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

       socket - Linux socket interface

       #include <sys/socket.h>

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

       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 like AF_INET, AF_IPX,
       AF_PACKET and socket types like 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

               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
              indicates that this is not a listening socket, the value 1
              indicates that this is a listening socket.  This socket option is

              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

              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.

              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.

              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

              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.

              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

       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

              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).

       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.i

              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

              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 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.

              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.

              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

              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.

              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.

              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.

              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

              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

              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.

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

       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/.

              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 buffer.

              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 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).

              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.

              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

              Argument is an integer boolean flag.  (This operation is
              synonymous 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).

       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.

       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.

       are not documented.  The suggested interface to use them is via the
       libpcap library.

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

       This page is part of release 3.53 of the Linux man-pages project.  A
       description of the project, and information about reporting bugs, can be
       found at

Linux                              2013-06-21                          SOCKET(7)