socket(7) — Linux manual page


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

       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.

              Returns a value indicating whether or not this socket has been
              marked to accept connections with listen(2).  The value 0 in‐
              dicates that this is not a listening socket, the value 1 indi‐
              cates that this is a listening socket.  This socket option is

       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 re‐
              turned 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

              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

              For use with the SO_REUSEPORT option, these options allow the
              user to set a classic BPF (SO_ATTACH_REUSEPORT_CBPF) or an ex‐
              tended 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 us‐
              ing 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 re‐
              turns an invalid index, socket selection will fall back to the
              plain SO_REUSEPORT mechanism.

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

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

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

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

              Bind this socket to a particular device like “eth0”, as 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-ter‐
              minated interface name string with the maximum size of IFNAM‐
              SIZ.  If a socket is bound to an interface, only packets re‐
              ceived from that particular interface are processed by the
              socket.  Note that this works only for some socket types, par‐
              ticularly 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.

              Set or get the broadcast flag.  When enabled, datagram sockets
              are allowed to send packets to a broadcast address.  This op‐
              tion has no effect on stream-oriented sockets.

              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.

              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

       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 op‐
              tion is read-only.

              Get and clear the pending socket error.  This socket option is
              read-only.  Expects an integer.

              Don't send via a gateway, send only to directly connected
              hosts.  The same effect can be achieved by setting the
              MSG_DONTROUTE flag on a socket send(2) operation.  Expects an
              integer boolean flag.

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

                  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_INCOMING_NAPI_ID (gettable since Linux 4.12)
              Returns a system level unique ID called NAPI ID that is asso‐
              ciated 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 pack‐
              ets 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 stream‐
              lines the flow of data from the NIC to the application.

              Enable sending of keep-alive messages on connection-oriented
              sockets.  Expects an integer boolean flag.

              Sets or gets the SO_LINGER option.  The argument is a linger

                  struct linger {
                      int l_onoff;    /* linger active */
                      int l_linger;   /* how many seconds to linger for */

              When enabled, a close(2) or shutdown(2) will not return until
              all queued messages for the socket have been successfully sent
              or the linger timeout has been reached.  Otherwise, the call
              returns immediately and the closing is done in the background.
              When the socket is closed as part of exit(2), it always
              lingers in the background.

              When set, this option will prevent changing the filters asso‐
              ciated with the socket.  These filters include any set using
              the socket options SO_ATTACH_FILTER, SO_ATTACH_BPF, SO_AT‐

              The typical use case is for a privileged process to set up a
              raw socket (an operation that requires the CAP_NET_RAW capa‐
              bility), 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

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

       SO_MARK (since Linux 2.6.25)
              Set the mark for each packet sent through this socket (similar
              to the netfilter MARK target but socket-based).  Changing the
              mark can be used for mark-based routing without netfilter or
              for packet filtering.  Setting this option requires the
              CAP_NET_ADMIN capability.

              If this option is enabled, out-of-band data is directly placed
              into the receive data stream.  Otherwise, out-of-band data is
              passed only when the MSG_OOB flag is set during receiving.

              Enable or disable the receiving of the SCM_CREDENTIALS control
              message.  For more information see unix(7).

              Enable or disable the receiving of the SCM_SECURITY control
              message.  For more information see unix(7).

       SO_PEEK_OFF (since Linux 3.4)
              This option, which is currently supported only for unix(7)
              sockets, sets the value of the "peek offset" for the recv(2)
              system call when used with MSG_PEEK flag.

              When this option is set to a negative value (it is set to -1
              for all new sockets), traditional behavior is provided:
              recv(2) with the MSG_PEEK flag will peek data from the front
              of the queue.

              When the option is set to a value greater than or equal to
              zero, then the next peek at data queued in the socket will oc‐
              cur 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 mid‐
              dle of a packet, the data returned will be marked with the
              MSG_TRUNC flag.

              The following example serves to illustrate the use of
              SO_PEEK_OFF.  Suppose a stream socket has the following queued
              input data:


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

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

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

              Set the protocol-defined priority for all packets to be sent
              on this socket.  Linux uses this value to order the networking
              queues: packets with a higher priority may be processed first
              depending on the selected device queueing discipline.  Setting
              a priority outside the range 0 to 6 requires the CAP_NET_ADMIN

       SO_PROTOCOL (since Linux 2.6.32)
              Retrieves the socket protocol as an integer, returning a value
              such as IPPROTO_SCTP.  See socket(2) for details.  This socket
              option is read-only.

              Sets or gets the maximum socket receive buffer in bytes.  The
              kernel doubles this value (to allow space for bookkeeping
              overhead) when it is set using setsockopt(2), and this doubled
              value is returned by getsockopt(2).  The default value is set
              by the /proc/sys/net/core/rmem_default file, and the maximum
              allowed value is set by the /proc/sys/net/core/rmem_max file.
              The minimum (doubled) value for this option is 256.

       SO_RCVBUFFORCE (since Linux 2.6.14)
              Using this socket option, a privileged (CAP_NET_ADMIN) process
              can perform the same task as SO_RCVBUF, but the rmem_max limit
              can be overridden.

