socket


SYNOPSIS
       #include <sys/socket.h>

       sockfd = socket(int socket_family, int socket_type, int protocol);

DESCRIPTION
       This  manual  page  describes  the  Linux  networking socket layer user
       interface.  The  BSD  compatible  sockets  are  the  uniform  interface
       between the user process and the network protocol stacks in the kernel.
       The protocol modules are grouped into protocol families  like  AF_INET,
       AF_IPX, AF_PACKET and socket types like SOCK_STREAM or SOCK_DGRAM.  See
       socket(2) for more information on families and types.

   Socket Layer Functions
       These functions are used by the user process to send or receive packets
       and  to  do  other  socket  operations.  For more information see their
       respective manual pages.

       socket(2) creates a socket, connect(2) connects a socket  to  a  remote
       socket  address,  the bind(2) function binds a socket to a local socket
       address, listen(2) tells the  socket  that  new  connections  shall  be
       accepted, and accept(2) is used to get a new socket with a new incoming
       connection.  socketpair(2)  returns  two  connected  anonymous  sockets
       (only implemented for a few local families like AF_UNIX)

       send(2),  sendto(2),  and  sendmsg(2)  send  data  over  a  socket, and
       recv(2), recvfrom(2), recvmsg(2) receive data from a  socket.   poll(2)
       and  select(2)  wait for arriving data or a readiness to send data.  In
       addition, the standard I/O operations like write(2),  writev(2),  send-
       file(2), read(2), and readv(2) can be used to read and write data.

       getsockname(2)  returns  the  local  socket  address and getpeername(2)
       returns the remote socket address.  getsockopt(2) and setsockopt(2) are
       used  to  set or get socket layer or protocol options.  ioctl(2) can be
       used to set or read some other options.

       close(2) is used to close a socket.   shutdown(2)  closes  parts  of  a
       full-duplex socket connection.

       Seeking,  or  calling  pread(2) or pwrite(2) with a nonzero position is
       not supported on sockets.

       It is possible to do nonblocking I/O on sockets by setting  the  O_NON-
       BLOCK flag on a socket file descriptor using fcntl(2).  Then all opera-
       tions that would block will (usually)  return  with  EAGAIN  (operation
       should  be  retried  later);  connect(2) will return EINPROGRESS error.
       The user can then wait for various events via poll(2) or select(2).

       +--------------------------------------------------------------------+
       |                            I/O events                              |
       +-----------+-----------+--------------------------------------------+
       |Event      | Poll flag | Occurrence                                 |
       +-----------+-----------+--------------------------------------------+
       |Write      | POLLOUT   | Socket has enough send  buffer  space  for |
       |           |           | writing new data.                          |
       +-----------+-----------+--------------------------------------------+
       |Read/Write | POLLIN|   | An outgoing connect(2) finished.           |
       |           | POLLOUT   |                                            |
       +-----------+-----------+--------------------------------------------+
       |Read/Write | POLLERR   | An asynchronous error occurred.            |
       +-----------+-----------+--------------------------------------------+
       |Read/Write | POLLHUP   | The other end has shut down one direction. |
       +-----------+-----------+--------------------------------------------+
       |Exception  | POLLPRI   | Urgent data arrived.  SIGURG is sent then. |
       +-----------+-----------+--------------------------------------------+

       An alternative to poll(2) and select(2) is to let the kernel inform the
       application about events via a SIGIO signal.  For that the O_ASYNC flag
       must be set on a socket file descriptor via fcntl(2) and a valid signal
       handler for SIGIO must be installed via sigaction(2).  See the  Signals
       discussion below.

   Socket Options
       These  socket  options  can be set by using setsockopt(2) and read with
       getsockopt(2) with the socket level set to SOL_SOCKET for all sockets:

       SO_ACCEPTCONN
              Returns a value indicating whether or not this socket  has  been
              marked  to accept connections with listen(2).  The value 0 indi-
              cates that this is not a listening socket, the value 1 indicates
              that  this  is  a listening socket.  This socket option is read-
              only.

       SO_BINDTODEVICE
              Bind this socket to a particular device like "eth0",  as  speci-
              fied  in  the  passed  interface  name.  If the name is an empty
              string or the option length is zero, the socket  device  binding
              is  removed.  The passed option is a variable-length null-termi-
              nated interface name string with the maximum size  of  IFNAMSIZ.
              If a socket is bound to an interface, only packets received from
              that particular interface are processed  by  the  socket.   Note
              that this only works for some socket types, particularly AF_INET
              sockets.  It is not supported for  packet  sockets  (use  normal
              bind(2) there).

       SO_BROADCAST
              Set  or  get the broadcast flag.  When enabled, datagram sockets
              receive packets sent to a broadcast address and they are allowed
              to  send  packets  to  a  broadcast address.  This option has no
              effect on stream-oriented sockets.

