Man Linux: Main Page and Category List


       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  like  AF_INET,
       AF_IPX, AF_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 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),
       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  nonblocking  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

       |                            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:

              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).

              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).

              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.

              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

              Enable socket debugging.  Only allowed for  processes  with  the
              CAP_NET_ADMIN capability or an effective user ID of 0.

              Get  and  clear  the  pending  socket  error.   Only  valid as a
              getsockopt(2).  Expects an integer.

              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

              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.

              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.

              Enable or disable the receiving of the  SCM_CREDENTIALS  control
              message.  For more information see unix(7).

              Return  the credentials of the foreign process connected to this
              socket.  This is only  possible  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).  Argument is a ucred structure.  Only valid as
              a getsockopt(2).

              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.

              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   /proc/sys/net/core/rmem_default   file,  and  the  maximum
              allowed value is set by  the  /proc/sys/net/core/rmem_max  file.
              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.

              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.

              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  nonblocking.   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.

              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.

              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 /proc/sys/net/core/wmem_default file and the maximum allowed
              value is  set  by  the  /proc/sys/net/core/wmem_max  file.   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.

              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.

              Gets the socket type as an integer (like SOCK_STREAM).  Can only
              be read with getsockopt(2).

       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
       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

   /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

              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 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).

              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.

              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.)

              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_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.


       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.


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


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       description of the project, and information about reporting  bugs,  can
       be found at