              Specify the minimum number of bytes in the buffer until the
              socket layer will pass the data to the protocol (SO_SNDLOWAT)
              or the user on receiving (SO_RCVLOWAT).  These two values are
              initialized to 1.  SO_SNDLOWAT is not changeable on Linux
              (setsockopt(2) fails with the error ENOPROTOOPT).  SO_RCVLOWAT
              is changeable only since Linux 2.4.

              Before Linux 2.6.28 select(2), poll(2), and epoll(7) did not
              respect the SO_RCVLOWAT setting on Linux, and indicated a
              socket as readable when even a single byte of data was avail‐
              able.  A subsequent read from the socket would then block un‐
              til 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.

              Specify the receiving or sending timeouts until reporting an
              error.  The argument is a struct timeval.  If an input or out‐
              put 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 trans‐
              ferred 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 non‐
              blocking.  If the timeout is set to zero (the default), then
              the operation will never timeout.  Timeouts only have effect
              for system calls that perform socket I/O (e.g., read(2),
              recvmsg(2), send(2), sendmsg(2)); timeouts have no effect for
              select(2), poll(2), epoll_wait(2), and so on.

              Indicates that the rules used in validating addresses supplied
              in a bind(2) call should allow reuse of local addresses.  For
              AF_INET sockets this means that a socket may bind, except when
              there is an active listening socket bound to the address.
              When the listening socket is bound to INADDR_ANY with a spe‐
              cific 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 pro‐
              cesses binding to the same address must have the same effec‐
              tive UID.  This option can be employed with both TCP and UDP

              For TCP sockets, this option allows accept(2) load distribu‐
              tion in a multi-threaded server to be improved by using a dis‐
              tinct 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 connec‐
              tions, or having multiple threads that compete to accept(2)
              from the same socket.

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

       SO_RXQ_OVFL (since Linux 2.6.33)
              Indicates that an unsigned 32-bit value ancillary message
              (cmsg) should be attached to received skbs indicating the num‐
              ber 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 notifica‐
              tions is no longer necessary.  This option is nevertheless re‐
              tained for backwards compatibility.

              Sets or gets the maximum socket send buffer in bytes.  The
              kernel doubles this value (to allow space for bookkeeping
              overhead) when it is set using setsockopt(2), and this doubled
              value is returned by getsockopt(2).  The default value is set
              by the /proc/sys/net/core/wmem_default file and the maximum
              allowed value is set by the /proc/sys/net/core/wmem_max file.
              The minimum (doubled) value for this option is 2048.

       SO_SNDBUFFORCE (since Linux 2.6.14)
              Using this socket option, a privileged (CAP_NET_ADMIN) process
              can perform the same task as SO_SNDBUF, but the wmem_max limit
              can be overridden.

              Enable or disable the receiving of the SO_TIMESTAMP control
              message.  The timestamp control message is sent with level
              SOL_SOCKET and a cmsg_type of SCM_TIMESTAMP.  The cmsg_data
              field is a struct timeval indicating the reception time of the
              last packet passed to the user in this call.  See cmsg(3) for
              details on control 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 de‐
              tails on control messages.

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

              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 oper‐
              ate 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

              While busy polling may improve latency of some applications,
              care must be taken when using it since this will increase both
              CPU utilization and power usage.

       When writing onto a connection-oriented socket that has been shut
       down (by the local or the remote end) SIGPIPE is sent to the writing
       process and EPIPE is returned.  The signal is not sent when the write
       call specified the MSG_NOSIGNAL flag.

       When requested with the FIOSETOWN fcntl(2) or SIOCSPGRP ioctl(2), SI‐
       GIO 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 sig‐
       nal 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 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/.

              contains the default setting in bytes of the socket receive

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

              contains the default setting in bytes of the socket send buf‐

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

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

              Maximum number of packets in the global input queue.

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

       These operations can be accessed using ioctl(2):

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

              Return a struct timeval with the receive timestamp of the last
              packet passed to the user.  This is useful for accurate round
              trip time measurements.  See setitimer(2) for a description of
              struct timeval.  This ioctl should be used only if the socket
              options SO_TIMESTAMP and SO_TIMESTAMPNS are not set on the
              socket.  Otherwise, it returns the timestamp of the last
              packet that was received while SO_TIMESTAMP and SO_TIMESTAMPNS
              were not set, or it fails if no such packet has been received,
              (i.e., ioctl(2) returns -1 with errno set to ENOENT).

              Set the process or process group that is to receive SIGIO or
              SIGURG signals when I/O becomes possible or urgent data is
              available.  The argument is a pointer to a pid_t.  For further
              details, see the description of F_SETOWN in fcntl(2).

              Change the O_ASYNC flag to enable or disable asynchronous I/O
              mode of the socket.  Asynchronous I/O mode means that the SI‐
              GIO 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 syn‐
              onymous with the use of fcntl(2) to set the O_ASYNC flag.)

              Get the current process or process group that receives SIGIO
              or SIGURG signals, or 0 when none is set.

       Valid fcntl(2) operations:

              The same as the SIOCGPGRP ioctl(2).

              The same as the SIOCSPGRP ioctl(2).

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 5.09 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

Linux                            2020-08-13                        SOCKET(7)

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