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man:socket

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  compatible  sockets  are  the  uniform  interface
     between the user process and the network protocol stacks in the kernel.
     The protocol  modules  are  grouped  into  protocol  families  such  as
     AF_INET, AF_IPX, and AF_PACKET, and socket types such as SOCK_STREAM or
     SOCK_DGRAM.  See socket(2) for more information on families and  types.
 Socket-layer functions
     These functions are used by the user process to send or receive packets
     and to do other socket operations.   For  more  information  see  their
     respective manual pages.
     socket(2)  creates  a  socket, connect(2) connects a socket to a remote
     socket address, the bind(2) function binds a socket to a  local  socket
     address,  listen(2)  tells  the  socket  that  new connections shall be
     accepted, and accept(2) is used to get a new socket with a new incoming
     connection.   socketpair(2)  returns  two  connected  anonymous sockets
     (implemented only for a few local families like AF_UNIX)
     send(2), sendto(2),  and  sendmsg(2)  send  data  over  a  socket,  and
     recv(2),  recvfrom(2),  recvmsg(2) receive data from a socket.  poll(2)
     and select(2) wait for arriving data or a readiness to send  data.   In
     addition,  the  standard I/O operations like write(2), writev(2), send-
     file(2), read(2), and readv(2) can be used to read and write data.
     getsockname(2) returns the  local  socket  address  and  getpeername(2)
     returns the remote socket address.  getsockopt(2) and setsockopt(2) are
     used to set or get socket layer or protocol options.  ioctl(2)  can  be
     used to set or read some other options.
     close(2)  is  used  to  close  a socket.  shutdown(2) closes parts of a
     full-duplex socket connection.
     Seeking, or calling pread(2) or pwrite(2) with a  nonzero  position  is
     not supported on sockets.
     It  is  possible to do nonblocking I/O on sockets by setting the O_NON-
     BLOCK flag on a socket file descriptor using fcntl(2).  Then all opera-
     tions  that  would  block  will (usually) return with EAGAIN (operation
     should be retried later); connect(2)  will  return  EINPROGRESS  error.
     The user can then wait for various events via poll(2) or select(2).
     +--------------------------------------------------------------------+
     |                            I/O events                              |
     +-----------+-----------+--------------------------------------------+
     |Event      | Poll flag | Occurrence                                 |
     +-----------+-----------+--------------------------------------------+
     |Read       | POLLIN    | New data arrived.                          |
     +-----------+-----------+--------------------------------------------+
     |Read       | POLLIN    | A connection setup has been completed (for |
     |           |           | connection-oriented sockets)               |
     +-----------+-----------+--------------------------------------------+
     |Read       | POLLHUP   | A disconnection request has been initiated |
     |           |           | by the other end.                          |
     +-----------+-----------+--------------------------------------------+
     |Read       | POLLHUP   | A  connection  is broken (only for connec- |
     |           |           | tion-oriented protocols).  When the socket |
     |           |           | is written SIGPIPE is also sent.           |
     +-----------+-----------+--------------------------------------------+
     |Write      | POLLOUT   | Socket  has  enough  send buffer space for |
     |           |           | writing new data.                          |
     +-----------+-----------+--------------------------------------------+
     |Read/Write | POLLIN |  | An outgoing connect(2) finished.           |
     |           | POLLOUT   |                                            |
     +-----------+-----------+--------------------------------------------+
     |Read/Write | POLLERR   | An asynchronous error occurred.            |
     +-----------+-----------+--------------------------------------------+
     |Read/Write | POLLHUP   | The other end has shut down one direction. |
     +-----------+-----------+--------------------------------------------+
     |Exception  | POLLPRI   | Urgent data arrived.  SIGURG is sent then. |
     +-----------+-----------+--------------------------------------------+
     An alternative to poll(2) and select(2) is to let the kernel inform the
     application about events via a SIGIO signal.  For that the O_ASYNC flag
     must be set on a socket file descriptor via fcntl(2) and a valid signal
     handler for SIGIO must be installed via sigaction(2).  See the  Signals
     discussion below.
 Socket address structures
     Each  socket  domain  has  its  own format for socket addresses, with a
     domain-specific address structure.  Each  of  these  structures  begins
     with  an  integer  "family" field (typed as sa_family_t) that indicates
     the type of the address structure.   This  allows  the  various  system
     calls  (e.g.,  connect(2), bind(2), accept(2), getsockname(2), getpeer-
     name(2)), which are generic to all socket  domains,  to  determine  the
     domain of a particular socket address.
     To  allow  any type of socket address to be passed to interfaces in the
     sockets API, the type struct sockaddr is defined.  The purpose of  this
     type is purely to allow casting of domain-specific socket address types
     to a "generic" type, so as to avoid compiler warnings about  type  mis-
     matches in calls to the sockets API.
     In  addition,  the  sockets  API  provides  the  data type struct sock-
     addr_storage.  This type  is  suitable  to  accommodate  all  supported
     domain-specific  socket  address  structures; it is large enough and is
