UNIX(7) Linux Programmer's Manual UNIX(7)
NAME
unix - sockets for local interprocess communication
SYNOPSIS
#include <sys/socket.h>
#include <sys/un.h>
unix_socket = socket(AF_UNIX, type, 0);
error = socketpair(AF_UNIX, type, 0, int *sv);
DESCRIPTION
The AF_UNIX (also known as AF_LOCAL) socket family is used to communi-
cate 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.
Valid socket types in the UNIX domain are: SOCK_STREAM, for a stream-
oriented socket; SOCK_DGRAM, for a datagram-oriented socket that pre-
serves 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) SOCK_SEQPACKET, for a sequenced-packet socket that
is connection-oriented, preserves message boundaries, and delivers mes-
sages in the order that they were sent.
UNIX domain sockets support passing file descriptors or process creden-
tials to other processes using ancillary data.
Address format
A UNIX domain socket address is represented in the following structure:
struct sockaddr_un {
sa_family_t sun_family; /* AF_UNIX */
char sun_path[108]; /* pathname */
};
The sun_family field always contains AF_UNIX. On Linux sun_path is 108
bytes in size; see also NOTES, below.
Various systems calls (for example, bind(2), connect(2), and sendto(2))
take a sockaddr_un argument as input. Some other system calls (for
example, getsockname(2), getpeername(2), recvfrom(2), and accept(2))
return an argument of this type.
Three types of address are distinguished in the sockaddr_un structure:
* pathname: a UNIX domain socket can be bound to a null-terminated
filesystem pathname using bind(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 sun_path contains the null-terminated pathname. (On Linux, the
above offsetof() expression equates to the same value as
sizeof(sa_family_t), but some other implementations include other
fields before sun_path, so the offsetof() expression more portably
describes the size of the address structure.)
For further details of pathname sockets, see below.
* unnamed: A stream socket that has not been bound to a pathname using
bind(2) has no name. Likewise, the two sockets created by socket-
pair(2) are unnamed. When the address of an unnamed socket is
returned, its length is sizeof(sa_family_t), and sun_path should not
be inspected.
* abstract: an abstract socket address is distinguished (from a path-
name socket) by the fact that sun_path[0] is a null byte ('\0').
The socket's address in this namespace is given by the additional
bytes in sun_path that are covered by the specified length of the
address structure. (Null bytes in the name have no special signifi-
cance.) The name has no connection with filesystem pathnames. When
the address of an abstract socket is returned, the returned addrlen
is greater than sizeof(sa_family_t) (i.e., greater than 2), and the
name of the socket is contained in the first (addrlen -
sizeof(sa_family_t)) bytes of sun_path.
Pathname sockets
When binding a socket to a pathname, a few rules should be observed for
maximum portability and ease of coding:
* The pathname in sun_path should be null-terminated.
* The length of the pathname, including the terminating null byte,
should not exceed the size of sun_path.
* The addrlen argument that describes the enclosing sockaddr_un struc-
ture should have a value of at least:
offsetof(struct sockaddr_un, sun_path)+strlen(addr.sun_path)+1
or, more simply, addrlen can be specified as sizeof(struct sock-
addr_un).
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 append a null terminator if none is
present in the supplied sun_path.
When coding portable applications, keep in mind that some implementa-
tions have sun_path as short as 92 bytes.
Various system calls (accept(2), recvfrom(2), getsockname(2), getpeer-
name(2)) return socket address structures. When applied to UNIX domain
sockets, the value-result addrlen argument supplied to the call should
be initialized as above. Upon return, the argument is set to indicate
the 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
sun_path. (See BUGS.)
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.
On Linux, connecting to a stream socket object requires write permis-
sion 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.
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 umask(2).
The owner, group, and permissions of a pathname socket can be changed
(using chown(2) and chmod(2)).
Abstract sockets
Socket permissions have no meaning for abstract sockets: the process
umask(2) has no effect when binding an abstract socket, and changing
the ownership and permissions of the object (via fchown(2) and fch-
mod(2)) has no effect on the accessibility of the socket.
Abstract sockets automatically disappear when all open references to
the socket are closed.
The abstract socket namespace is a nonportable Linux extension.
Socket options
For historical reasons, these socket options are specified with a
SOL_SOCKET type even though they are AF_UNIX specific. They can be set
with setsockopt(2) and read with getsockopt(2) by specifying SOL_SOCKET
as the socket family.
