socket(7) — Linux manual page

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

NAME         top

       socket - Linux socket interface

SYNOPSIS         top

       #include <sys/socket.h>

       sockfd = socket(int socket_family, int socket_type, int protocol);

DESCRIPTION         top

       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 connection.

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

       It is possible to do nonblocking I/O on sockets by setting the
       O_NONBLOCK flag on a socket file descriptor using fcntl(2).  Then
       all operations that would block will (usually) return with EAGAIN
       (operation should be retried later); connect(2) will return
       EINPROGRESS error.  The user can then wait for various events via
       poll(2) or select(2).

       ┌────────────────────────────────────────────────────────────────┐
       │                          I/O events                            │
       ├───────────┬───────────┬────────────────────────────────────────┤
       │Event      │ Poll flag │ Occurrence                             │
       ├───────────┼───────────┼────────────────────────────────────────┤
       │Read       │ POLLIN    │ New data arrived.                      │
       ├───────────┼───────────┼────────────────────────────────────────┤
       │Read       │ POLLIN    │ A connection setup has been completed  │
       │           │           │ (for connection-oriented sockets)      │
       ├───────────┼───────────┼────────────────────────────────────────┤
       │Read       │ POLLHUP   │ A disconnection request has been       │
       │           │           │ initiated by the other end.            │
       ├───────────┼───────────┼────────────────────────────────────────┤
       │Read       │ POLLHUP   │ A connection is broken (only for       │
       │           │           │ connection-oriented protocols).  When  │
       │           │           │ the socket is written SIGPIPE is also  │
       │           │           │ sent.                                  │
       ├───────────┼───────────┼────────────────────────────────────────┤
       │Write      │ POLLOUT   │ Socket has enough send buffer space    │
       │           │           │ for writing new data.                  │
       ├───────────┼───────────┼────────────────────────────────────────┤
       │Read/Write │ POLLIN |  │ An outgoing connect(2) finished.       │
       │           │ POLLOUT   │                                        │
       ├───────────┼───────────┼────────────────────────────────────────┤
       │Read/Write │ POLLERR   │ An asynchronous error occurred.        │
       ├───────────┼───────────┼────────────────────────────────────────┤
       │Read/Write │ POLLHUP   │ The other end has shut down one        │
       │           │           │ direction.                             │
       ├───────────┼───────────┼────────────────────────────────────────┤
       │Exception  │ POLLPRI   │ Urgent data arrived.  SIGURG is sent   │
       │           │           │ then.                                  │
       └───────────┴───────────┴────────────────────────────────────────┘
       An alternative to poll(2) and select(2) is to let the kernel
       inform the application about events via a SIGIO signal.  For that
       the O_ASYNC flag must be set on a socket file descriptor via
       fcntl(2) and a valid signal handler for SIGIO must be installed
       via sigaction(2).  See the Signals discussion below.

   Socket 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.

       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/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 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 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
              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.

       SO_BINDTODEVICE
              Bind this socket to a particular device like “eth0”, as
              specified in the passed interface name.  If the name is an
              empty string or the option length is zero, the socket
              device binding is removed.  The passed option is a
              variable-length null-terminated interface name string with
              the maximum size of IFNAMSIZ.  If a socket is bound to an
              interface, only packets received from that particular
              interface are processed by the socket.  Note that this
              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 argument.

       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 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 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
              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.

       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_ATTACH_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 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.

       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 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:

                  aabbccddeeff

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

                  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_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).

       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 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 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 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
              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 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 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_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.

       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 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 messages.

       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 exclusive.

       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 condition that caused the SIGIO may have
       already disappeared when the process reacts to the signal.  If
       this happens, the process should wait again because Linux will
       resend the signal later.

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

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

       rmem_max
              contains the maximum socket receive buffer size in bytes
              which a user may set by using the SO_RCVBUF socket option.

       wmem_default
              contains the default setting in bytes of the socket send
              buffer.

       wmem_max
              contains the maximum socket send buffer size in bytes
              which a user may set by using the SO_SNDBUF socket option.

       message_cost and message_burst
              configure the token bucket filter used to load limit
              warning messages caused by external network events.

       netdev_max_backlog
              Maximum number of packets in the global input queue.

       optmem_max
              Maximum length of ancillary data and user control data
              like the iovecs per socket.

   Ioctls
       These operations can be accessed using ioctl(2):

           error = ioctl(ip_socket, ioctl_type, &value_result);

       SIOCGSTAMP
              Return a struct timeval with the receive timestamp of the
              last packet passed to the user.  This is useful for
              accurate round trip time measurements.  See setitimer(2)
              for a description of struct timeval.  This ioctl should 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).

       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         top

       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         top

       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         top

       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)

COLOPHON         top

       This page is part of release 5.11 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                          2021-03-22                      SOCKET(7)

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