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

NAME
       socket - Linux socket interface

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				    │
       ├───────────┼───────────┼────────────────────────────────────────────┤
       │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:

       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
	      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
	      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
	      2.2.

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

       SO_DOMAIN (since Linux 2.6.32)
	      Retrieves	 the  socket  domain  as an integer, returning a value
	      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_MARK (since Linux 2.6.25)
	      Set the mark for each packet sent through this  socket  (similar
	      to  the  netfilter  MARK target but socket-based).  Changing the
	      mark can be used for mark-based routing without netfilter or for
	      packet	filtering.    Setting	this   option	requires   the
	      CAP_NET_ADMIN capability.

       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
	      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.

       SO_PROTOCOL (since Linux 2.6.32)
	      Retrieves	 the  socket protocol as an integer, returning a value
	      such as IPPROTO_SCTP.  See socket(2) for details.	  This	socket
	      option is read-only.

       SO_RCVBUF
	      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), and so on.

       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
	      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.

       SO_TIMESTAMP
	      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.

       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
	      iovecs per socket.

   Ioctls
       These operations can be accessed using ioctl(2):

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

       SIOCGSTAMP
	      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).

       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
       invisible,  since,  for example, a server program is designed to always
       set this option.

BUGS
       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.

SEE ALSO
       getsockopt(2),  setsockopt(2),  socket(2),   capabilities(7),   ddp(7),
       ip(7), packet(7), tcp(7), udp(7), unix(7)

COLOPHON
       This  page  is  part of release 3.44 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				  2012-07-07			     SOCKET(7)
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