     aligned properly.  (In particular, it is  large  enough  to  hold  IPv6
     socket  addresses.)   The structure includes the following field, which
     can be used to identify the type of socket address actually  stored  in
     the structure:
             sa_family_t ss_family;
     The  sockaddr_storage  structure is useful in programs that must handle
     socket addresses in a generic way (e.g., programs that must  deal  with
     both IPv4 and IPv6 socket addresses).
 Socket options
     The  socket  options listed below can be set by using setsockopt(2) and
     read with getsockopt(2) with the socket level set to SOL_SOCKET for all
     sockets.  Unless otherwise noted, optval is a pointer to an int.
     SO_ACCEPTCONN
            Returns  a  value indicating whether or not this socket has been
            marked to accept connections with listen(2).  The value 0  indi-
            cates that this is not a listening socket, the value 1 indicates
            that this is a listening socket.  This socket  option  is  read-
            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 pro-
            gram 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/filter.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 pre-
            vious 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 Documentation/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)  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 repre-
            senting 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.
     SO_BINDTODEVICE
            Bind this socket to a particular device like "eth0",  as  speci-
            fied  in  the  passed  interface  name.  If the name is an empty
            string or the option length is zero, the socket  device  binding
            is  removed.  The passed option is a variable-length null-termi-
            nated interface name string with the maximum size  of  IFNAMSIZ.
            If a socket is bound to an interface, only packets received from
            that particular interface are processed  by  the  socket.   Note
            that this works only for some socket types, particularly AF_INET
            sockets.  It is not supported for  packet  sockets  (use  normal
            bind(2) there).
            Before Linux 3.8, this socket option could be set, but could not
            retrieved with getsockopt(2).  Since Linux 3.8, it is  readable.
            The  optlen argument should contain the buffer size available to
            receive the device name and is recommended to be 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 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  pro-
            gram.   In  later  kernel  versions, support for this option has
            been phased out: Linux 2.4 silently ignores it,  and  Linux  2.6
            generates  a  kernel  warning  (printk()) if a program uses this
            option.  Linux 2.0 also  enabled  BSD  bug-to-bug  compatibility
            options (random header changing, skipping of the broadcast flag)
            for raw sockets with this option, but that was removed in  Linux
            2.2.
     SO_DEBUG
            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.
     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 pack-
            ets 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 han-
            dling 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  associ-
            ated  with  the socket.  These filters include any set using the
            socket options SO_ATTACH_FILTER, SO_ATTACH_BPF, SO_ATTACH_REUSE-
            PORT_CBPF and SO_ATTACH_REUSEPORT_EPBF.
            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 descrip-
            tor 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.
     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_PEEK_OFF (since Linux 3.4)
            This option, which is currently supported only for unix(7) sock-
            ets,  sets the value of the "peek offset" for the recv(2) system
            call when used with MSG_PEEK flag.
            When this option is set to a negative value (it is set to -1 for
            all new sockets), traditional behavior is provided: recv(2) with
            the MSG_PEEK flag will peek data from the front of the queue.
            When the option is set to a value greater than or equal to zero,
            then  the  next  peek at data queued in the socket will occur at
            the byte offset specified by the  option  value.   At  the  same
            time,  the  "peek  offset"  will be incremented by the number of
            bytes that were peeked from the queue, so that a subsequent peek
            will return the next data in the queue.
            If  data  is  removed  from the front of the queue via a call to
            recv(2) (or similar) without the MSG_PEEK flag, the  "peek  off-
            set" will be decreased by the number of bytes removed.  In other
            words, receiving data without the MSG_PEEK flag will  cause  the
            "peek  offset"  to  be adjusted to maintain the correct relative
            position in the queued data, so  that  a  subsequent  peek  will
            retrieve  the  data  that would have been retrieved had the data
            not been removed.
            For datagram sockets, if the "peek offset" points to the  middle
            of a packet, the data returned will be marked with the MSG_TRUNC
            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 comments:
                int  ov  =  4;                  // Set peek offset to 4 set-
                sockopt(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 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.
     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  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 over-
            head) when it is set using setsockopt(2), and this doubled value
            is  returned  by getsockopt(2).  The default value is set by the
            /proc/sys/net/core/rmem_default file, and  the  maximum  allowed
