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

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 com-
       patible sockets are the uniform interface between the user process and the network  proto-
       col 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 con-
       nection.

       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)  re-
       turn  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  de-
       scriptor  via  fcntl(2)  and a valid signal handler for SIGIO must be installed via sigac-
       tion(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 iden-
       tify 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  getsock-
       opt(2)  with  the socket level set to SOL_SOCKET for all sockets.  Unless otherwise noted,
       optval is a pointer to an int.

       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_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 <linux/fil-
              ter.h>:

                  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 Documenta-
              tion/networking/filter.txt

       SO_ATTACH_REUSEPORT_CBPF, SO_ATTACH_REUSEPORT_EBPF
              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) pro-
              gram 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 ad-
              dress 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 re-
              place 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.

       SO_BINDTODEVICE
              Bind this socket to a particular device like "eth0", as specified in the passed in-
              terface  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-ter-
              minated  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, particu-
              larly 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  con-
              tain  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  argu-
              ment.

       SO_BROADCAST
              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 sock-
              ets.

       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.  Allowed only for processes with the CAP_NET_ADMIN capa-
              bility 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  ex-
              tended  BPF  program  attached  to  a socket with either SO_ATTACH_FILTER or SO_AT-
              TACH_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.

       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, 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, sizeof(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  in-
              coming  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_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_LOCK_FILTER
              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, SO_AT-
              TACH_BPF, SO_ATTACH_REUSEPORT_CBPF, and SO_ATTACH_REUSEPORT_EBPF.

              The typical use case is for a privileged process to set up a raw socket (an  opera-
              tion 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  rout-
              ing  without  netfilter  or for packet filtering.  Setting this option requires the
              CAP_NET_ADMIN capability.

       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 passed only 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_PASSSEC
              Enable  or disable the receiving of the SCM_SECURITY control message.  For more in-
              formation 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:

                  aabbccddeeff

              The following sequence of recv(2) calls would have the effect  noted  in  the  com-
              ments:

                  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

       SO_PEERCRED
              Return  the  credentials of the peer process connected to this socket.  For further
              details, see unix(7).

       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.  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 IP-
              PROTO_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.

              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 un-
              til 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, or EIN-
              PROGRESS (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.

       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  ad-
              dress.   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  in-
              teger boolean flag.

       SO_REUSEPORT (since Linux 3.9)
              Permits multiple AF_INET or AF_INET6 sockets to be bound to an identical socket ad-
              dress.  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 us-
              ing 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  tech-
              nique of having multiple 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  since  its
              creation.

       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.

       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.

   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  condi-
       tion  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 sig-
       nal 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 ex-
              ternal 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 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).

       SIOCSPGRP
              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).

       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 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.

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 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.

SEE ALSO
       wireshark(1), bpf(2), connect(2), getsockopt(2), setsockopt(2),  socket(2),  pcap(3),  ad-
       dress_families(7),  capabilities(7),  ddp(7),  ip(7),  packet(7), tcp(7), udp(7), unix(7),
       tcpdump(8)

COLOPHON
       This page is part of release 5.05 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 https://www.kernel.org/doc/man-pages/.

Linux                                       2019-08-02                                  SOCKET(7)

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