UNIX(7)                    Linux Programmer's 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.

   Address format
       A UNIX domain socket address is represented in the following structure:

           struct sockaddr_un {
               sa_family_t sun_family;               /* AF_UNIX */
               char        sun_path[108];            /* Pathname */

       The sun_family field always contains AF_UNIX.  On Linux, sun_path is
       108 bytes in size; see also NOTES, below.

       Various systems calls (for example, bind(2), connect(2), and sendto(2))
       take a sockaddr_un argument as input.  Some other system calls (for
       example, getsockname(2), getpeername(2), recvfrom(2), and accept(2))
       return an argument of this type.

       Three types of address are distinguished in the sockaddr_un structure:

       *  pathname: a UNIX domain socket can be bound to a null-terminated
          filesystem pathname using bind(2).  When the address of a pathname
          socket is returned (by one of the system calls noted above), its
          length is

              offsetof(struct sockaddr_un, sun_path) + strlen(sun_path) + 1

          and sun_path contains the null-terminated pathname.  (On Linux, the
          above offsetof() expression equates to the same value as
          sizeof(sa_family_t), but some other implementations include other
          fields before sun_path, so the offsetof() expression more portably
          describes the size of the address structure.)

          For further details of pathname sockets, see below.

       *  unnamed: A stream socket that has not been bound to a pathname using
          bind(2) has no name.  Likewise, the two sockets created by
          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

       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.

              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

              The value given as an argument to setsockopt(2) and returned as
              the result of getsockopt(2) is an integer boolean flag.

              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.

              See socket(7).

              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).

   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

       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).

              Send or receive a set of open file descriptors from another
              process.  The data portion contains an integer array of the file

              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

              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.

              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:

              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).

              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).

              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.

              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

              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.

              Socket operation needs a target address, but the socket is not

              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).

              Passed protocol is not AF_UNIX.

              Remote socket does not match the local socket type (SOCK_DGRAM
              versus SOCK_STREAM).

              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.

              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

       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

       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

       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

       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

       #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"

       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) {

            * 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) {

            * 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) {

           /* This is the main loop for handling connections. */

           for (;;) {

               /* Wait for incoming connection. */

               data_socket = accept(connection_socket, NULL, NULL);
               if (data_socket == -1) {

               result = 0;
               for (;;) {

                   /* Wait for next data packet. */

                   ret = read(data_socket, buffer, BUFFER_SIZE);
                   if (ret == -1) {

                   /* Ensure buffer is 0-terminated. */

                   buffer[BUFFER_SIZE - 1] = 0;

                   /* Handle commands. */

                   if (!strncmp(buffer, "DOWN", BUFFER_SIZE)) {
                       down_flag = 1;

                   if (!strncmp(buffer, "END", BUFFER_SIZE)) {

                   /* Add received summand. */

                   result += atoi(buffer);

               /* Send result. */

               sprintf(buffer, "%d", result);
               ret = write(data_socket, buffer, BUFFER_SIZE);
               if (ret == -1) {

               /* Close socket. */


               /* Quit on DOWN command. */

               if (down_flag) {


           /* Unlink the socket. */



        * 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"

       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) {

            * 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");

           /* Send arguments. */

           for (i = 1; i < argc; ++i) {
               ret = write(data_socket, argv[i], strlen(argv[i]) + 1);
               if (ret == -1) {

           /* Request result. */

           strcpy (buffer, "END");
           ret = write(data_socket, buffer, strlen(buffer) + 1);
           if (ret == -1) {

           /* Receive result. */

           ret = read(data_socket, buffer, BUFFER_SIZE);
           if (ret == -1) {

           /* Ensure buffer is 0-terminated. */

           buffer[BUFFER_SIZE - 1] = 0;

           printf("Result = %s\n", buffer);

           /* Close socket. */



       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)

       This page is part of release 5.06 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

Linux                             2020-02-09                           UNIX(7)