linux, this is required to successfully send ancillary data over
a UNIX domain stream socket.
When sending ancillary data over a UNIX domain datagram socket,
it is not necessary on Linux to send any accompanying real data.
However, portable applications should also include at least one byte
of real data when sending ancillary data over a datagram socket.
.PP
When receiving from a stream socket,
ancillary data forms a kind of barrier for the received data.
For example, suppose that the sender transmits as follows:
.PP
.RS
.PD 0
.IP 1. 3
.BR sendmsg (2)
of four bytes, with no ancillary data.
.IP 2.
.BR sendmsg (2)
of one byte, with ancillary data.
.IP 3.
.BR sendmsg (2)
of four bytes, with no ancillary data.
.PD
.RE
.PP
Suppose that the receiver now performs
.BR recvmsg (2)
calls each with a buffer size of 20 bytes.
The first call will receive five bytes of data,
along with the ancillary data sent by the second
.BR sendmsg (2)
call.
The next call will receive the remaining four bytes of data.
.PP
If the space allocated for receiving incoming ancillary data is too small
then the ancillary data is truncated to the number of headers
that will fit in the supplied buffer (or, in the case of an
.BR SCM_RIGHTS
file descriptor list, the list of file descriptors may be truncated).
If no buffer is provided for incoming ancillary data (i.e., the
.I msg_control
field of the
.I msghdr
structure supplied to
.BR recvmsg (2)
is NULL),
then the incoming ancillary data is discarded.
In both of these cases, the
.BR MSG_CTRUNC
flag will be set in the
.I msg.msg_flags
value returned by
.BR recvmsg (2).
.\"
.SS Ioctls
The following
.BR ioctl (2)
calls return information in
.IR value .
The correct syntax is:
.PP
.RS
.nf
.BI int " value";
.IB error " = ioctl(" unix_socket ", " ioctl_type ", &" value ");"
.fi
.RE
.PP
.I ioctl_type
can be:
.TP
.B SIOCINQ
For
.B SOCK_STREAM
sockets, this call returns the number of unread bytes in the receive buffer.
The socket must not be in LISTEN state, otherwise an error
.RB ( EINVAL )
is returned.
.B SIOCINQ
is defined in
.IR <linux/sockios.h> .
.\" 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
.BR FIONREAD ,
defined in
.IR <sys/ioctl.h> .
.\" SIOCOUTQ also has an effect for UNIX domain sockets, but not
.\" quite what userland might expect. It seems to return the number
.\" of bytes allocated for buffers containing pending output.
.\" That number is normally larger than the number of bytes of pending
.\" output. Since this info is, from userland's point of view, imprecise,
.\" and it may well change, probably best not to document this now.
For
.B SOCK_DGRAM
sockets,
the returned value is the same as
for Internet domain datagram sockets;
see
.BR udp (7).
.SH ERRORS
.TP
.B EADDRINUSE
The specified local address is already in use or the filesystem socket
object already exists.
.TP
.B EBADF
This error can occur for
.BR sendmsg (2)
when sending a file descriptor as ancillary data over
a UNIX domain socket (see the description of
.BR SCM_RIGHTS ,
above), and indicates that the file descriptor number that
is being sent is not valid (e.g., it is not an open file descriptor).
.TP
.B ECONNREFUSED
The remote address specified by
.BR connect (2)
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 EFAULT
User memory address was not valid.
.TP
.B EINVAL
Invalid argument passed.
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 was in an
invalid state for the applied operation.
.TP
.B EISCONN
.BR connect (2)
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
.B ENOTCONN
Socket operation needs a target address, but the socket is not connected.
.TP
.B EOPNOTSUPP