       SO_BSDCOMPAT
              Enable BSD bug-to-bug compatibility.  This is used  by  the  UDP
              protocol  module  in  Linux 2.0 and 2.2.  If enabled ICMP errors
              received for a UDP socket will not be passed to  the  user  pro-
              gram.   In  later  kernel  versions, support for this option has
              been phased out: Linux 2.4 silently ignores it,  and  Linux  2.6
              such  as  AF_INET6.   See  socket(2)  for  details.  This socket
              option is read-only.

       SO_ERROR
              Get and clear the pending socket error.  This socket  option  is
              read-only.  Expects an integer.

       SO_DONTROUTE
              Don't send via a gateway, only send to directly connected hosts.
              The same effect can be achieved  by  setting  the  MSG_DONTROUTE
              flag  on a socket send(2) operation.  Expects an integer boolean
              flag.

       SO_KEEPALIVE
              Enable sending of  keep-alive  messages  on  connection-oriented
              sockets.  Expects an integer boolean flag.

       SO_LINGER
              Sets  or  gets  the  SO_LINGER option.  The argument is a linger
              structure.

                  struct linger {
                      int l_onoff;    /* linger active */
                      int l_linger;   /* how many seconds to linger for */
                  };

              When enabled, a close(2) or shutdown(2) will  not  return  until
              all  queued  messages for the socket have been successfully sent
              or the linger timeout has been  reached.   Otherwise,  the  call
              returns  immediately  and the closing is done in the background.
              When the socket is closed as part of exit(2), it always  lingers
              in the background.

       SO_OOBINLINE
              If  this  option is enabled, out-of-band data is directly placed
              into the receive data stream.   Otherwise  out-of-band  data  is
              only passed when the MSG_OOB flag is set during receiving.

       SO_PASSCRED
              Enable  or  disable the receiving of the SCM_CREDENTIALS control
              message.  For more information see unix(7).

       SO_PEERCRED
              Return the credentials of the foreign process connected to  this
              socket.   This  is  only  possible  for connected AF_UNIX stream
              sockets and AF_UNIX stream and  datagram  socket  pairs  created
              using  socketpair(2); see unix(7).  The returned credentials are
              those that were in effect at the time of the call to  connect(2)
              or  socketpair(2).   Argument is a ucred structure.  This socket
              option is read-only.

       SO_PRIORITY
              Set the protocol-defined priority for all packets to be sent  on
              this  socket.   Linux  uses  this  value to order the networking
              Sets or gets the maximum socket receive buffer  in  bytes.   The
              kernel  doubles this value (to allow space for bookkeeping over-
              head) when it is set using setsockopt(2), and this doubled value
              is  returned  by getsockopt(2).  The default value is set by the
              /proc/sys/net/core/rmem_default file, and  the  maximum  allowed
              value is set by the /proc/sys/net/core/rmem_max file.  The mini-
              mum (doubled) value for this option is 256.

       SO_RCVBUFFORCE (since Linux 2.6.14)
              Using this socket option, a privileged  (CAP_NET_ADMIN)  process
              can  perform  the same task as SO_RCVBUF, but the rmem_max limit
              can be overridden.

       SO_RCVLOWAT and SO_SNDLOWAT
              Specify the minimum number of bytes  in  the  buffer  until  the
              socket layer will pass the data to the protocol (SO_SNDLOWAT) or
              the user on receiving (SO_RCVLOWAT).  These two values are  ini-
              tialized to 1.  SO_SNDLOWAT is not changeable on Linux (setsock-
              opt(2)  fails  with  the  error  ENOPROTOOPT).   SO_RCVLOWAT  is
              changeable only since Linux 2.4.  The select(2) and poll(2) sys-
              tem calls currently do not respect the  SO_RCVLOWAT  setting  on
              Linux,  and  mark  a  socket readable when even a single byte of
              data is available.  A subsequent read from the socket will block
              until SO_RCVLOWAT bytes are available.

       SO_RCVTIMEO and SO_SNDTIMEO
              Specify  the  receiving  or  sending timeouts until reporting an
              error.  The argument is a struct timeval.  If an input or output
              function  blocks for this period of time, and data has been sent
              or received, the return value  of  that  function  will  be  the
              amount  of data transferred; if no data has been transferred and
              the timeout has been reached then -1 is returned with errno  set
              to  EAGAIN or EWOULDBLOCK just as if the socket was specified to
              be nonblocking.  If the timeout is set  to  zero  (the  default)
              then  the  operation  will  never  timeout.   Timeouts only have
              effect for system calls that perform socket I/O (e.g.,  read(2),
              recvmsg(2),  send(2),  sendmsg(2));  timeouts have no effect for
              select(2), poll(2), epoll_wait(2), etc.

       SO_REUSEADDR
              Indicates that the rules used in validating  addresses  supplied
              in  a  bind(2)  call should allow reuse of local addresses.  For
              AF_INET sockets this means that a socket may bind,  except  when
              there  is an active listening socket bound to the address.  When
              the listening socket is bound to INADDR_ANY with a specific port
              then  it  is  not  possible  to  bind to this port for any local
              address.  Argument is an integer boolean flag.

