| UNIX(7) | Miscellaneous Information Manual | UNIX(7) |
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 BUGS, below.
Various system 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:
offsetof(struct sockaddr_un, sun_path) + strlen(sun_path) + 1
When binding a socket to a pathname, a few rules should be observed for maximum portability and ease of coding:
offsetof(struct sockaddr_un, sun_path)+strlen(addr.sun_path)+1
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.
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).
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 */
};
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:
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:
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 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 append 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(void)
{
int down_flag = 0;
int ret;
int connection_socket;
int data_socket;
int result;
ssize_t r, w;
struct sockaddr_un name;
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. */
r = read(data_socket, buffer, sizeof(buffer));
if (r == -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;
continue;
}
if (!strncmp(buffer, "END", sizeof(buffer))) {
break;
}
if (down_flag) {
continue;
}
/* Add received summand. */
result += atoi(buffer);
}
/* Send result. */
sprintf(buffer, "%d", result);
w = write(data_socket, buffer, sizeof(buffer));
if (w == -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 <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[])
{
int ret;
int data_socket;
ssize_t r, w;
struct sockaddr_un addr;
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.\n");
exit(EXIT_FAILURE);
}
/* Send arguments. */
for (int i = 1; i < argc; ++i) {
w = write(data_socket, argv[i], strlen(argv[i]) + 1);
if (w == -1) {
perror("write");
break;
}
}
/* Request result. */
strcpy(buffer, "END");
w = write(data_socket, buffer, strlen(buffer) + 1);
if (w == -1) {
perror("write");
exit(EXIT_FAILURE);
}
/* Receive result. */
r = read(data_socket, buffer, sizeof(buffer));
if (r == -1) {
perror("read");
exit(EXIT_FAILURE);
}
/* Ensure buffer is 0-terminated. */
buffer[sizeof(buffer) - 1] = 0;
printf("Result = %s\n", buffer);
/* Close socket. */
close(data_socket);
exit(EXIT_SUCCESS);
}
For examples of the use of SCM_RIGHTS, see cmsg(3) and seccomp_unotify(2).
recvmsg(2), sendmsg(2), socket(2), socketpair(2), cmsg(3), capabilities(7), credentials(7), socket(7), udp(7)
| 2024-03-16 | Linux man-pages 6.7 |