            value is set by the /proc/sys/net/core/rmem_max file.  The mini-
            mum (doubled) value for this option is 256.
     SO_RCVBUFFORCE (since Linux 2.6.14)
            Using this socket option, a privileged  (CAP_NET_ADMIN)  process
            can  perform  the same task as SO_RCVBUF, but the rmem_max limit
            can be overridden.
     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  ini-
            tialized to 1.  SO_SNDLOWAT is not changeable on Linux (setsock-
            opt(2)  fails  with  the  error  ENOPROTOOPT).   SO_RCVLOWAT  is
            changeable only since Linux 2.4.  The select(2) and poll(2) sys-
            tem calls currently do not respect the  SO_RCVLOWAT  setting  on
            Linux,  and  mark  a  socket readable when even a single byte of
            data is available.  A subsequent read from the socket will block
            until SO_RCVLOWAT bytes are available.
     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 EINPROGRESS (for connect(2)) just
            as if the socket was specified to be nonblocking.  If the  time-
            out  is set to zero (the default), then the operation will never
            timeout.  Timeouts only have effect for system calls  that  per-
            form   socket   I/O   (e.g.,   read(2),   recvmsg(2),   send(2),
            sendmsg(2)); timeouts have no  effect  for  select(2),  poll(2),
            epoll_wait(2), and so on.
     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_REUSEPORT (since Linux 3.9)
            Permits  multiple  AF_INET or AF_INET6 sockets to be bound to an
            identical socket address.  This  option  must  be  set  on  each
            socket  (including the first socket) prior to calling bind(2) on
            the socket.  To prevent port hijacking,  all  of  the  processes
            binding  to  the  same address must have the same effective UID.
            This option can be employed with both TCP and UDP sockets.
            For TCP sockets, this option allows accept(2) load  distribution
            in  a  multi-threaded  server to be improved by using a distinct
            listener socket for each thread.  This  provides  improved  load
            distribution  as compared to traditional techniques such using a
            single accept(2)ing thread that distributes connections, or hav-
            ing  multiple  threads  that  compete to accept(2) from the same
            socket.
            For UDP sockets, the use of this option can provide better  dis-
            tribution  of  incoming  datagrams  to  multiple  processes  (or
            threads) as compared to the traditional technique of having mul-
            tiple processes compete to receive datagrams on the same socket.
     SO_RXQ_OVFL (since Linux 2.6.33)
            Indicates that an unsigned 32-bit value ancillary message (cmsg)
            should  be  attached  to  received skbs indicating the number of
            packets dropped by the socket since its creation.
     SO_SNDBUF
            Sets or gets the maximum socket send buffer in bytes.  The  ker-
            nel doubles this value (to allow space for bookkeeping overhead)
            when it is set using setsockopt(2), and this  doubled  value  is
            returned  by  getsockopt(2).   The  default  value is set by the
            /proc/sys/net/core/wmem_default file  and  the  maximum  allowed
            value is set by the /proc/sys/net/core/wmem_max file.  The mini-
            mum (doubled) value for this option is 2048.
     SO_SNDBUFFORCE (since Linux 2.6.14)
            Using this socket option, a privileged  (CAP_NET_ADMIN)  process
            can  perform  the same task as SO_SNDBUF, but the wmem_max limit
            can be overridden.
     SO_TIMESTAMP
            Enable or disable the receiving of the SO_TIMESTAMP control mes-
            sage.    The  timestamp  control  message  is  sent  with  level
            SOL_SOCKET and the cmsg_data field is a struct timeval  indicat-
            ing  the reception time of the last packet passed to the user in
            this call.  See cmsg(3) for details on control messages.
     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 con-
            trolled by the /proc/sys/net/core/busy_read file.
            The value in the  /proc/sys/net/core/busy_poll  file  determines
            how  long select(2) and poll(2) will busy poll when they operate
            on sockets with SO_BUSY_POLL set and no  events  to  report  are
            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 sig-
     nal using the F_SETSIG fcntl(2); the handler of the  real  time  signal
     will  be called with the file descriptor in the si_fd field of its sig-
     info_t.  See fcntl(2) for more information.
     Under some circumstances (e.g., multiple processes accessing  a  single
     socket),  the  condition  that caused the SIGIO may have already disap-
     peared when the process reacts to the signal.   If  this  happens,  the
     process should wait again because Linux will resend the signal later.
 /proc interfaces
     The  core socket networking parameters can be accessed via files in the
     directory /proc/sys/net/core/.
     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 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 synony-
            mous 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_RCV-
     TIMEO and SO_SNDTIMEO are supported since Linux 2.3.41.  Earlier, time-
     outs  were  fixed to a protocol-specific setting, and could not be read
     or written.

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), capabilities(7), ddp(7), ip(7), packet(7), tcp(7),
     udp(7), unix(7), tcpdump(8)

COLOPHON

     This page is part of release 4.16 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 2018-02-02 SOCKET(7)

/data/webs/external/dokuwiki/data/pages/man/socket.txt · Last modified: 2019/05/17 09:47 by 127.0.0.1

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