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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  such  as
       AF_INET, AF_IPX, and AF_PACKET, and socket types such as 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
       (implemented only 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 address structures
       Each  socket  domain  has  its  own format for socket addresses, with a
       domain-specific address structure.  Each	 of  these  structures	begins
       with  an	 integer  "family" field (typed as sa_family_t) that indicates
       the type of the address structure.   This  allows  the  various	system
       calls  (e.g.,  connect(2), bind(2), accept(2), getsockname(2), getpeer‐
       name(2)), which are generic to all socket  domains,  to	determine  the
       domain of a particular socket address.

       To  allow  any type of socket address to be passed to interfaces in the
       sockets API, the type struct sockaddr is defined.  The purpose of  this
       type is purely to allow casting of domain-specific socket address types
       to a "generic" type, so as to avoid compiler warnings about  type  mis‐
       matches in calls to the sockets API.

       In  addition,  the  sockets  API	 provides  the	data type struct sock‐
       addr_storage.  This type	 is  suitable  to  accommodate	all  supported
       domain-specific	socket	address	 structures; it is large enough and is
       aligned properly.  (In particular, it is	 large	enough	to  hold  IPv6
       socket  addresses.)   The structure includes the following field, which
       can be used to identify the type of socket address actually  stored  in
       the structure:

	       sa_family_t ss_family;

       The  sockaddr_storage  structure is useful in programs that must handle
       socket addresses in a generic way (e.g., programs that must  deal  with
       both IPv4 and IPv6 socket addresses).

   Socket options
       The  socket  options listed below can be set by using setsockopt(2) and
       read with getsockopt(2) with the socket level set to SOL_SOCKET for all
       sockets.	 Unless otherwise noted, optval is a pointer to an int.

	      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-

	      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 works only for some socket types, particularly AF_INET
	      sockets.	 It  is	 not  supported for packet sockets (use normal
	      bind(2) there).

	      Before Linux 3.8, this socket option could be set, but could not
	      retrieved	 with getsockopt(2).  Since Linux 3.8, it is readable.
	      The optlen argument should contain the buffer size available  to
	      receive  the device name and is recommended to be IFNAMSZ bytes.
	      The real device name length is reported back in the optlen argu‐

	      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.

	      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.

       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.

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

	      Don't send via a gateway, send only 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.

       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.

	      If this option is enabled, out-of-band data is  directly	placed
	      into  the	 receive  data	stream.	 Otherwise out-of-band data is
	      passed only 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).

       SO_PEEK_OFF (since Linux 3.4)
	      This option, which is currently supported only for unix(7) sock‐
	      ets, sets the value of the "peek offset" for the recv(2)	system
	      call when used with MSG_PEEK flag.

	      When this option is set to a negative value (it is set to -1 for
	      all new sockets), traditional behavior is provided: recv(2) with
	      the MSG_PEEK flag will peek data from the front of the queue.

	      When the option is set to a value greater than or equal to zero,
	      then the next peek at data queued in the socket  will  occur  at
	      the  byte	 offset	 specified  by	the option value.  At the same
	      time, the "peek offset" will be incremented  by  the  number  of
	      bytes that were peeked from the queue, so that a subsequent peek
	      will return the next data in the queue.

	      If data is removed from the front of the queue  via  a  call  to
	      recv(2)  (or  similar) without the MSG_PEEK flag, the "peek off‐
	      set" will be decreased by the number of bytes removed.  In other
	      words,  receiving	 data without the MSG_PEEK flag will cause the
	      "peek offset" to be adjusted to maintain	the  correct  relative
	      position	in  the	 queued	 data,	so that a subsequent peek will
	      retrieve the data that would have been retrieved	had  the  data
	      not been removed.

	      For  datagram sockets, if the "peek offset" points to the middle
	      of a packet, the data returned will be marked with the MSG_TRUNC

	      The   following	example	  serves  to  illustrate  the  use  of
	      SO_PEEK_OFF.  Suppose a stream socket has the  following	queued
	      input data:


	      The  following  sequence	of recv(2) calls would have the effect
	      noted in the comments:

		  int ov = 4;		       // Set peek offset to 4
		  setsockopt(fd, SOL_SOCKET, SO_PEEK_OFF, &ov, sizeof(ov));

		  recv(fd, buf, 2, MSG_PEEK);  // Peeks "cc"; offset set to 6
		  recv(fd, buf, 2, MSG_PEEK);  // Peeks "dd"; offset set to 8
		  recv(fd, buf, 2, 0);	       // Reads "aa"; offset set to 6
		  recv(fd, buf, 2, MSG_PEEK);  // Peeks "ee"; offset set to 8

	      Return the credentials of the foreign process connected to  this
	      socket.	This  is  possible  only  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).	 The argument is a ucred structure; define the
	      _GNU_SOURCE feature test macro to obtain the definition of  that
	      structure from <sys/socket.h>.  This socket option is read-only.

	      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.

	      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, or EINPROGRESS (for  connect(2))  just
	      as  if the socket was specified to be nonblocking.  If the time‐
	      out is set to zero (the default), then the operation will	 never
	      timeout.	 Timeouts  only have effect for system calls that per‐
	      form   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.

	      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_RXQ_OVFL (since Linux 2.6.33)
	      Indicates that an unsigned 32-bit value  ancilliary  msg	(cmsg)
	      should  be  attached  to	received skbs indicating the number of
	      packets dropped by the socket between the last  received	packet
	      and this received packet

	      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 (e.g., SOCK_STREAM).  This
	      socket option is read-only.

       SO_BUSY_POLL (since Linux 3.11)
	      Sets the approximate time in microseconds	 to  busy  poll	 on  a
	      blocking	receive	 when there is no data.	 Increasing this value
	      requires CAP_NET_ADMIN.  The default for	this  option  is  con‐
	      trolled by the /proc/sys/net/core/busy_read file.

	      The  value  in  the /proc/sys/net/core/busy_poll file determines
	      how long select(2) and poll(2) will busy poll when they  operate
	      on  sockets  with	 SO_BUSY_POLL  set and no events to report are

	      In both cases, busy polling will only be done  when  the	socket
	      last  received  data  from  a  network device that supports this

	      While busy polling may improve  latency  of  some	 applications,
	      care  must  be taken when using it since this will increase both
	      CPU utilization and power usage.

       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 be used only 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 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.

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

       connect(2), 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 3.65 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at

Linux				  2014-02-21			     SOCKET(7)

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