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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
       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), 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.

       SO_ATTACH_FILTER (since Linux 2.2), SO_ATTACH_BPF (since Linux 3.19)
              Attach a classic BPF (SO_ATTACH_FILTER) or an extended BPF (SO_ATTACH_BPF)  program
              to the socket for use as a filter of incoming packets.  A packet will be dropped if
              the filter program returns zero.  If the filter program  returns  a  nonzero  value
              which  is  less  than the packet's data length, the packet will be truncated to the
              length returned.  If the value returned by the filter is greater than or  equal  to
              the packet's data length, the packet is allowed to proceed unmodified.

              The   argument   for   SO_ATTACH_FILTER  is  a  sock_fprog  structure,  defined  in

                  struct sock_fprog {
                      unsigned short      len;
                      struct sock_filter *filter;

              The argument for SO_ATTACH_BPF is a file descriptor returned by the  bpf(2)  system
              call and must refer to a program of type BPF_PROG_TYPE_SOCKET_FILTER.

              These options may be set multiple times for a given socket, each time replacing the
              previous filter program.  The classic and extended versions may be  called  on  the
              same  socket,  but  the  previous filter will always be replaced such that a socket
              never has more than one filter defined.

              Both  classic  and  extended  BPF  are  explained  in  the   kernel   source   file

              For use with the SO_REUSEPORT option, these options allow the user to set a classic
              BPF  (SO_ATTACH_REUSEPORT_CBPF)  or  an  extended  BPF   (SO_ATTACH_REUSEPORT_EBPF)
              program  which  defines  how  packets  are assigned to the sockets in the reuseport
              group (that is, all sockets which have SO_REUSEPORT set  and  are  using  the  same
              local address to receive packets).

              The  BPF  program  must  return  an index between 0 and N-1 representing the socket
              which should receive the packet (where N is the number of sockets  in  the  group).
              If the BPF program returns an invalid index, socket selection will fall back to the
              plain SO_REUSEPORT mechanism.

              Sockets are numbered in the order in which they are added to the  group  (that  is,
              the  order of bind(2) calls for UDP sockets or the order of listen(2) calls for TCP
              sockets).  New sockets added to a reuseport group will  inherit  the  BPF  program.
              When  a socket is removed from a reuseport group (via close(2)), the last socket in
              the group will be moved into the closed socket's position.

              These options may be set repeatedly at any time on  any  socket  in  the  group  to
              replace the current BPF program used by all sockets in the group.

              SO_ATTACH_REUSEPORT_CBPF  takes  the  same  argument  type  as SO_ATTACH_FILTER and
              SO_ATTACH_REUSEPORT_EBPF takes the same argument type as SO_ATTACH_BPF.

              UDP support for this feature is available since Linux 4.5; TCP support is available
              since Linux 4.6.

              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  IFNAMSIZ  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.   Allowed  only  for  processes  with  the  CAP_NET_ADMIN
              capability or an effective user ID of 0.

       SO_DETACH_FILTER (since Linux 2.2), SO_DETACH_BPF (since Linux 3.19)
              These  two  options,  which  are  synonyms,  may  be  used to remove the classic or
              extended  BPF  program  attached  to  a  socket  with  either  SO_ATTACH_FILTER  or
              SO_ATTACH_BPF.  The option value is ignored.

       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.

       SO_INCOMING_CPU (gettable since Linux 3.19, settable since Linux 4.4)
              Sets or gets the CPU affinity of a socket.  Expects an integer flag.

                  int cpu = 1;
                  setsockopt(fd, SOL_SOCKET, SO_INCOMING_CPU, &cpu,

              Because  all  of  the  packets  for a single stream (i.e., all packets for the same
              4-tuple) arrive on the single RX queue that is associated with  a  particular  CPU,
              the  typical  use  case  is  to employ one listening process per RX queue, with the
              incoming flow being handled by a listener on the same CPU that is handling  the  RX
              queue.  This provides optimal NUMA behavior and keeps CPU caches hot.

       SO_INCOMING_NAPI_ID (gettable since Linux 4.12)
              Returns  a system-level unique ID called NAPI ID that is associated with a RX queue
              on which the last packet associated with that socket is received.

              This can be used by an application to split the incoming flows among worker threads
              based  on the RX queue on which the packets associated with the flows are received.
              It allows each worker thread to be associated with  a  NIC  HW  receive  queue  and
              service  all  the  connection  requests  received  on  that RX queue.  This mapping
              between a app thread and a HW NIC queue streamlines the flow of data from  the  NIC
              to the application.

