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 connection-oriented socket
       that preserves message boundaries and delivers messages  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.  The abstract socket  names-
          pace is a nonportable Linux extension.

   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.)

   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.

       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 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 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

       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.

              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 sendmsg(2) or recvmsg(2).

              Passed protocol is not AF_UNIX.

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

              Unknown socket type.

       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

       In  the Linux implementation, sockets which are visible in the filesys-
       tem honor the permissions of the directory they are in.   Their  owner,
       group,  and  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 domain sockets.
       Portable programs should not rely on this feature for security.

       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, 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

       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 *) 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.

       See bind(2).

       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 4.04 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                             2015-05-07                           UNIX(7)
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