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        unix,  PF_UNIX,  AF_UNIX, PF_LOCAL, AF_LOCAL - Sockets for local inter‐
        process communication


        #include <sys/socket.h>
        #include <sys/un.h>
        unix_socket = socket(PF_UNIX, type, 0);
        error = socketpair(PF_UNIX, type, 0, int *sv);


        The PF_UNIX (also known as PF_LOCAL) socket family is used to  communi‐
        cate  between  processes  on the same machine efficiently. Unix sockets
socketpair(2)) or associated with a
        file  of  type socket.  Linux also supports an abstract namespace which
        is independent of the file system.
        Valid  types  are:  SOCK_STREAM,  for  a  stream-oriented  socket   and
        SOCK_DGRAM,  for  a  datagram-oriented  socket  that  preserves message
        boundaries (as on most Unix implementations, Unix domain datagram sock‐
        ets are always reliable and don’t reorder datagrams); and (since kernel
        2.6.4) SOCK_SEQPACKET, for a connection-oriented socket that  preserves
        message  boundaries  and  delivers messages in the order that they were
        Unix sockets support passing file descriptors or process credentials to
        other processes using ancillary data.
        A  Unix  address  is  defined  as  a filename in the filesystem or as a
        unique string in the abstract namespace.  Sockets  created  by  socket     
pair(2) are anonymous. For non-anonymous sockets the target address can
        When  a socket is connected and it doesn’t already have a local address
        a unique address in the abstract namespace will be generated  automati‐
          #define UNIX_PATH_MAX    108
          struct sockaddr_un {
              sa_family_t    sun_family;               /* AF_UNIX */
              char           sun_path[UNIX_PATH_MAX];  /* pathname */
        sun_family  always contains AF_UNIX.  sun_path contains the zero-termi‐
        nated pathname of the socket in the file system.   If  sun_path  starts
        with a null byte (’’ ’), then it refers to the abstract namespace main‐
        tained by the Unix protocol  module.   The  socket’s  address  in  this
        namespace  is  given  by  the rest of the bytes in sun_path.  Note that
        names in the abstract namespace are not zero-terminated.
        For historical reasons  these  socket  options  are  specified  with  a
        SOL_SOCKET type even though they are PF_UNIX specific.  They can be set
getsockopt(2) by specifying SOL_SOCKET
        as the socket family.
               Enables  the receiving of the credentials of the sending process
               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 flag.
        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
send(2) MSG_MORE flag is 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‐
socket(7)) option
        value less 32 bytes used for overhead.
        For  historical  reasons  the  ancillary message types listed below are
        specified with a SOL_SOCKET type even though they are PF_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
               Send or receive a set of open file descriptors from another pro‐
               cess.  The data portion contains an integer array  of  the  file
               descriptors.   The passed file descriptors behave as though they
               Send or receive Unix credentials.  This can be used for  authen‐
               tication.   The  credentials are passed as a struct ucred ancil‐
               lary message.
                 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 */
               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 user ID, effective user ID, or saved set-user-ID (unless  it
               has  CAP_SETUID), and its 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


        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


        In  the Linux implementation, sockets which are visible in the filesys‐
        tem honour the permissions of the directory they are in.  Their  owner,
        group  and  their permissions can be changed.  Creation of a new socket
        will fail if the process does not have write and search (execute)  per‐
        mission  on  the directory the socket is created in.  Connecting to the
        socket object requires read/write permission.   This  behavior  differs
        from  many  BSD-derived systems which ignore permissions for Unix sock‐
        ets. Portable programs should not rely on this feature for security.
        Binding to a socket with a filename creates a socket in the file system
        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 file
        system when the last reference to it is closed.
        To pass file descriptors or credentials over a SOCK_STREAM, you need to
        send  or  receive  at  least one byte of non-ancillary data in the same
        sendmsg() or recvmsg() call.
        Unix domain stream sockets do not support  the  notion  of  out-of-band


        ENOMEM Out of memory.
connect(2)  called  with  a  socket object that isn’t listening.
               This can happen when the remote socket does  not  exist  or  the
               filename is not a socket.
        EINVAL Invalid  argument  passed. A common cause is the missing setting
               of AF_UNIX in the sun_type field  of  passed  addresses  or  the
               socket being in an invalid state for the applied operation.
               Stream  operation  called on non-stream oriented socket or tried
               to use the out-of-band data option.
               Passed protocol is not PF_UNIX.
               Unknown socket type.
               Remote socket does not match the local socket  type  (SOCK_DGRAM
               vs.  SOCK_STREAM)
               Selected  local  address  is  already taken or filesystem socket
               object already exists.
connect(2) called on an already connected  socket  or  a  target
               address was specified on a connected socket.
               Socket  operation  needs a target address, but the socket is not
               Remote socket was unexpectedly closed.
        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
        EFAULT User memory address was not valid.
        EPERM  The sender passed invalid credentials in the struct ucred.
        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.
cmsg(3),  capabili     


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