unix - sockets for local interprocess communication
#include <sys/socket.h>
#include <sys/un.h>
unix_socket = socket(AF_UNIX, type, 0);
error = socketpair(AF_UNIX, type, 0, int *sv);
The
AF_UNIX (also known as
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.
Valid socket types in the UNIX domain are:
SOCK_STREAM, for a
stream-oriented socket;
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)
SOCK_SEQPACKET, for a sequenced-packet socket that is
connection-oriented, preserves message boundaries, and delivers messages in
the order that they were sent.
UNIX domain sockets support passing file descriptors or process credentials to
other processes using ancillary data.
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
socketpair(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
pathname 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 significance.)
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.
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 structure 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
sockaddr_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 implementations have
sun_path as short as 92 bytes.
Various system calls (
accept(2),
recvfrom(2),
getsockname(2),
getpeername(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.)
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 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.
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)).
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
fchmod(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.
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 ancillary data.
- When this option is set and the socket is not yet connected, a unique name
in the abstract namespace will be generated automatically.
- 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 sockets 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
definition 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).
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.)
The following paragraphs describe domain-specific details and unsupported
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 sockets,
but the
SO_RCVBUF option does not. For datagram sockets, the
SO_SNDBUF value imposes an upper limit on the size of outgoing
datagrams. This limit is calculated as the doubled (see
socket(7))
option value less 32 bytes used for overhead.
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
descriptor" 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
different file descriptor number will be used. Semantically, this
operation is equivalent to duplicating (dup(2)) a file descriptor
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 automatically 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 authentication. The
credentials are passed as a struct ucred ancillary 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 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
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 allocate at least
NAME_MAX bytes in the data portion of the ancillary 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 include 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 returned
by
recvmsg(2).
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 defined
in <linux/sockios.h>. Alternatively, you can use the
synonymous 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).
- 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 descriptor
as ancillary data over a UNIX domain socket (see the description of
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).
- 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 SIGPIPE
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 descriptor
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
capability. 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 appropriate manual pages
for more information.
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.)
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.
When binding a socket to an address, Linux is one of the implementations 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 pathname 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
implementations, there is no null terminator in
sun_path.
Applications that retrieve socket addresses can (portably) code to handle 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 allocating 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.
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 background and
repeatedly executing the client. Execution of the server program ends when it
receives the "DOWN" command.
$ ./server &
[1] 25887
$ ./client 3 4
Result = 7
$ ./client 11 -5
Result = 6
$ ./client DOWN
Result = 0
[1]+ Done ./server
$
/*
* 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).
recvmsg(2),
sendmsg(2),
socket(2),
socketpair(2),
cmsg(3),
capabilities(7),
credentials(7),
socket(7),
udp(7)