SO_PASSCRED
Enables the receiving of the credentials of the sending process
in an 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.
Autobind feature
If a bind(2) call specifies addrlen as sizeof(sa_family_t), or the
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 [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.)
Sockets API
The following paragraphs describe domain-specific details and unsup-
ported features of the sockets API for UNIX domain sockets on Linux.
UNIX domain sockets do not support the transmission of out-of-band data
(the MSG_OOB flag for send(2) and recv(2)).
The send(2) MSG_MORE flag is not supported by UNIX domain sockets.
Before Linux 3.4, the use of MSG_TRUNC in the flags argument of recv(2)
was not supported by UNIX domain sockets.
The SO_SNDBUF socket option does have an effect for UNIX domain sock-
ets, but the SO_RCVBUF option does not. For datagram sockets, the
SO_SNDBUF value imposes an upper limit on the size of outgoing data-
grams. This limit is calculated as the doubled (see socket(7)) option
value less 32 bytes used for overhead.
Ancillary messages
Ancillary data is sent and received using sendmsg(2) and recvmsg(2).
For historical reasons the ancillary message types listed below are
specified with a SOL_SOCKET type even though they are AF_UNIX specific.
To send them set the cmsg_level field of the struct cmsghdr to
SOL_SOCKET and the cmsg_type field to the type. For more information
see cmsg(3).
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 dup(2).
SCM_CREDENTIALS
Send or receive UNIX credentials. This can be used for authen-
tication. The credentials are passed as a struct ucred ancil-
lary message. Thus structure is defined in <sys/socket.h> as
follows:
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 */
};
Since glibc 2.8, the _GNU_SOURCE feature test macro must be
defined (before including any header files) in order to obtain
the definition of this structure.
The credentials which the sender specifies are checked by the
kernel. A process with effective user ID 0 is allowed to spec-
ify values that do not match its own. The sender must specify
its own process ID (unless it has the capability CAP_SYS_ADMIN),
its real user ID, effective user ID, or saved set-user-ID
(unless it has CAP_SETUID), and its real group ID, effective
group ID, or saved set-group-ID (unless it has CAP_SETGID). To
receive a struct ucred message the SO_PASSCRED option must be
enabled on the socket.
Ioctls
The following ioctl(2) calls return information in value. The correct
syntax is:
int value;
error = ioctl(unix_socket, ioctl_type, &value);
ioctl_type can be:
SIOCINQ
For SOCK_STREAM socket the function returns the amount of queued
unread data in the receive buffer. The socket must not be in
LISTEN state, otherwise an error (EINVAL) is returned. SIOCINQ
is defined in <linux/sockios.h>. Alternatively, you can use the
synonymous FIONREAD, defined in <sys/ioctl.h>. For SOCK_DGRAM
socket, the returned value is the same as for Internet domain
datagram socket; see udp(7).
ERRORS
EADDRINUSE
The specified local address is already in use or the filesystem
socket object already exists.
ECONNREFUSED
The remote address specified by connect(2) was not a listening
socket. This error can also occur if the target pathname is not
a socket.
ECONNRESET
Remote socket was unexpectedly closed.
EFAULT User memory address was not valid.
EINVAL Invalid argument passed. A common cause is that the value
AF_UNIX was not specified in the sun_type field of passed
addresses, or the socket was in an invalid state for the applied
operation.
EISCONN
connect(2) called on an already connected socket or a target
address was specified on a connected socket.
ENOENT The pathname in the remote address specified to connect(2) did
not exist.
ENOMEM Out of memory.
ENOTCONN
Socket operation needs a target address, but the socket is not
connected.
EOPNOTSUPP
Stream operation called on non-stream oriented socket or tried
to use the out-of-band data option.
EPERM The sender passed invalid credentials in the struct ucred.
EPIPE Remote socket was closed on a stream socket. If enabled, a SIG-
PIPE is sent as well. This can be avoided by passing the
MSG_NOSIGNAL flag to send(2) or sendmsg(2).
EPROTONOSUPPORT
Passed protocol is not AF_UNIX.
EPROTOTYPE
Remote socket does not match the local socket type (SOCK_DGRAM
versus SOCK_STREAM).
ESOCKTNOSUPPORT
Unknown socket type.