Stream operation called on non-stream oriented socket or tried to
use the out-of-band data option.
.TP
.B EPERM
The sender passed invalid credentials in the
.IR "struct ucred" .
.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 send (2)
or
.BR sendmsg (2).
.TP
.B EPROTONOSUPPORT
Passed protocol is not
.BR AF_UNIX .
.TP
.B EPROTOTYPE
Remote socket does not match the local socket type
.RB ( SOCK_DGRAM
versus
.BR SOCK_STREAM ).
.TP
.B ESOCKTNOSUPPORT
Unknown socket type.
.TP
.B ESRCH
While sending an ancillary message containing credentials
.RB ( SCM_CREDENTIALS ),
the caller specified a PID that does not match any existing process.
.TP
.B ETOOMANYREFS
This error can occur for
.BR sendmsg (2)
when sending a file descriptor as ancillary data over
a UNIX domain socket (see the description of
.BR SCM_RIGHTS ,
above).
It occurs if the number of "in-flight" file descriptors exceeds the
.B RLIMIT_NOFILE
resource limit and the caller does not have the
.BR CAP_SYS_RESOURCE
capability.
An in-flight file descriptor is one that has been sent using
.BR sendmsg (2)
but has not yet been accepted in the recipient process using
.BR recvmsg (2).
.IP
This error is diagnosed since mainline Linux 4.5
(and in some earlier kernel versions where the fix has been backported).
.\" commit 712f4aad406bb1ed67f3f98d04c044191f0ff593
In earlier kernel versions,
it was possible to place an unlimited number of file descriptors in flight,
by sending each file descriptor with
.BR sendmsg (2)
and then closing the file descriptor so that it was not accounted against the
.B RLIMIT_NOFILE
resource limit.
.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 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
Binding to a socket with a filename creates a socket
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 filesystem when the last
reference to it is closed.
.PP
To pass file descriptors or credentials over a
.BR SOCK_STREAM
socket, you must
send or receive at least one byte of nonancillary data in the same
.BR sendmsg (2)
or
.BR recvmsg (2)
call.
.PP
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 .
.PP
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 .
.PP
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:
.PP
    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
.PP
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 :
.PP
.in +4n
.EX
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\en", ((struct sockaddr_un *) addrp)\->sun_path);
.EE
.in
.PP
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 EXAMPLES
The following code demonstrates the use of sequenced-packet
sockets for local interprocess communication.
It consists of two programs.
The server program waits for a connection from the client program.
The client sends each of its command-line arguments in separate messages.
The server treats the incoming messages as integers and adds them up.
The client sends the command string "END".
The server sends back a message containing the sum of the client's integers.
The client prints the sum and exits.
The server waits for the next client to connect.
To stop the server, the client is called with the command-line argument "DOWN".
.PP
The following output was recorded while running the server in the background
and repeatedly executing the client.
Execution of the server program ends when it receives the "DOWN" command.
.SS Example output
.in +4n
.EX
$ \fB./server &\fP
[1] 25887
$ \fB./client 3 4\fP
Result = 7
$ \fB./client 11 \-5\fP
Result = 6
$ \fB./client DOWN\fP
Result = 0
[1]+  Done                    ./server
$
.EE
.in
.SS Program source
\&
.EX
/*
 * File connection.h
 */

