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

       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  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.

              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-

              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-

              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.

              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

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

       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.

              Get  and  clear the pending socket error.  This socket option is
              read-only.  Expects an integer.

              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

              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.

       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.

              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.

              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

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


              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

              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.

              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

       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.

              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.

              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.

              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.

              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

              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.

              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.

              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.

              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

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

              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.

       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/.

              contains the default setting in bytes of the socket receive buf-

              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 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).

              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.

              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.)

              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 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.

       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.

       are not documented.  The suggested interface to use  them  is  via  the
       libpcap library.

       bpf(2),  connect(2), getsockopt(2), setsockopt(2), socket(2), capabili-
       ties(7), ddp(7), ip(7), packet(7), tcp(7), udp(7), unix(7)

       This page is part of release 4.04 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

Linux                             2015-05-07                         SOCKET(7)
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