ETOOMANYREFS
This error can occur for sendmsg(2) when sending a file descrip-
tor as ancillary data over a UNIX domain socket (see the
description of SCM_RIGHTS, above). It occurs if the number of
"in-flight" file descriptors exceeds the RLIMIT_NOFILE resource
limit and the caller does not have the CAP_SYS_RESOURCE capabil-
ity. An in-flight file descriptor is one that has been sent
using sendmsg(2) but has not yet been accepted in the recipient
process using recvmsg(2).
This error is diagnosed since mainline Linux 4.5 (and in some
earlier kernel versions where the fix has been backported). In
earlier kernel versions, it was possible to place an unlimited
number of file descriptors in flight, by sending each file
descriptor with sendmsg(2) and then closing the file descriptor
so that it was not accounted against the RLIMIT_NOFILE resource
limit.
Other errors can be generated by the generic socket layer or by the
filesystem while generating a filesystem socket object. See the appro-
priate manual pages for more information.
VERSIONS
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.)
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
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.
To pass file descriptors or credentials over a SOCK_STREAM, you need to
send or receive at least one byte of nonancillary data in the same
sendmsg(2) or recvmsg(2) call.
UNIX domain stream sockets do not support the notion of out-of-band
data.
BUGS
When binding a socket to an address, Linux is one of the implementa-
tions that appends a null terminator if none is supplied in 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 path-
name beyond sizeof(sun_path). Consequently, when retrieving the socket
address (for example, via accept(2)), if the input addrlen argument for
the retrieving call is specified as sizeof(struct sockaddr_un), then
the returned address structure won't have a null terminator in
sun_path.
In addition, some implementations don't require a null terminator when
binding a socket (the addrlen argument is used to determine the length
of sun_path) and when the socket address is retrieved on these imple-
mentations, there is no null terminator in sun_path.
Applications that retrieve socket addresses can (portably) code to han-
dle the possibility that there is no null terminator in 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))
Alternatively, an application can retrieve the socket address by allo-
cating a buffer of size sizeof(struct sockaddr_un)+1 that is zeroed out
before the retrieval. The retrieving call can specify addrlen as
sizeof(struct sockaddr_un), and the extra zero byte ensures that there
will be a null terminator for the string returned in sun_path:
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);
This sort of messiness can be avoided if it is guaranteed that the
applications that create pathname sockets follow the rules outlined
above under Pathname sockets.
EXAMPLE
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".
The following output was recorded while running the server in the back-
ground and repeatedly executing the client. Execution of the server
program ends when it receives the "DOWN" command.
Example output
$ ./server &
[1] 25887
$ ./client 3 4
Result = 7
$ ./client 11 -5
Result = 6
$ ./client DOWN
Result = 0
[1]+ Done ./server
$
Program source
/*
* 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];
/*
* In case the program exited inadvertently on the last run,
* remove the socket.
*/
unlink(SOCKET_NAME);
/* 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(struct sockaddr_un));
/* 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(struct sockaddr_un));
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, BUFFER_SIZE);
if (ret == -1) {
perror("read");
exit(EXIT_FAILURE);
}
/* Ensure buffer is 0-terminated. */
buffer[BUFFER_SIZE - 1] = 0;
/* Handle commands. */
if (!strncmp(buffer, "DOWN", BUFFER_SIZE)) {
down_flag = 1;
break;
}
if (!strncmp(buffer, "END", BUFFER_SIZE)) {
break;
}
/* Add received summand. */
result += atoi(buffer);
}
/* Send result. */
sprintf(buffer, "%d", result);
ret = write(data_socket, buffer, BUFFER_SIZE);
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 i;
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(struct sockaddr_un));
/* 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(struct sockaddr_un));
if (ret == -1) {
fprintf(stderr, "The server is down.\n");
exit(EXIT_FAILURE);
}
/* Send arguments. */
for (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, BUFFER_SIZE);
if (ret == -1) {
perror("read");
exit(EXIT_FAILURE);
}
/* Ensure buffer is 0-terminated. */
buffer[BUFFER_SIZE - 1] = 0;
printf("Result = %s\n", buffer);
/* Close socket. */
close(data_socket);
exit(EXIT_SUCCESS);
}
For an example of the use of SCM_RIGHTS see cmsg(3).
SEE ALSO
recvmsg(2), sendmsg(2), socket(2), socketpair(2), cmsg(3), capabili-
ties(7), credentials(7), socket(7), udp(7)
COLOPHON
This page is part of release 4.15 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
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