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SOCKET(7)		   Linux Programmers Manual		    SOCKET(7)

       socket - Linux socket interface


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

       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  PF_INET,
       PF_IPX, PF_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 PF_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 non-zero position is
       not supported on sockets.

       It is possible to do non-blocking 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				    |
       |Read	   | POLLIN    | New data arrived.			    |
       |Read	   | POLLIN    | A connection setup has been completed (for |
       |	   |	       | connection-oriented sockets)		    |
       |Read	   | POLLHUP   | A disconnection request has been initiated |
       |	   |	       | by the other end.			    |
       |Read	   | POLLHUP   | A connection is broken (only  for  connec |
       |	   |	       | tion-oriented protocols).  When the socket |
       |	   |	       | is written SIGPIPE is also sent.	    |
       |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:

	      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.	Can only be read with getsock

	      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(8) there).

	      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.

	      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
	      generates a kernel warning (printk()) if	a  program  uses  this
	      option.	Linux  2.0  also  enabled BSD bug-to-bug compatibility
	      options (random header changing, skipping of the broadcast flag)
	      for  raw sockets with this option, but that was removed in Linux

	      Enable socket debugging.	Only allowed for  processes  with  the
	      CAP_NET_ADMIN capability or an effective user ID of 0.

	      Get  and	clear  the pending socket error.  Only valid as a get
	      sockopt(2).  Expects an integer.

	      Dont 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

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

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

		  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.

	      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.

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

	      Return  the credentials of the foreign process connected to this
	      socket.  This is only  possible  for  connected  PF_UNIX	stream
	      sockets  and  PF_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.  Only valid as
	      a getsockopt(2).

	      Set  the protocol-defined priority for all packets to be sent on
	      this socket.  Linux uses this  value  to	order  the  networking
	      queues:  packets	with  a higher priority may be processed first
	      depending on  the  selected  device  queueing  discipline.   For
	      ip(7),  this  also  sets	the IP type-of-service (TOS) field for
	      outgoing packets.  Setting a priority outside the range 0  to  6
	      requires the CAP_NET_ADMIN capability.

	      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
	      rmem_default  sysctl and the maximum allowed value is set by the
	      rmem_max sysctl.	The minimum (doubled) value for this option is

       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.

	      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.

	      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  non-blocking.   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.

	      Indicates  that  the rules used in validating addresses supplied
	      in a bind(2) call should allow reuse of  local  addresses.   For
	      PF_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.

	      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
	      wmem_default  sysctl and the maximum allowed value is set by the
	      wmem_max sysctl.	The minimum (doubled) value for this option is

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

	      Enable or disable the receiving of the SO_TIMESTAMP control mes
	      sage.   The  timestamp  control  message	is  sent  with	 level
	      SOL_SOCKET  and the cmsg_data field is a struct timeval indicat
	      ing the reception time of the last packet passed to the user  in
	      this call.  See cmsg(3) for details on control messages.

	      Gets the socket type as an integer (like SOCK_STREAM).  Can only
	      be read with getsockopt(2).

       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.

       The  core  socket  networking  sysctls  can  be	accessed   using   the
       /proc/sys/net/core/* files or with the sysctl(2) interface.

	      contains	the  default  setting  in  bytes of the socket receive

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

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

	      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.

	      Maximum number of packets in the global input queue.

	      Maximum length of ancillary data and user control data like  the
	      iovecs per socket.

       These operations can be accessed using ioctl(2):

	   error = ioctl(ip_socket, ioctl_type, &value_result);

	      Return  a  struct timeval with the receive timestamp of the last
	      packet passed to the user.  This is useful  for  accurate  round
	      trip  time  measurements.  See setitimer(2) for a description of
	      struct timeval.  This ioctl should only be used  if  the	socket
	      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).

	      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.

	      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.)

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

       Valid fcntl(2) operations:

	      The same as the SIOCGPGRP ioctl(2).

	      The same as the SIOCSPGRP ioctl(2).

       SO_BINDTODEVICE was introduced in Linux 2.0.30.	SO_PASSCRED is new  in
       Linux  2.2.  The sysctls are new in Linux 2.2.  SO_RCVTIMEO and SO_SND
       TIMEO are supported since Linux 2.3.41.	Earlier, timeouts  were  fixed
       to a protocol-specific setting, and could not be read or written.

       Linux assumes that half of the send/receive buffer is used for internal
       kernel structures; thus the sysctls are twice what can be  observed  on
       the wire.

       Linux  will  only  allow  port re-use 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 re-use 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 invisible,  since,  for  example,	a  server  program  is
       designed to always set this option.

       are not documented.  The suggested interface to use  them  is  via  the
       libpcap library.

       getsockopt(2),	setsockopt(2),	 socket(2),  capabilities(7),  ddp(7),
       ip(7), packet(7)

       This page is part of release 3.05 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				  2007-12-28			     SOCKET(7)

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