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 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.

  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 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:

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.

  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 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.)

  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 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)).

  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 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.

  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 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), listen(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).

  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 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 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 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 before Linux 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).

  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 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).

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 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.

ENFILE
    The system-wide limit on the total number of open files has been reached.

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.

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 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 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.

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 background 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
     */
    #ifndef CONNECTION_H
    #define CONNECTION_H

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

    #endif  // include guard

    /*
     * File server.c
     */

    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>
    #include <sys/socket.h>
    #include <sys/types.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/types.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).

SEE ALSO
recvmsg(2), sendmsg(2), socket(2), socketpair(2), cmsg(3), capabilities(7), credentials(7), socket(7), udp(7)