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



NAME
       socket - Linux socket interface

SYNOPSIS
       #include <&lt;sys/socket.h>&gt;

       mysocket = 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  PF_INET,
       PF_IPX, PF_PACKET and socket types like SOCK_STREAM or SOCK_DGRAM.  See
       socket(2) for more information on families and types.

   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 PF_UNIX)

       send(2),  sendto(2),  and  sendmsg(2)  send  data  over  a  socket, and
       recv(2), recvfrom(2), recvmsg(2) receive data from a  socket.   poll(2)
       and  select(2)  wait for arriving data or a readiness to send data.  In
       addition, the standard I/O operations like write(2),  writev(2),  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).

       tab(:) allbox; c s s l l  l.   I/O  events  Event:Poll  flag:Occurrence
       Read:POLLIN:T{  New data arrived.  T} Read:POLLIN:T{ A connection setup
       has been completed (for connection-oriented sockets) T} Read:POLLHUP:T{
       A  disconnection  request  has  been  initiated  by  the other end.  T}
       Read:POLLHUP:T{ A connection is broken  (only  for  connection-oriented
       protocols).   When  the  socket  is  written  SIGPIPE is also sent.  T}
       Write:POLLOUT:T{ Socket has enough send buffer space  for  writing  new
       data.  T} Read/Write:T{ POLLIN|
       POLLOUT     T}:T{     An     outgoing    connect(2)    finished.     T}
       Read/Write:POLLERR:An asynchronous  error  occurred.   Read/Write:POLL-
       HUP:The  other  end  has shut down one direction.  Exception:POLLPRI:T{
       Urgent data arrived.  SIGURG is sent then.  T}


       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.  Can only be read with getsock-
              opt(2).

       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(8) there).

       SO_BROADCAST
              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.

       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_ERROR
              Get  and  clear  the pending socket error.  Only valid as a get-
              sockopt(2).  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_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  PF_UNIX  stream
              sockets  and  PF_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).

       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_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
              rmem_default  sysctl and the maximum allowed value is set by the
              rmem_max sysctl.  The minimum (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  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.

       SO_REUSEADDR
              Indicates  that  the rules used in validating addresses supplied
              in a bind(2) call should allow reuse of  local  addresses.   For
              PF_INET  sockets  this means that a socket may bind, except when
              there is an active listening socket bound to the address.   When
              the listening socket is bound to INADDR_ANY with a specific port
              then it is not possible to bind  to  this  port  for  any  local
              address.  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
              wmem_default  sysctl and the maximum allowed value is set by the
              wmem_max sysctl.  The minimum (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 (like SOCK_STREAM).  Can only
              be read with getsockopt(2).

   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.

   Sysctls
       The  core  socket  networking  sysctls  can  be  accessed   using   the
       /proc/sys/net/core/* files or with the sysctl(2) interface.

       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, &amp;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 sysctls are new in Linux 2.2.  SO_RCVTIMEO and SO_SND-
       TIMEO are supported since Linux 2.3.41.  Earlier, timeouts  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 sysctls 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.

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)

COLOPHON
       This page is part of release 3.05 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                             2007-12-28                         SOCKET(7)