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

       socket - Linux socket interface

       #include <sys/socket.h>
       mysocket = socket(int socket_family, int socket_type, int protocol);

       This  manual page describes the Linux networking socket layer user interface. The BSD com-
       patible sockets are the uniform interface between the user process and the network  proto-
       col  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.

       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 imple-
       mented 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), sendfile(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 con-

       Seeking, or calling pread(2) or pwrite(2) with a non-zero position  is  not  supported  on

       It is possible to do non-blocking IO on sockets by setting the O_NONBLOCK flag on a socket
       file descriptor using fcntl(2).	Then all  operations  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/select is to let the kernel inform the application about events via
       a SIGIO signal. For that the FASYNC 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.

       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:

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

	      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.

	      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 not changeable in Linux and	their  argument  size  is  always
	      fixed  to  1  byte.  getsockopt is able to read them; setsockopt will always return

	      Specify the sending or receiving timeouts until reporting an error.  They are fixed
	      to  a protocol specific setting in Linux and cannot be read or written. Their func-
	      tionality can be emulated using alarm(2) or setitimer(2).

	      Enable BSD bug-to-bug compatibility. This is used only by the UDP  protocol  module
	      and  scheduled  to be removed in future.	If enabled ICMP errors received for a UDP
	      socket will not be passed to the user program. 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 has been removed in  Linux  2.2.  It  is
	      better to fix the user programs than to enable this flag.

	      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.  Only  use-
	      ful for PF_UNIX sockets; see unix(7).  Argument is a ucred structure. Only valid as
	      a getsockopt.

	      Bind this socket to a particular device like "eth0", as  specified  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  ter-
	      minated  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, particu-
	      larly AF_INET sockets. It is not supported for packet sockets (use  normal  bind(8)

	      Enable socket debugging. Only allowed for processes with the CAP_NET_ADMIN capabil-
	      ity or an effective user id of 0.

	      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.

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

	      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.

	      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.

	      Sets  or gets the maximum socket send buffer in bytes.  The default value is set by
	      the wmem_default sysctl and the maximum  allowed	value  is  set	by  the  wmem_max

	      Sets  or	gets the maximum socket receive buffer in bytes. The default value is set
	      by the rmem_default sysctl and the maximum allowed value is  set	by  the  rmem_max

	      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.

	      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.

	      Get  and	clear  the  pending socket error. Only valid as a getsockopt.  Expects an

       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 or SIOCSPGRP ioctl, 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  realtime
       signal  using  the F_SETSIG fcntl; the handler of the real time signal will be called with
       the file descriptor in the si_fd field of its siginfo_t.  See fcntl(2) for  more  informa-

       Under  some  circumstances (e.g. multiple processes accessing a single socket), the condi-
       tion that caused the SIGIO may have already disappeared when the  process  reacts  to  the
       signal.	If this happens, the process should wait again because Linux will resend the sig-
       nal 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 buffer.

	      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 ioctls 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.

	      Set the process or process group to send SIGIO or SIGURG signals to when	an  asyn-
	      chronous 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 signals unless it has the CAP_KILL capability or an effec-
	      tive UID of 0.

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

	      Argument is a integer boolean flag.

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

       Valid fcntls:

	      The same as the SIOCGPGRP ioctl.

	      The same as the SIOCSPGRP ioctl

       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.

       The CONFIG_FILTER socket options SO_ATTACH_FILTER and SO_DETACH_FILTER are not documented.
       The suggested interface to use them is via the libpcap library.

       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.

       This man page was written by Andi Kleen.

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

Linux Man Page				    1999-05-07					SOCKET(7)
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