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NAME

       socket - Linux socket interface

SYNOPSIS

       #include <sys/socket.h>
       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),
       sendfile(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  IO  on  sockets  by setting the
       O_NONBLOCK flag on a socket file descriptor using fcntl(2).   Then  all
       operations   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 |
       |           |           | connection-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() and select() is to let the kernel  inform  the
       application  about  events via a SIGIO signal. For that the FASYNC 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_KEEPALIVE
              Enable  sending  of  keep-alive  messages on connection-oriented
              sockets. Expects an integer boolean flag.

       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_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
              initialised  to  1.   SO_SNDLOWAT  is  not  changeable  on Linux
              (setsockopt fails with the error ENOPROTOOPT).   SO_RCVLOWAT  is
              changeable  only  since  Linux  2.4.   The select(2) and poll(2)
              system 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 parameter 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.

       SO_BSDCOMPAT
              Enable  BSD  bug-to-bug  compatibility. This is used only by the
              UDP protocol module and scheduled to be removed in  future.   If
              enabled ICMP errors received for a UDP socket will not be passed
              to the user program.  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  has
              been removed in Linux 2.2. It is better to fix the user programs
              than to enable this flag.

       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().

       SO_BINDTODEVICE
              Bind  this  socket  to  a  particular  device  like  “eth0”,  as
              specified 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
              terminated  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_DEBUG
              Enable socket debugging. Only allowed  for  processes  with  the
              CAP_NET_ADMIN capability or an effective user ID of 0.

       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.

       SO_TYPE
              Gets the socket type as an integer (like SOCK_STREAM).  Can only
              be read with getsockopt().

       SO_ACCEPTCONN
              Returns a value indicating whether or not this socket  has  been
              marked  to  accept  connections  with  listen().   The  value  0
              indicates that this is not  a  listening  socket,  the  value  1
              indicates  that  this  is  a listening socket.  Can only be read
              with getsockopt().

       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_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_SNDBUF
              Sets or gets the maximum  socket  send  buffer  in  bytes.   The
              default  value is set by the wmem_default sysctl and the maximum
              allowed value is set by the wmem_max sysctl.

       SO_RCVBUF
              Sets or gets the maximum socket receive  buffer  in  bytes.  The
              default  value is set by the rmem_default sysctl and the maximum
              allowed value is set by the rmem_max sysctl.

       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_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_ERROR
              Get and  clear  the  pending  socket  error.  Only  valid  as  a
              getsockopt().  Expects an integer.

       SO_TIMESTAMP
              Enable  or  disable  the  receiving  of the SO_TIMESTAMP control
              message.  The timestamp  control  message  is  sent  with  level
              SOL_SOCKET   and   the  cmsg_data  field  is  a  struct  timeval
              indicating the reception time of the last packet passed  to  the
              user in this call.  See cmsg(3) for details on control messages.

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() or SIOCSPGRP ioctl(), 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 realtime signal
       using the F_SETSIG fcntl(); the handler of the real time signal will be
       called with the file descriptor in the si_fd field  of  its  siginfo_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
       disappeared  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
              buffer.

       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() 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
              signals unless it has the CAP_KILL capability  or  an  effective
              UID of 0.

       FIOASYNC
              Change  the  O_ASYNC  flag  to enable or disable asynchronous IO
              mode of the socket. Asynchronous IO 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.

       SIOCGPGRP
              Get the current process or process group that receives SIGIO  or
              SIGURG signals, or 0 when none is set.

       Valid fcntl() operations:

       FIOGETOWN
              The same as the SIOCGPGRP ioctl().

       FIOSETOWN
              The same as the SIOCSPGRP ioctl().

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()  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.

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_SNDTIMEO  are  supported  since Linux 2.3.41. Earlier, timeouts were
       fixed to a protocol specific setting, and could not be read or written.

SEE ALSO

       getsockopt(2),   setsockopt(2),   socket(2),  capabilities(7),  ddp(7),
       ip(7), packet(7)