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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 communi-
     cate between processes on the same machine efficiently.  Traditionally,
     UNIX  domain  sockets  can  be either unnamed, or bound to a filesystem
     pathname (marked as being of type  socket).   Linux  also  supports  an
     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  pre-
     serves message boundaries (as on most UNIX implementations, UNIX domain
     datagram sockets are always reliable and don't reorder datagrams);  and
     (since  Linux 2.6.4) SOCK_SEQPACKET, for a sequenced-packet socket that
     is connection-oriented, preserves message boundaries, and delivers mes-
     sages in the order that they were sent.
     UNIX domain sockets support passing file descriptors or process creden-
     tials to other processes using ancillary data.
 Address format
     A UNIX domain socket address is represented in the following structure:
         struct sockaddr_un {
             sa_family_t sun_family;               /* AF_UNIX */
             char        sun_path[108];            /* pathname */ };
     The sun_family field always contains AF_UNIX.  On Linux sun_path is 108
     bytes in size; see also NOTES, below.
     Various systems calls (for example, bind(2), connect(2), and sendto(2))
     take  a  sockaddr_un  argument  as input.  Some other system calls (for
     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 socket-

        pair(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 path-

name socket) by the fact that sun_path[0] is a null byte ('\0').

        The socket's address in this namespace is given  by  the  additional
        bytes  in  sun_path  that are covered by the specified length of the
        address structure.  (Null bytes in the name have no special signifi-
        cance.)  The name has no connection with filesystem pathnames.  When
        the address of an abstract socket is returned, the returned  addrlen
        is  greater than sizeof(sa_family_t) (i.e., greater than 2), and the
        name  of  the  socket  is  contained  in  the   first   (addrlen   -
        sizeof(sa_family_t)) bytes of sun_path.
 Pathname sockets
     When binding a socket to a pathname, a few rules should be observed for
     maximum portability and ease of coding:
  • The pathname in sun_path should be null-terminated.
  • The length of the pathname, including the terminating null byte,

should not exceed the size of sun_path.

  • The addrlen argument that describes the enclosing sockaddr_un struc-

ture should have a value of at least:

