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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_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
              IFNAMSZ 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.  Only allowed for processes with the
              CAP_NET_ADMIN capability or an effective user ID of 0.

       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_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_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)
              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 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.  For
              ip(7), this also sets the IP type-of-service (TOS) field for
              outgoing packets.  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.  The select(2) and poll(2)
              system 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 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_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 between the last
              received packet and this received packet.

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

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

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

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

       SO_BUSY_POLL (since Linux 3.11)
              Sets the approximate time in microseconds to busy poll on a
              blocking receive when there is no data.  Increasing this value
              requires CAP_NET_ADMIN.  The default for this option is
              controlled by the /proc/sys/net/core/busy_read file.

              The value in the /proc/sys/net/core/busy_poll file determines
              how long select(2) and poll(2) will busy poll when they
              operate on sockets with SO_BUSY_POLL set and no events to
              report are found.

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

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

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

       When requested with the FIOSETOWN fcntl(2) or SIOCSPGRP ioctl(2),
       SIGIO is sent when an I/O event occurs.  It is possible to use
       poll(2) or select(2) in the signal handler to find out which socket
       the event occurred on.  An alternative (in Linux 2.2) is to set a
       real-time signal using the F_SETSIG fcntl(2); the handler of the real
       time signal will be called with the file descriptor in the si_fd
       field of its siginfo_t.  See fcntl(2) for more information.

       Under some circumstances (e.g., multiple processes accessing a single
       socket), the 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
              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 to send SIGIO or SIGURG
              signals to when an asynchronous I/O operation has finished or
              urgent data is available.  The argument is a pointer to a
              pid_t.  If the argument is positive, send the signals to that
              process.  If the argument is negative, send the signals to the
              process group with the ID of the absolute value of the
              argument.  The process may only choose itself or its own
              process group to receive signals unless it has the CAP_KILL
              capability or an effective UID of 0.

       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 was 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 only allow port reuse 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.

BUGS         top

       The CONFIG_FILTER socket options SO_ATTACH_FILTER and
       SO_DETACH_FILTER are not documented.  The suggested interface to use
       them is via the libpcap library.

SEE ALSO         top

       connect(2), getsockopt(2), setsockopt(2), socket(2), capabilities(7),
       ddp(7), ip(7), packet(7), tcp(7), udp(7), unix(7)

COLOPHON         top

       This page is part of release 3.70 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
       http://www.kernel.org/doc/man-pages/.

Linux                            2014-07-08                        SOCKET(7)