#define SOCKET_NAME "/tmp/9Lq7BNBnBycd6nxy.socket"
#define BUFFER_SIZE 12

/*
 * File server.c
 */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <unistd.h>
#include "connection.h"

int
main(int argc, char *argv[])
{
    struct sockaddr_un name;
    int down_flag = 0;
    int ret;
    int connection_socket;
    int data_socket;
    int result;
    char buffer[BUFFER_SIZE];

    /* Create local socket. */

    connection_socket = socket(AF_UNIX, SOCK_SEQPACKET, 0);
    if (connection_socket == \-1) {
        perror("socket");
        exit(EXIT_FAILURE);
    }

    /*
     * For portability clear the whole structure, since some
     * implementations have additional (nonstandard) fields in
     * the structure.
     */

    memset(&name, 0, sizeof(name));

    /* Bind socket to socket name. */

    name.sun_family = AF_UNIX;
    strncpy(name.sun_path, SOCKET_NAME, sizeof(name.sun_path) \- 1);

    ret = bind(connection_socket, (const struct sockaddr *) &name,
               sizeof(name));
    if (ret == \-1) {
        perror("bind");
        exit(EXIT_FAILURE);
    }

    /*
     * Prepare for accepting connections. The backlog size is set
     * to 20. So while one request is being processed other requests
     * can be waiting.
     */

    ret = listen(connection_socket, 20);
    if (ret == \-1) {
        perror("listen");
        exit(EXIT_FAILURE);
    }

    /* This is the main loop for handling connections. */

    for (;;) {

        /* Wait for incoming connection. */

        data_socket = accept(connection_socket, NULL, NULL);
        if (data_socket == \-1) {
            perror("accept");
            exit(EXIT_FAILURE);
        }

        result = 0;
        for (;;) {

            /* Wait for next data packet. */

            ret = read(data_socket, buffer, sizeof(buffer));
            if (ret == \-1) {
                perror("read");
                exit(EXIT_FAILURE);
            }

            /* Ensure buffer is 0\-terminated. */

            buffer[sizeof(buffer) \- 1] = 0;

            /* Handle commands. */

            if (!strncmp(buffer, "DOWN", sizeof(buffer))) {
                down_flag = 1;
                break;
            }

            if (!strncmp(buffer, "END", sizeof(buffer))) {
                break;
            }

            /* Add received summand. */

            result += atoi(buffer);
        }

        /* Send result. */

        sprintf(buffer, "%d", result);
        ret = write(data_socket, buffer, sizeof(buffer));
        if (ret == \-1) {
            perror("write");
            exit(EXIT_FAILURE);
        }

        /* Close socket. */

        close(data_socket);

        /* Quit on DOWN command. */

        if (down_flag) {
            break;
        }
    }

    close(connection_socket);

    /* Unlink the socket. */

    unlink(SOCKET_NAME);

    exit(EXIT_SUCCESS);
}

/*
 * File client.c
 */

#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <unistd.h>
#include "connection.h"

int
main(int argc, char *argv[])
{
    struct sockaddr_un addr;
    int ret;
    int data_socket;
    char buffer[BUFFER_SIZE];

    /* Create local socket. */

    data_socket = socket(AF_UNIX, SOCK_SEQPACKET, 0);
    if (data_socket == \-1) {
        perror("socket");
        exit(EXIT_FAILURE);
    }

    /*
     * For portability clear the whole structure, since some
     * implementations have additional (nonstandard) fields in
     * the structure.
     */

    memset(&addr, 0, sizeof(addr));

    /* Connect socket to socket address. */

    addr.sun_family = AF_UNIX;
    strncpy(addr.sun_path, SOCKET_NAME, sizeof(addr.sun_path) \- 1);

    ret = connect(data_socket, (const struct sockaddr *) &addr,
                   sizeof(addr));
    if (ret == \-1) {
        fprintf(stderr, "The server is down.\en");
        exit(EXIT_FAILURE);
    }

    /* Send arguments. */

    for (int i = 1; i < argc; ++i) {
        ret = write(data_socket, argv[i], strlen(argv[i]) + 1);
        if (ret == \-1) {
            perror("write");
            break;
        }
    }

    /* Request result. */

    strcpy(buffer, "END");
    ret = write(data_socket, buffer, strlen(buffer) + 1);
    if (ret == \-1) {
        perror("write");
        exit(EXIT_FAILURE);
    }

    /* Receive result. */

    ret = read(data_socket, buffer, sizeof(buffer));
    if (ret == \-1) {
        perror("read");
        exit(EXIT_FAILURE);
    }

    /* Ensure buffer is 0\-terminated. */

    buffer[sizeof(buffer) \- 1] = 0;

    printf("Result = %s\en", buffer);

    /* Close socket. */

    close(data_socket);

    exit(EXIT_SUCCESS);
}
.EE
.PP
For examples of the use of
.BR SCM_RIGHTS ,
see
.BR cmsg (3)
and
.BR seccomp_unotify (2).
.SH SEE ALSO
.BR recvmsg (2),
.BR sendmsg (2),
.BR socket (2),
.BR socketpair (2),
.BR cmsg (3),
.BR capabilities (7),
.BR credentials (7),
.BR socket (7),
.BR udp (7)