            offsetof(struct sockaddr_un, sun_path)+strlen(addr.sun_path)+1
        or, more simply, addrlen can be  specified  as  sizeof(struct  sock-
     There  is  some  variation  in  how  implementations handle UNIX domain
     socket addresses that do not follow the above rules.  For example, some
     (but  not  all)  implementations  append  a  null terminator if none is
     present in the supplied sun_path.
     When coding portable applications, keep in mind that  some  implementa-
     tions have sun_path as short as 92 bytes.
     Various  system calls (accept(2), recvfrom(2), getsockname(2), getpeer-
     name(2)) return socket address structures.  When applied to UNIX domain
     sockets,  the value-result addrlen argument supplied to the call should
     be initialized as above.  Upon return, the argument is set to  indicate
     the  actual size of the address structure.  The caller should check the
     value returned in this argument: if the output value exceeds the  input
     value,  then there is no guarantee that a null terminator is present in
     sun_path.  (See BUGS.)
 Pathname socket ownership and permissions
     In the Linux implementation, pathname sockets honor the permissions  of
     the  directory  they  are  in.   Creation  of a new socket fails if the
     process does not have write and  search  (execute)  permission  on  the
     directory in which the socket is created.
     On  Linux,  connecting to a stream socket object requires write permis-
     sion on that socket; sending a datagram to a datagram  socket  likewise
     requires  write  permission  on  that  socket.  POSIX does not make any
     statement about the effect of the permissions on a socket file, and  on
     some  systems  (e.g.,  older BSDs), the socket permissions are ignored.
     Portable programs should not rely on this feature for security.
     When creating a new socket, the owner and group of the socket file  are
     set  according to the usual rules.  The socket file has all permissions
     enabled, other than those that are turned off by the process  umask(2).
     The  owner,  group, and permissions of a pathname socket can be changed
     (using chown(2) and chmod(2)).
 Abstract sockets
     Socket permissions have no meaning for abstract  sockets:  the  process
     umask(2)  has  no  effect when binding an abstract socket, and changing
     the ownership and permissions of the object  (via  fchown(2)  and  fch-
     mod(2)) has no effect on the accessibility of the socket.
     Abstract  sockets  automatically  disappear when all open references to
     the socket are closed.
     The abstract socket namespace is a nonportable Linux extension.
 Socket options
     For historical reasons, these  socket  options  are  specified  with  a
     SOL_SOCKET type even though they are AF_UNIX specific.  They can be set
     with setsockopt(2) and read with getsockopt(2) by specifying SOL_SOCKET
     as the socket family.
            Enables  the receiving of the credentials of the sending process
            in an ancillary message.  When this option is set and the socket
            is  not  yet  connected  a unique name in the abstract namespace
            will be generated automatically.   Expects  an  integer  boolean
 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  unsup-
     ported features of the sockets API for UNIX domain sockets on Linux.
     UNIX domain sockets do not support the transmission of out-of-band data
     (the MSG_OOB flag for send(2) and recv(2)).
     The send(2) MSG_MORE flag is not supported by UNIX domain sockets.
     Before Linux 3.4, the use of MSG_TRUNC in the flags argument of recv(2)
     was not supported by UNIX domain sockets.
     The  SO_SNDBUF  socket option does have an effect for UNIX domain sock-
     ets, but the SO_RCVBUF option does  not.   For  datagram  sockets,  the
     SO_SNDBUF  value  imposes  an upper limit on the size of outgoing data-
     grams.  This limit is calculated as the doubled (see socket(7))  option
     value less 32 bytes used for overhead.
 Ancillary messages
     Ancillary  data  is  sent and received using sendmsg(2) and recvmsg(2).
     For historical reasons the ancillary message  types  listed  below  are
     specified with a SOL_SOCKET type even though they are AF_UNIX specific.
     To send them  set  the  cmsg_level  field  of  the  struct  cmsghdr  to
     SOL_SOCKET  and  the cmsg_type field to the type.  For more information
     see cmsg(3).
            Send or receive a set of  open  file  descriptors  from  another
            process.  The data portion contains an integer array of the file
            descriptors.  The passed file descriptors behave as though  they
            have been created with dup(2).
            Send  or receive UNIX credentials.  This can be used for authen-
            tication.  The credentials are passed as a struct  ucred  ancil-
            lary  message.   Thus  structure is defined in <sys/socket.h> as
                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 process with effective user ID 0 is allowed to spec-
            ify values that do not match its own.  The sender  must  specify
            its own process ID (unless it has the capability CAP_SYS_ADMIN),
            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.
     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 socket the function returns the amount of queued
            unread data 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
            socket, the returned value is the same as  for  Internet  domain
            datagram socket; 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 descrip-
            tor  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 SIG-
            PIPE 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.
            This error can occur for sendmsg(2) when sending a file descrip-
            tor  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 capabil-
            ity.   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 appro-
     priate 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 implementa-
     tions that appends a null terminator if none is supplied  in  sun_path.
     In  most  cases  this  is  unproblematic:  when  the  socket address is
     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 path-
     name 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
     In addition, some implementations don't require a null terminator  when
     binding  a socket (the addrlen argument is used to determine the length
     of sun_path) and when the socket address is retrieved on  these  imple-
     mentations, there is no null terminator in sun_path.
     Applications that retrieve socket addresses can (portably) code to han-
     dle the possibility that there is no null  terminator  in  sun_path  by
     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 allo-
     cating a buffer of size sizeof(struct sockaddr_un)+1 that is zeroed out
     before  the  retrieval.   The  retrieving  call  can specify addrlen as
     sizeof(struct sockaddr_un), and the extra zero byte ensures that  there
     will be a null terminator for the string returned in sun_path:
         void *addrp;
         addrlen  = sizeof(struct sockaddr_un); addrp = malloc(addrlen + 1);
         if (addrp == NULL)
             /* Handle error */ ; memset(addrp, 0, addrlen + 1);
         if (getsockname(sfd, (struct sockaddr *) addrp, &addrlen)) == -1)
             /* handle error */ ;
         printf("sun_path    =    %s\n",     ((struct     sockaddr_un     *)
     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 back-
     ground  and  repeatedly  executing the client.  Execution of the server
     program ends when it receives the "DOWN" command.
 Example output
         $ ./server & [1] 25887 $ ./client 3 4 Result = 7 $ ./client  11  -5
         Result   =   6   $   ./client   DOWN   Result   =   0   [1]+   Done
         ./server $
 Program source
      * File connection.h
     #define SOCKET_NAME "/tmp/9Lq7BNBnBycd6nxy.socket" #define  BUFFER_SIZE
      * File server.c
     #include  <stdio.h>  #include  <stdlib.h>  #include <string.h> #include
     <sys/socket.h> #include <sys/un.h> #include <unistd.h>  #include  "con-
     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.
         /* 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. */
         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) {
          * 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. */
         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), capabili-
     ties(7), credentials(7), socket(7), udp(7)


     This page is part of release 4.16 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

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