       SO_SNDBUF
              Sets or gets the maximum socket send buffer in bytes.  The  ker-
              nel doubles this value (to allow space for bookkeeping overhead)
              when it is set using setsockopt(2), and this  doubled  value  is
              returned  by  getsockopt(2).   The  default  value is set by the
              /proc/sys/net/core/wmem_default file  and  the  maximum  allowed
              ing  the reception time of the last packet passed to the user in
              this call.  See cmsg(3) for details on control messages.

       SO_TYPE
              Gets the socket type as an integer  (e.g.,  SOCK_STREAM).   This
              socket option is read-only.

   Signals
       When  writing onto a connection-oriented socket that has been shut down
       (by the local or the remote end) SIGPIPE is sent to the writing process
       and  EPIPE  is  returned.   The  signal is not sent when the write call
       specified the MSG_NOSIGNAL flag.

       When requested with the FIOSETOWN fcntl(2) or SIOCSPGRP ioctl(2), SIGIO
       is  sent  when  an  I/O event occurs.  It is possible to use poll(2) or
       select(2) in the signal handler to find  out  which  socket  the  event
       occurred  on.  An alternative (in Linux 2.2) is to set a real-time sig-
       nal using the F_SETSIG fcntl(2); the handler of the  real  time  signal
       will  be called with the file descriptor in the si_fd field of its sig-
       info_t.  See fcntl(2) for more information.

       Under some circumstances (e.g., multiple processes accessing  a  single
       socket),  the  condition  that caused the SIGIO may have already disap-
       peared when the process reacts to the signal.   If  this  happens,  the
       process should wait again because Linux will resend the signal later.

   /proc interfaces
       The  core socket networking parameters can be accessed via files in the
       directory /proc/sys/net/core/.

       rmem_default
              contains the default setting in bytes of the socket receive buf-
              fer.

       rmem_max
              contains the maximum socket receive buffer size in bytes which a
              user may set by using the SO_RCVBUF socket option.

       wmem_default
              contains the default setting in bytes of the socket send buffer.

       wmem_max
              contains the maximum socket send buffer size in  bytes  which  a
              user may set by using the SO_SNDBUF socket option.

       message_cost and message_burst
              configure  the  token  bucket  filter used to load limit warning
              messages caused by external network events.

       netdev_max_backlog
              Maximum number of packets in the global input queue.

       optmem_max
              Maximum length of ancillary data and user control data like  the
              option  SO_TIMESTAMP  is  not  set on the socket.  Otherwise, it
              returns the timestamp of the last packet that was received while
              SO_TIMESTAMP was not set, or it fails if no such packet has been
              received, (i.e., ioctl(2) returns -1 with errno set to ENOENT).

       SIOCSPGRP
              Set the process or process group to send SIGIO or SIGURG signals
              to  when  an  asynchronous  I/O operation has finished or urgent
              data is available.  The argument is a pointer to  a  pid_t.   If
              the  argument is positive, send the signals to that process.  If
              the argument is negative, send the signals to the process  group
              with  the ID of the absolute value of the argument.  The process
              may only choose itself or its own process group to receive  sig-
              nals  unless  it has the CAP_KILL capability or an effective UID
              of 0.

       FIOASYNC
              Change the O_ASYNC flag to enable or  disable  asynchronous  I/O
              mode  of the socket.  Asynchronous I/O mode means that the SIGIO
              signal or the signal set with F_SETSIG is raised when a new  I/O
              event occurs.

              Argument is an integer boolean flag.  (This operation is synony-
              mous with the use of fcntl(2) to set the O_ASYNC flag.)

       SIOCGPGRP
              Get the current process or process group that receives SIGIO  or
              SIGURG signals, or 0 when none is set.

       Valid fcntl(2) operations:

       FIOGETOWN
              The same as the SIOCGPGRP ioctl(2).

       FIOSETOWN
              The same as the SIOCSPGRP ioctl(2).

VERSIONS
       SO_BINDTODEVICE  was introduced in Linux 2.0.30.  SO_PASSCRED is new in
       Linux 2.2.  The /proc interfaces was introduced in Linux 2.2.   SO_RCV-
       TIMEO and SO_SNDTIMEO are supported since Linux 2.3.41.  Earlier, time-
       outs were fixed to a protocol-specific setting, and could not  be  read
       or written.

NOTES
       Linux assumes that half of the send/receive buffer is used for internal
       kernel structures; thus the values in the corresponding /proc files are
       twice what can be observed on the wire.

       Linux will only allow port reuse with the SO_REUSEADDR option when this
       option was set both in the previous program that performed a bind(2) to
       the port and in the program that wants to reuse the port.  This differs
       from some implementations (e.g., FreeBSD) where only the later  program
       needs  to  set  the  SO_REUSEADDR option.  Typically this difference is

COLOPHON
       This page is part of release 3.35 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at http://man7.org/linux/man-pages/.



Linux                             2010-06-13                         SOCKET(7)
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