socket() - create an endpoint for communication
#include <sys/types.h>
#include <sys/socket.h>
int socket (int protofamily, int type, int protocol)
The socket(2) function creates an endpoint for communication and returns a descriptor.
The protofamily parameter specifies a communications protofamily within which communication will take place; this selects the protocol family which should be used. These families are defined in the include file <sys/socket.h>. Currently, the sockets understand only two formats:
The socket has the indicated type, which specifies the semantics of communication. Currently defined types are:
SOCK_STREAM
SOCK_DGRAM
SOCK_RAW
SOCK_SEQPACKET
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 exist, in which case a particular protocol must be specified in this manner. Since only AF_INET protocols are supported in this implementation and because sockets of type SOCK_STREAM and SOCK_DGRAM only support one protocol each, use the value 0 for the protocol.
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 can be sent or received on it. A connection to another socket is created with a connect(2) call. Once connected, data can 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) can be performed. Out-of-band data can also be transmitted as described in send(2) and received as described in recv(2).
The communications protocols used to implement a SOCK_STREAM insure 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 broken and calls will indicate an error with -1 returns and with [ETIMEDOUT] as the specific code in the global variable errno The protocols optionally keep sockets warm by forcing transmissions roughly every minute in the absence of other activity. An error is then indicated if no response can be elicited on an otherwise idle connection for a extended period (such as five minutes). A SIGPIPE signal is raised if a process sends 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.
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.
An fcntl(2) call can be used to specify a process group to receive a SIGURG signal when the out-of-band data arrives. It might also enable non-blocking I/O and asynchronous notification of I/O events through SIGIO.
The operation of sockets is controlled by socket level options These options are defined in the file <sys/socket.h>. The setsockopt(2) and getsockopt(2) functions are used to set and get options, respectively.
The function returns a descriptor referencing the socket; if the function fails, it returns -1 and sets errno to indicate the error.
The socket(2) call fails if:
accept(2)
bind(2)
connect(2)
getsockname(2)
getsockopt(2)
listen(2)
read(2)
recv(2)
select(2)
send(2)
write(2)