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Linux / Unix Command: socket
Command Library

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 IO 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
EventPoll flagOccurrence
ReadPOLLIN New data arrived.
ReadPOLLIN A connection setup has been completed (for connection-oriented sockets)
ReadPOLLHUP A disconnection request has been initiated by the other end.
ReadPOLLHUP A connection is broken (only for connection-oriented protocols). When the socket is written SIGPIPE is also sent.
WritePOLLOUT Socket has enough send buffer space for writing new data.
Read/Write POLLIN|
POLLOUT
An outgoing connect(2) finished.
Read/WritePOLLERRAn asynchronous error occurred.
Read/WritePOLLHUPThe other end has shut down one direction.
ExceptionPOLLPRI Urgent data arrived. SIGURG is sent then.

An alternative to poll/select 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_KEEPALIVE
Enable sending of keep-alive messages on connection-oriented sockets. Expects a integer boolean flag.
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_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 not changeable in Linux and their argument size is always fixed to 1 byte. getsockopt is able to read them; setsockopt will always return ENOPROTOOPT.
SO_RCVTIMEO and SO_SNDTIMEO
Specify the sending or receiving timeouts until reporting an error. They are fixed to a protocol specific setting in Linux and cannot be read or written. Their functionality can be emulated using alarm(2) or setitimer(2).
SO_BSDCOMPAT
Enable BSD bug-to-bug compatibility. This is used only by the UDP protocol module and scheduled to be removed in future. If enabled ICMP errors received for a UDP socket will not be passed to the user program. 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 has been removed in Linux 2.2. It is better to fix the user programs than to enable this flag.
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. Only useful for PF_UNIX sockets; see unix(7). Argument is a ucred structure. Only valid as a getsockopt.
SO_BINDTODEVICE
Bind this socket to a particular device like lqeth0rq, 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_DEBUG
Enable socket debugging. Only allowed for processes with the CAP_NET_ADMIN capability or an effective user id of 0.
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_TYPE
Gets the socket type as an integer (like SOCK_STREAM). Can be only read with getsockopt.
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_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_SNDBUF
Sets or gets the maximum socket send buffer in bytes. The default value is set by the wmem_default sysctl and the maximum allowed value is set by the wmem_max sysctl.
SO_RCVBUF
Sets or gets the maximum socket receive buffer in bytes. The default value is set by the rmem_default sysctl and the maximum allowed value is set by the rmem_max sysctl.
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_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.
SO_ERROR
Get and clear the pending socket error. Only valid as a getsockopt. Expects an integer.

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 or SIOCSPGRP ioctl, 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; 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 ioctls 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.
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 IO mode of the socket. Asynchronous IO mode means that the SIGIO signal or the signal set with F_SETSIG is raised when a new I/O event occurs.
Argument is a integer boolean flag.
SIOCGPGRP
Get the current process or process group that receives SIGIO or SIGURG signals, or 0 when none is set.

Valid fcntls:

FIOGETOWN
The same as the SIOCGPGRP ioctl.
FIOSETOWN
The same as the SIOCSPGRP ioctl

SEE ALSO

socket(2), ip(7), setsockopt(2), getsockopt(2), packet(7), ddp(7)


Important: Use the man command (% man) to see how a command is used on your particular computer.

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