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 ab-
stract 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 ex-
ample, getsockname(2), getpeername(2), recvfrom(2), and accept(2)) re-
turn 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 re-
turned, 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 di-
rectory 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
Enabling this socket option causes receipt of the credentials of
the sending process in an SCM_CREDENTIALS ancillary message in
each subsequently received message. The returned credentials
are those specified by the sender using SCM_CREDENTIALS, or a
default that includes the sender's PID, real user ID, and real
group ID, if the sender did not specify SCM_CREDENTIALS ancil-
lary data.
When this option is set and the socket is not yet connected, a
unique name in the abstract namespace will be generated automat-
ically.
The value given as an argument to setsockopt(2) and returned as
the result of getsockopt(2) is an integer boolean flag.
SO_PASSSEC
Enables receiving of the SELinux security label of the peer
socket in an ancillary message of type SCM_SECURITY (see below).
The value given as an argument to setsockopt(2) and returned as
the result of getsockopt(2) is an integer boolean flag.
The SO_PASSSEC option is supported for UNIX domain datagram
sockets since Linux 2.6.18; support for UNIX domain stream sock-
ets was added in Linux 4.2.
SO_PEEK_OFF
See socket(7).
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 connect(2)
or socketpair(2).
The argument to getsockopt(2) is a pointer to a ucred structure;
define the _GNU_SOURCE feature test macro to obtain the defini-
tion of that structure from <sys/socket.h>.
The use of this option is possible only for connected AF_UNIX
stream sockets and for AF_UNIX stream and datagram socket pairs
created using socketpair(2).
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 ex-
plicitly bound to an address, then the socket is autobound to an ab-
stract 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 ad-
dresses. 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.
Commonly, this operation is referred to as "passing a file de-
scriptor" to another process. However, more accurately, what is
being passed is a reference to an open file description (see
open(2)), and in the receiving process it is likely that a dif-
ferent file descriptor number will be used. Semantically, this
operation is equivalent to duplicating (dup(2)) a file descrip-
tor into the file descriptor table of another process.
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.
If the number of file descriptors received in the ancillary data
would cause the process to exceed its RLIMIT_NOFILE resource
limit (see getrlimit(2)), the excess file descriptors are auto-
matically closed in the receiving process.
The kernel constant 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 sendmsg(2) to fail with the error
EINVAL. SCM_MAX_FD has the value 253 (or 255 in kernels before
2.6.38).
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. This 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 de-
fined (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 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 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
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.
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 al-
locate at least NAME_MAX bytes in the data portion of the ancil-
lary message for this data.
To receive the security context, the SO_PASSSEC option must be
enabled on the socket (see above).
When sending ancillary data with sendmsg(2), only one item of each of
the above types may be included in the sent message.
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 in-
clude at least one byte of real data when sending ancillary data over a
datagram socket.
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:
1. sendmsg(2) of four bytes, with no ancillary data.
2. sendmsg(2) of one byte, with ancillary data.
3. sendmsg(2) of four bytes, with no ancillary data.
Suppose that the receiver now performs 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 sendmsg(2) call.
The next call will receive the remaining four bytes of data.
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 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
msg_control field of the msghdr structure supplied to recvmsg(2) is
NULL), then the incoming ancillary data is discarded. In both of these
cases, the MSG_CTRUNC flag will be set in the msg.msg_flags value re-
turned by recvmsg(2).
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 sockets, this call returns the number of unread
bytes in the receive buffer. The socket must not be in LISTEN
state, otherwise an error (EINVAL) is returned. SIOCINQ is de-
fined in <linux/sockios.h>. Alternatively, you can use the syn-
onymous FIONREAD, defined in <sys/ioctl.h>. For SOCK_DGRAM
sockets, the returned value is the same as for Internet domain
datagram sockets; see udp(7).
ERRORS
EADDRINUSE
The specified local address is already in use or the filesystem
socket object already exists.
EBADF This error can occur for sendmsg(2) when sending a file descrip-
tor as ancillary data over a UNIX domain socket (see the de-
scription of SCM_RIGHTS, above), and indicates that the file de-
scriptor number that is being sent is not valid (e.g., it is not
an open file descriptor).
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 ad-
dresses, or the socket was in an invalid state for the applied
operation.
EISCONN
connect(2) called on an already connected socket or a target ad-
dress 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.
ESRCH While sending an ancillary message containing credentials
(SCM_CREDENTIALS), the caller specified a PID that does not
match any existing process.
ETOOMANYREFS
This error can occur for sendmsg(2) when sending a file descrip-
tor as ancillary data over a UNIX domain socket (see the de-
scription 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 us-
ing 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 de-
scriptor 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 socket, you
must 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 re-
trieved, it will be one byte longer than that supplied when the socket
was bound. However, there is one case where confusing behavior can re-
sult: 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 sizeof(sun_path). Consequently, when retrieving the socket ad-
dress (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 re-
specting 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 ap-
plications that create pathname sockets follow the rules outlined above
under Pathname sockets.
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".
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];
/* 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 5.07 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/.
Linux 2020-06-09 UNIX(7)