              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.

              When set, this option will prevent changing the filters associated with the socket.
              These  filters  include  any  set  using  the  socket   options   SO_ATTACH_FILTER,

              The  typical  use  case  is  for  a  privileged  process to set up a raw socket (an
              operation that requires the CAP_NET_RAW capability), apply  a  restrictive  filter,
              set  the  SO_LOCK_FILTER  option,  and  then either drop its privileges or pass the
              socket file descriptor to an unprivileged process via a UNIX domain socket.

              Once the SO_LOCK_FILTER option has been enabled, attempts to change or  remove  the
              filter attached to a socket, or to disable the SO_LOCK_FILTER option will fail with
              the error EPERM.

       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).

              Enable or disable the receiving of the  SCM_SECURITY  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 peer process connected to this socket.   For  further
              details, see unix(7).

       SO_PEERSEC (since Linux 2.6.2)
              Return  the  security  context  of  the  peer socket connected to this socket.  For
              further details, see unix(7) and ip(7).

              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 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.

              Before  Linux  2.6.28  select(2),  poll(2),  and  epoll(7)  did  not  respect   the
              SO_RCVLOWAT setting on Linux, and indicated a socket as readable when even a single
              byte of data was available.  A subsequent read from the  socket  would  then  block
              until SO_RCVLOWAT bytes are available.  Since Linux 2.6.28, select(2), poll(2), and
              epoll(7) indicate a socket as readable only  if  at  least  SO_RCVLOWAT  bytes  are

              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.,
              accept(2),  connect(2), 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 having multiple
              threads that compete to accept(2) from the same socket.

              For UDP sockets, the use of this option can provide better distribution of incoming
              datagrams to multiple  processes  (or  threads)  as  compared  to  the  traditional
              technique  of  having  multiple  processes compete to receive datagrams on the same

       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 since its

       SO_SELECT_ERR_QUEUE (since Linux 3.10)
              When this option is set on  a  socket,  an  error  condition  on  a  socket  causes
              notification  not only via the exceptfds set of select(2).  Similarly, poll(2) also
              returns a POLLPRI whenever an POLLERR event is returned.

              Background: this option was added when waking up on  an  error  condition  occurred
              only via the readfds and writefds sets of select(2).  The option was added to allow
              monitoring for error conditions via the exceptfds argument  without  simultaneously
              having  to  receive  notifications  (via readfds) for regular data that can be read
              from the socket.  After changes in Linux 4.16, the use of this flag to achieve  the
              desired notifications is no longer necessary.  This option is nevertheless retained
              for backwards compatibility.

              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 a cmsg_type of SCM_TIMESTAMP.
              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

       SO_TIMESTAMPNS (since Linux 2.6.22)
              Enable or disable  the  receiving  of  the  SO_TIMESTAMPNS  control  message.   The
              timestamp  control  message  is  sent  with  level  SOL_SOCKET  and  a cmsg_type of
              SCM_TIMESTAMPNS.  The cmsg_data field is a struct timespec indicating the reception
              time  of  the  last packet passed to the user in this call.  The clock used for the
              timestamp is CLOCK_REALTIME.  See cmsg(3) for details on control messages.

              A socket cannot mix SO_TIMESTAMP and SO_TIMESTAMPNS: the  two  modes  are  mutually

              Gets  the  socket  type  as  an integer (e.g., SOCK_STREAM).  This socket option is

       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 controlled 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 found.

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

              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  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
              options  SO_TIMESTAMP  and SO_TIMESTAMPNS are not set on the socket.  Otherwise, it
              returns the timestamp of the last packet that was received while  SO_TIMESTAMP  and
              SO_TIMESTAMPNS  were  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 that is to receive SIGIO or  SIGURG  signals  when
              I/O  becomes  possible or urgent data is available.  The argument is a pointer to a
              pid_t.  For further details, see the description of F_SETOWN in fcntl(2).

              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 were 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  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.


       wireshark(1),   bpf(2),  connect(2),  getsockopt(2),  setsockopt(2),  socket(2),  pcap(3),
       address_families(7), capabilities(7), ddp(7), ip(7), ipv6(7), packet(7),  tcp(7),  udp(7),
       unix(7), tcpdump(8)