SOCKET

Section: Linux Programmer's Manual (2)
Updated: 24 July 1993
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NAME

socket, socket_secure - create an endpoint for communication

SYNOPSIS

#include <sys/types.h>
#include <sys/socket.h>

int socket(int domain, int type, int protocol);

int socket_secure(int domain, int type, int protocol, security_id_t sid);

DESCRIPTION

Socket and socket_secure create an endpoint for communication and return a descriptor. Socket_secure may be used to specify the security identifier for the new socket with the sid parameter. Otherwise, the security identifier is inherited from the creating process.

The domain parameter specifies a communications domain within which communication will take place; this selects the protocol family which should be used. These families are defined in <sys/socket.h>. The currently understood formats include:

PF_UNIX, PF_LOCAL: UNIX protocols for local communication
PF_INET: IPv4 Internet protocols; see ip(4)
PF_INET6: IPv6 Internet protocols
PF_IPX: IPX - Novell protocols
PF_NETLINK: Kernel user interface device
PF_X25: ITU-T X.25 / ISO-8208 protocol
PF_AX25: Amateur radio AX.25 protocol
PF_ATMPVC: Access to raw ATM PVCs
PF_APPLETALK: Appletalk; see ddp(4)
PF_PACKET: Low level packet interface; see packet(4)

The socket has the indicated type, which specifies the semantics of communication. Currently defined types are:

SOCK_STREAM: Provides sequenced, reliable, two-way connection-based byte streams. An out-of-band data transmission mechanism may be supported.
SOCK_DGRAM: Supports datagrams (connectionless, unreliable messages of a fixed maximum length).
SOCK_SEQPACKET: Provides a sequenced, reliable, two-way connection-based data transmission path for datagrams of fixed maximum length; a consumer is required to read an entire packet with each read system call.
SOCK_RAW: Provides raw network protocol access.
SOCK_RDM: Provides a reliable datagram layer that does not gurantee ordering.
SOCK_PACKET: Obsolete and should not be used in new programs; see packet(4).

Some socket types may not be implemented by all protocol families; for example, SOCK_SEQPACKET is currently not implemented in Linux for AF_INET.

The protocol specifies a particular protocol to be used with the socket. Normally only a single protocol exists to support a particular socket type within a given protocol family. However, it is possible that many protocols may exist, in which case a particular protocol must be specified in this manner. The protocol number to use is particular to the ``communication domain'' in which communication is to take place; see protocols(5). See getprotoent(3) on how to map protocol name strings to protocol numbers.

Sockets of type SOCK_STREAM are full-duplex byte streams, similar to pipes. A stream socket must be in a connected state before any data may be sent or received on it. A connection to another socket is created with a connect(2) call. Once connected, data may be transferred using read(2) and write(2) calls or some variant of the send(2) and recv(2) calls. When a session has been completed a close(2) may be performed. Out-of-band data may also be transmitted as described in send(2) and received as described in recv(2).

The communications protocols which implement a SOCK_STREAM ensure that data is not lost or duplicated. If a piece of data for which the peer protocol has buffer space cannot be successfully transmitted within a reasonable length of time, then the connection is considered When SO_KEEPALIVE is enabled on the socket the protocol checks in a protocol-specific manner if the other end is still alive. A SIGPIPE signal is raised if a process sends or receives on a broken stream; this causes naive processes, which do not handle the signal, to exit. SOCK_SEQPACKET sockets employ the same system calls as SOCK_STREAM sockets. The only difference is that read(2) calls will return only the amount of data requested, and any remaining in the arriving packet will be discarded. Also all message boundaries in incoming datagrams are preserved.

SOCK_DGRAM and SOCK_RAW sockets allow sending of datagrams to correspondents named in send(2) calls. Datagrams are generally received with recvfrom(2), which returns the next datagram with its return address.

SOCK_PACKET is an obsolete socket type to receive raw packets directly from the device driver. Use packet(4) instead.

An fcntl(2) call with the the F_SETOWN argument can be used to specify a process group to receive a SIGURG signal when the out-of-band data arrives or SIGPIPE signal when a SOCK_STREAM connection breaks unexpectedly. It may also be used to set the process or process group that receives the I/O and asynchronous notification of I/O events via SIGIO. Using F_SETOWN is equivalent to an ioctl(2) call with the SIOSETOWN argument.

When the network signals an error condition to the protocol module (e.g. using a ICMP message for IP) the pending error flag is set for the socket. The next operation on this socket will return the error code of the pending error. For some protocols it is possible to enable a per-socket error queue to retrieve detailed information about the error; see IP_RECVERR in ip(4).

The operation of sockets is controlled by socket level options. These options are defined in <sys/socket.h>. Setsockopt(2) and getsockopt(2) are used to set and get options, respectively.

RETURN VALUES

-1 is returned if an error occurs; otherwise the return value is a descriptor referencing the socket.

ERRORS

EPROTONOSUPPORT: The protocol type or the specified protocol is not supported within this domain.
ENFILE: Not enough kernel memory to allocate a new socket structure.
EMFILE: Process file table overflow.
EACCES: Permission to create a socket of the specified type and/or protocol is denied.
ENOBUFS or ENOMEM: Insufficient memory is available. The socket cannot be created until sufficient resources are freed.
EINVAL: Unknown protocol, or protocol family not available.

Other errors may be generated by the underlying protocol modules.

CONFORMING TO

4.4BSD (the socket function call appeared in 4.2BSD). Generally portable with non-BSD systems supporting clones of the BSD socket layer (including System V variants).

This document was created by man2html, using the manual pages.
Time: 17:08:29 GMT, December 18, 2000