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The socket interface is one of several application
programming interfaces (APIs) to the communication protocols. Designed to
be a generic communication programming interface, it was first introduced by
the 4.2BSD UNIX system. Although it has not been standardized, it has become a
de facto industry standard.

4.2BSD allowed two different communication domains: Internet and UNIX.
4.3BSD has added the Xerox Network System (XNS) protocols and 4.4BSD will add
an extended interface to support the ISO OSI protocols.

2.10.3 Basic Socket Calls

The following lists some basic socket interface calls. In the next section
we shall see an example scenario of using these socket interface calls.

lInitialize a socket


FORMAT: int sockfd = socket(int family, int
type, int protocol)


where:



lfamily stands for addressing family. It can take on values
such as AF_UNIX, AF_INET, AF_NS and AF_IUCV. Its purpose is to specify the
method of addressing used by the socket.
ltype stands for the type of socket interface to be used. It can take
on values such as SOCK_STREAM, SOCK_DGRAM, SOCK_RAW, and SOCK_SEQPACKET.
lprotocol can be UDP, TCP, IP or ICMP.
lsockfd is an integer (similar to a file descriptor) returned by the
socket call.


lBind (Register) a socket to a port address


FORMAT: int bind(int sockfd, struct sockaddr
*localaddr, int addrlen)


where:



lsockfd is the same integer returned by the socket call.
llocaladdr is the local address returned by the bind call.


Note that after the bind call, we now have values for the first three
parameters inside our 5-tuple association:


{protocol, local-address, local-process, foreign-address,
foreign-process}
lIndicate readiness to receive connections


FORMAT: int listen(int sockfd, int queue-size)


where:



lsockfd is the same integer returned by the socket call.
lqueue-size indicates the number of connection requests which can be
queued by the system while the local process has not yet issued the
accept call.


lAccept a connection


FORMAT: int accept(int sockfd, struct sockaddr
*foreign-address, int addrlen)


where:



lsockfd is the same integer returned by the socket call.
lforeign-address is the address of the foreign (client) process
returned by the accept call.




Note that this accept call is issued by a server process rather than
a client process. If there is a connection request waiting on the queue for
this socket connection, accept takes the first request on the queue and
creates another socket with the same properties as sockfd; otherwise,
accept will block the caller process until a connection request arrives.
lRequest connection to the server


FORMAT: int connect(int sockfd, struct sockaddr
*foreign-address, int addrlen)


where:



lsockfd is the same integer returned by the socket call.
lforeign-address is the address of the foreign (server) process
returned by the connect call.




Note that this call is issued by a client process rather than a server
process.
lSend and/or receive data


The read(), readv(sockfd, char *buffer, int
addrlen), recv(), readfrom(), send(sockfd, msg,
len, flags), write() calls can be used to receive and send data in
an established socket association (or connection).


Note that these calls are similar to the standard read and
write file I/O system calls.
lClose a socket


FORMAT: int close(int sockfd)


where:



lsockfd is the same integer returned by the socket call.