SOCKET(7)

NAME
socket - Linux socket interface

SYNOPSIS
#include <sys/socket.h>
mysocket = socket(int socket_family, int socket_type, int protocol);

DESCRIPTION
This manual page describes the Linux networking socket layer user interface. The BSD compatible sockets are the uniform interface between the user process and the network protocol stacks in the kernel. The protocol modules are grouped into protocol families like PF_INET, PF_IPX, PF_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 (only implemented for a few local families like PF_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 non-zero position is not supported on sockets.

It is possible to do non-blocking 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 is also sent.
Write | POLLOUT | Socket has enough send buffer space for writing new data.
Read/Write | POLLIN| POLLOUT | An outgoing finished.
Read/Write | POLLERR | An asynchronous error occurred.
Read/Write | POLLHUP | The other end has shut down one direction.
Exception | POLLPRI | Urgent data arrived. 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 FASYNC 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 OPTIONS
These socket options can be set by using setsockopt(2) and read with getsockopt(2) with the socket level set to SOL_SOCKET for all sockets:

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. Can only be read with getsockopt(2).

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 only works for some socket types, particularly AF_INET sockets. It is not supported for packet sockets (use normal bind(8) there).

SO_BROADCAST
    Set or get the broadcast flag. When enabled, datagram sockets receive packets sent to a broadcast address and they 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_ERROR
    Get and clear the pending socket error. Only valid as a getsockopt(2). Expects an integer.

SO_DONTROUTE
    Don't send via a gateway, only send 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_OOBINLINE
    If this option is enabled, out-of-band data is directly placed into the receive data stream. Otherwise out-of-band data is only passed 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_PEERCRED
    Return the credentials of the foreign process connected to this socket. This is only possible for connected PF_UNIX stream sockets and PF_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). Argument is a ucred structure. Only valid as a getsockopt(2).

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_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 rmem_default sysctl and the maximum allowed value is set by the rmem_max sysctl. 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 initialised to 1. SO_SNDLOWAT is not changeable on Linux (setsockopt 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 parameter 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 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.

SO_REUSEADDR
    Indicates that the rules used in validating addresses supplied in a bind(2) call should allow reuse of local addresses. For PF_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.

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 wmem_default sysctl and the maximum allowed value is set by the wmem_max sysctl. 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 (like SOCK_STREAM). Can only be read with getsockopt(2).

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 realtime 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.

SYSCTLS
The core socket networking sysctls can be accessed using the /proc/sys/net/core/* files or with the sysctl(2) interface.

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 only be used 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.

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).

NOTES
Linux assumes that half of the send/receive buffer is used for internal kernel structures; thus the sysctls are twice what can be observed on the wire. Linux will only allow port re-use 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 re-use 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
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.

VERSIONS
SO_BINDTODEVICE was introduced in Linux 2.0.30. SO_PASSCRED is new in Linux 2.2. The sysctls are new 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.

SEE ALSO
getsockopt(2), setsockopt(2), socket(2), capabilities(7), ddp(7), ip(7), packet(7)