Provided by: manpages_2.77-1_all bug

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  nonzero  position  is
       not supported on sockets.

       It  is  possible  to  do  non-blocking  I/O  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(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
              indicates  that  this  is  not  a  listening socket, the value 1
              indicates that this is a listening socket.   Can  only  be  read
              with getsockopt(2).

       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_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
              program.  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
              getsockopt(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
              overhead)  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
              initialized to  1.   SO_SNDLOWAT  is  not  changeable  on  Linux
              (setsockopt(2)  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 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
              kernel 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
              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.

       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 realtime signal
       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 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(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
              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 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
              synonymous 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_SNDTIMEO 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 2.77 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/.