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NAME

       socket - Linux socket interface

SYNOPSIS

       #include <sys/socket.h>

       sockfd = 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 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 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.  This socket option
              is read-only.

       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(2) 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_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.

       SO_ERROR
              Get  and clear the pending socket error.  This socket option is read-only.  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 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.  This socket option is read-only.

       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_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.

       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  /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.

       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 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.

       SO_REUSEADDR
              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_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  /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.

       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 (e.g., SOCK_STREAM).  This socket option is
              read-only.

   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  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.

   /proc interfaces
       The core socket  networking  parameters  can  be  accessed  via  files  in  the  directory
       /proc/sys/net/core/.

       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
       /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.

NOTES

       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.

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),
       tcp(7), udp(7), unix(7)

COLOPHON

       This  page  is  part of release 3.35 of the Linux man-pages project.  A description of the
       project, and information about reporting bugs, can be found at  http://man7.org/linux/man-
       pages/.