socket api

NAME

socket - create an endpoint for communication

SYNOPSIS

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

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

DESCRIPTION

socket() creates an endpoint for communication and returns a descriptor.

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

NamePurposeMan pagePF_UNIX, PF_LOCALLocal communicationunix(7) PF_INETIPv4 Internet protocolsip(7) PF_INET6IPv6 Internet protocolsipv6(7) PF_IPXIPX - Novell protocols PF_NETLINKKernel user interface devicenetlink(7) PF_X25ITU-T X.25 / ISO-8208 protocolx25(7) PF_AX25Amateur radio AX.25 protocol PF_ATMPVCAccess to raw ATM PVCs PF_APPLETALKAppletalkddp(7) PF_PACKETLow level packet interfacepacket(7)

The socket has the indicated type, which specifies the communication semantics. 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 guarantee ordering.

SOCK_PACKET

Obsolete and should not be used in new programs; see packet(7).

Some socket types may not be implemented by all protocol families; for example,SOCK_SEQPACKET is not implemented 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, in which caseprotocol can be specified as 0. 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 specific to the ``communication domain'' in which communication is to take place; see protocols(5). Seegetprotoent(3) on how to map protocol name strings to protocol numbers.

Sockets of type SOCK_STREAM are full-duplex byte streams, similar to pipes. They do not preserve record boundaries. A stream socket must be in aconnected state before any data may be sent or received on it. A connection to another socket is created with aconnect(2) call. Once connected, data may be transferred usingread(2) and write(2) calls or some variant of thesend(2) and recv(2) calls. When a session has been completed aclose(2) may be performed. Out-of-band data may also be transmitted as described insend(2) and received as described inrecv(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 to be dead. When SO_KEEPALIVE is enabled on the socket the protocol checks in a protocol-specific manner if the other end is still alive. ASIGPIPE 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 thatread(2) calls will return only the amount of data requested, and any data 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 insendto(2) calls. Datagrams are generally received withrecvfrom(2), which returns the next datagram along with the address of its sender.

SOCK_PACKET is an obsolete socket type to receive raw packets directly from the device driver. Usepacket(7) instead.

An fcntl(2) F_SETOWN operation can be used to specify a process or process group to receive aSIGURG signal when the out-of-band data arrives or SIGPIPE signal when aSOCK_STREAM connection breaks unexpectedly. This operation may also be used to set the process or process group that receives the I/O and asynchronous notification of I/O events viaSIGIO. Using F_SETOWN is equivalent to an ioctl(2) call with the FIOSETOWN orSIOCSPGRP 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; seeIP_RECVERR in ip(7).

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

RETURN VALUE

On success, a file descriptor for the new socket is returned. On error, -1 is returned, anderrno is set appropriately.

ERRORS

EACCES

Permission to create a socket of the specified type and/or protocol is denied.

EAFNOSUPPORT

The implementation does not support the specified address family.

EINVAL

Unknown protocol, or protocol family not available.

EMFILE

Process file table overflow.

ENFILE

The system limit on the total number of open files has been reached.

ENOBUFS or ENOMEM

Insufficient memory is available. The socket cannot be created until sufficient resources are freed.

EPROTONOSUPPORT

The protocol type or the specified protocol is not supported within this domain.

Other errors may be generated by the underlying protocol modules. 

#include <stdio.h>#include <stdlib.h>#include <sys/types.h> #include <sys/socket.h>#define EXIT_FALIURE    -1int main(void){    int listen_fd;    listen_fd = socket(PF_INET, SOCK_STREAM, 0);    if (listen_fd < 0)    {        perror("socket");        exit(EXIT_FALIURE);    }    return 0;}