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  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.

       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_ATTACH_FILTER (since Linux 2.2), SO_ATTACH_BPF (since Linux 3.19)
	      Attach  a	 classic  BPF  (SO_ATTACH_FILTER)  or  an extended BPF
	      (SO_ATTACH_BPF) program to the socket for use  as	 a  filter  of
	      incoming	packets.   A packet will be dropped if the filter pro-
	      gram returns zero.  If the filter	 program  returns  a  non-zero
	      value  which  is	less than the packet's data length, the packet
	      will be truncated to the length returned.	 If the value returned
	      by  the  filter  is  greater  than or equal to the packet's data
	      length, the packet is allowed to proceed unmodified.

	      The argument for SO_ATTACH_FILTER	 is  a	sock_fprog  structure,
	      defined in <linux/filter.h>:

		  struct sock_fprog {
		      unsigned short	  len;
		      struct sock_filter *filter;
		  };

	      The  argument for SO_ATTACH_BPF is a file descriptor returned by
	      the bpf(2) system call and must  refer  to  a  program  of  type
	      BPF_PROG_TYPE_SOCKET_FILTER.

	      These options may be set multiple times for a given socket, each
	      time replacing the previous filter  program.   The  classic  and
	      extended versions may be called on the same socket, but the pre-
	      vious filter will always be replaced such that  a	 socket	 never
	      has more than one filter defined.

	      Both classic and extended BPF are explained in the kernel source
	      file Documentation/networking/filter.txt

       SO_ATTACH_REUSEPORT_CBPF, SO_ATTACH_REUSEPORT_EBPF
	      For use with the SO_REUSEPORT option, these  options  allow  the
	      user  to	set  a	classic	 BPF  (SO_ATTACH_REUSEPORT_CBPF) or an
	      extended BPF (SO_ATTACH_REUSEPORT_EBPF)  program	which  defines
	      how  packets  are assigned to the sockets in the reuseport group
	      (that is, all sockets which have SO_REUSEPORT set and are	 using
	      the same local address to receive packets).

	      The  BPF	program	 must return an index between 0 and N-1 repre-
	      senting the socket which should receive the packet (where	 N  is
	      the number of sockets in the group).  If the BPF program returns
	      an invalid index, socket selection will fall back to  the	 plain
	      SO_REUSEPORT mechanism.

	      Sockets are numbered in the order in which they are added to the
	      group (that is, the order of bind(2) calls for  UDP  sockets  or
	      the  order  of  listen(2)	 calls	for TCP sockets).  New sockets
	      added to a reuseport group will inherit the BPF program.	When a
	      socket  is  removed  from	 a reuseport group (via close(2)), the
	      last socket in the group will be moved into the closed  socket's
	      position.

	      These options may be set repeatedly at any time on any socket in
	      the group to replace the current BPF program used by all sockets
	      in the group.

	      SO_ATTACH_REUSEPORT_CBPF	 takes	 the  same  argument  type  as
	      SO_ATTACH_FILTER and  SO_ATTACH_REUSEPORT_EBPF  takes  the  same
	      argument type as SO_ATTACH_BPF.

	      UDP  support  for this feature is available since Linux 4.5; TCP
	      support is available since Linux 4.6.

       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 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-
	      ment.

       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.   Allowed only for processes with the
	      CAP_NET_ADMIN capability or an effective user ID of 0.

       SO_DETACH_FILTER (since Linux 2.2), SO_DETACH_BPF (since Linux 3.19)
	      These two options, which are synonyms, may be used to remove the
	      classic or extended BPF program attached to a socket with either
	      SO_ATTACH_FILTER or SO_ATTACH_BPF.  The option value is ignored.

       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, 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
	      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_LOCK_FILTER
	      When  set, this option will prevent changing the filters associ-
	      ated with the socket.  These filters include any set  using  the
	      socket options SO_ATTACH_FILTER, SO_ATTACH_BPF, SO_ATTACH_REUSE-
	      PORT_CBPF and SO_ATTACH_REUSEPORT_EPBF.

	      The typical use case is for a privileged process to set up a raw
	      socket  (an operation that requires the CAP_NET_RAW capability),
	      apply a restrictive filter, set the SO_LOCK_FILTER  option,  and
	      then either drop its privileges or pass the socket file descrip-
	      tor to an unprivileged process via a UNIX domain socket.

	      Once the SO_LOCK_FILTER option has  been	enabled,  attempts  to
	      change  or remove the filter attached to a socket, or to disable
	      the SO_LOCK_FILTER option will fail with the error EPERM.

       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
	      passed only 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_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
	      flag.

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

		  aabbccddeeff

	      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

       SO_PEERCRED
	      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.

       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.  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, 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.

       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_REUSEPORT (since Linux 3.9)
	      Permits  multiple	 AF_INET or AF_INET6 sockets to be bound to an
	      identical socket address.	 This  option  must  be	 set  on  each
	      socket  (including the first socket) prior to calling bind(2) on
	      the socket.  To prevent port hijacking,  all  of	the  processes
	      binding  to  the	same address must have the same effective UID.
	      This option can be employed with both TCP and UDP sockets.

	      For TCP sockets, this option allows accept(2) load  distribution
	      in  a  multi-threaded  server to be improved by using a distinct
	      listener socket for each thread.	This  provides	improved  load
	      distribution  as compared to traditional techniques such using a
	      single accept(2)ing thread that distributes connections, or hav-
	      ing  multiple  threads  that  compete to accept(2) from the same
	      socket.

	      For UDP sockets, the use of this option can provide better  dis-
	      tribution	 of  incoming  datagrams  to  multiple	processes  (or
	      threads) as compared to the traditional technique of having mul-
	      tiple processes compete to receive datagrams on the same socket.

       SO_RXQ_OVFL (since Linux 2.6.33)
	      Indicates that an unsigned 32-bit value ancillary message (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.

       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.

       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
	      found.

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

	      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.

   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 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).

       SIOCSPGRP
	      Set the process or process group that is	to  receive  SIGIO  or
	      SIGURG  signals  when  I/O  becomes  possible  or urgent data is
	      available.  The argument is a pointer to a pid_t.	  For  further
	      details, see the description of F_SETOWN in fcntl(2).

       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 were 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 allow port reuse only 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.

SEE ALSO
       wireshark(1),   bpf(2),	 connect(2),   getsockopt(2),	setsockopt(2),
       socket(2),  pcap(3), capabilities(7), ddp(7), ip(7), packet(7), tcp(7),
       udp(7), unix(7), tcpdump(8)

COLOPHON
       This page is part of release 4.10 of the Linux  man-pages  project.   A
       description  of	the project, information about reporting bugs, and the
       latest	 version    of	  this	  page,	   can	   be	  found	    at
       https://www.kernel.org/doc/man-pages/.



Linux				  2016-10-08			     SOCKET(7)