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

       socket - Linux socket interface

       #include <sys/socket.h>

       sockfd = 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  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 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.

	      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

	      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 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.	 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
	      /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.

	      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
	      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.

	      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
	      value is set by the /proc/sys/net/core/wmem_max file.  The mini‐
	      mum (doubled) value for this option is 2048.

       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.

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

	      contains the default setting in bytes of the socket receive buf‐

	      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 /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.

       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 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), tcp(7), udp(7), unix(7)

       This page is part of release 3.22 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at

Linux				  2008-12-03			     SOCKET(7)

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