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       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 (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), 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

       Seeking, or calling pread(2) or pwrite(2) with a nonzero  position  is  not  supported  on

       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 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), getpeername(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 mismatches in calls to the sockets API.

       In addition, the sockets API provides the data type struct sockaddr_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 indicates that this is not a listening
              socket, the value 1 indicates that this is a listening socket.  This socket  option
              is read-only.

              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  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

              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

              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.

              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 flag.

              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 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_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) sockets, 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 offset" 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 flag.

              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

                  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.   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.

              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,  or
              EINPROGRESS (for connect(2)) 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), 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.

              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 (e.g., SOCK_STREAM).  This socket option is

       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

              contains the default setting in bytes of the socket receive buffer.

              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

              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

       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.


       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.


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


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