UNIX(7)                                             Linux Programmer's Manual                                            UNIX(7)



NAME
       unix, AF_UNIX, AF_LOCAL - 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 effi-
       ciently.  Traditionally, Unix sockets can be either unnamed, or bound to a file system pathname (marked as being 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 sockets are always reliable and  don't  reorder
       datagrams);  and  (since  Linux 2.6.4) SOCK_SEQPACKET, for a connection-oriented socket that preserves message boundaries
       and delivers messages in the order that they were sent.

       Unix 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:

           #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.

       Three types of address are distinguished in this structure:

       *  pathname: a Unix domain socket can be bound to a null-terminated file system pathname using bind(2).  When the address
          of  the  socket  is  returned  by  getsockname(2),  getpeername(2), and accept(2), its length is sizeof(sa_family_t) +
          strlen(sun_path) + 1, and sun_path contains the null-terminated pathname.

       *  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 by getsockname(2), getpeer-
          name(2), and accept(2), its length is sizeof(sa_family_t), and sun_path should not be inspected.

       *  abstract: an abstract socket address is distinguished by the fact that sun_path[0] is a null byte ('\0').  All of  the
          remaining  bytes  in sun_path define the "name" of the socket.  (Null bytes in the name have no special significance.)
          The name has no connection with file system pathnames.  The socket's address in this namespace is given by the rest of
          the  bytes  in  sun_path.   When  the address of an abstract socket is returned by getsockname(2), getpeername(2), and
          accept(2), its length is sizeof(struct sockaddr_un), and sun_path contains the abstract  name.   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
              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.

   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.

       The use of MSG_TRUNC in the flags argument of recv(2) is 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.  The passed file descriptors behave as though they have been created with dup(2).

       SCM_CREDENTIALS
              Send or receive Unix credentials.  This can be used for authentication.  The credentials are passed  as  a  struct
              ucred ancillary message.  Thus 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 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  specify  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 socket.

ERRORS
       EADDRINUSE
              Selected local address is already taken or file system socket object already exists.

       ECONNREFUSED
              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.

       ECONNRESET
              Remote socket was unexpectedly closed.

       EFAULT User memory address was not valid.

       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.

       EISCONN
              connect(2) called on an already connected socket or a target address was specified on a connected socket.

       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 pass-
              ing the MSG_NOSIGNAL flag to sendmsg(2) or recvmsg(2).

       EPROTONOSUPPORT
              Passed protocol is not AF_UNIX.

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

       ESOCKTNOSUPPORT
              Unknown socket type.

       Other  errors  can  be  generated by the generic socket layer or by the file system while generating a file system 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
       In the Linux implementation, sockets which are visible in the file system honor 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) permission 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
       sockets.  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 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.

EXAMPLE
       See bind(2).

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

COLOPHON
       This page is part of release 3.25 of the Linux man-pages project.  A description of the project,  and  information  about
       reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                                                      2008-12-01                                                    UNIX(7)