Socket Programming Functions | Introduction of System function for Socket Programming | Socket Programming basics

     Introduction to Socket Programming Functions

What is Socket?

A socket is one endpoint of a two-way communication link between two programs running on the network. The application creates a socket. Socket is an interface between application layer and transport layer.

It is an interface (a “door”) into which an application process can both send and receive message to/from another application process (remote/local application process). Socket is also referred as the application programmer’s interface (API) between the application and the network.

What is Socket Programming?

Socket programming is a way of connecting two nodes on a network to communicate with each other.

Which functions are used in socket programming?

The main functions in <sys/socket.h> are: 

socket ()

bind ()

listen ()

connect ()

accept ()

send ()/recv ()/read ()/write ()/sendto ()/recvfrom ()

close ()

Socket () – A Connection Endpoint

Purpose: It creates socket. This function is used by client and server.

Syntax:

int socket (int family, int type, int protocol)

Example:

int socket (AF_INET, SOCK_STREAM, 0);

 

Bind () – Attaching to an IP and Port

Purpose: Attach itself to a specific port and IP address. This function is used by server.

Syntax:

int bind (int sockfd, struct sockaddr *serv_addr, int addrlen)

sockfd = socket descriptor returned by socket ()

serv_addr = It contains server IP address and port.

addrlen = length of the address in bytes.

Example:

struct sockaddr_in serv_addr;

serv_addr.sin_family = AF_INET;

serv_addr.sin_addr.s_addr = INADDR_ANY;

serv_addr.sin_port = htons(port);

int bind (sockfd, (struct sockaddr *) &serv_addr, sizeof(serv_addr));

 

Connect () – connect to a service

Purpose: Connect to a server port. This function is used by client.

Syntax:

int connect (int sockfd, struct sockaddr *serv_addr, int addrlen)

sockfd: socket descriptor returned by socket ()

serv_addr: filled with all the remote server details

addrlen: size of the server_addr struct

Example:

int connect (sockfd, (struct sockaddr *) &serv_addr, sizeof (serv_addr));

 

Listen () – Wait for a connection

Purpose: The server process calls listen to tell the kernel to initialize a wait queue of connections for this socket. This function is used by server.

Syntax:

int listen (int sockfd, int backlog)

sockfd = socket descriptor returned by socket ()

backlog = Maximum length of the pending connections queue

Example:

int listen (sockfd, 10);

This will allow a maximum of 10 connections to be in pending state.

 

Accept () – A new connection!

Purpose: Accept new connections from new clients. This function is used by server.

Syntax:

int accept (int sockfd, struct sockaddr *cli_addr, int addrlen)

sockfd = socket descriptor returned by socket ()

cli_addr = will hold the new client’s information when accept returns

addrlen= pointer to size of the client structure

Example:

struct sockaddr_in cli_addr;

int len = sizeof(client);

int accept (sock, (struct sockaddr *) &cli_addr, &len);

 

Send () / Recv () – Stream Socket

Purpose: Send, Recv functions are used to send and receive data over stream socket. This function is used by client and server.

Syntax:

int send (int sockfd, void *msg, int len, int flags)

int recv (int sockfd, void *msg, int len, int flags)

sockfd = socket descriptor returned by socket ()

msg = It is the pointer to a data you want to send/recv.

len = It is the length of data; you want to send/recv.

flags = It is set to 0.

Example:

char send_msg [1024], recv_msg [1024];

int sent_bytes, recv_bytes;

sent_bytes = int send (sockfd, send_msg, 1024, 0);

recv_bytes= int recv (sockfd, recv_msg, 1024, 0);

 

Sendto () / Recvfrom () – Datagram Socket

Purpose: Sendto and Recvfrom functions are used to send and receive data over datagram socket. This function is used by client and server.

Syntax:

int sendto (int sockfd, void *msg, int len, int flags, struct sockaddr *to, int tolen);

int recvfrom (int sockfd, void *msg, int len, int flags, struct sockaddr *from, int fromlen);

sockfd = socket descriptor returned by socket ()

msg = It is the pointer to a data you want to send/recv.

len = It is the length of data; you want to send/recv.

flags = It is set to 0.

to = socket address for the host where data has to be sent.

tolen = size of socket address (where data to be sent)

from = socket address for the host where data has to be recv.

fromlen = size of sock address (where data to be received)

 

Close () – Bye...Bye!

Purpose: Close signals the end of communication between a server-client pair. This effectively closes the socket. This function is used by client and server.

Syntax:

int close (int sockfd)

sockfd = socket descriptor returned by socket ()

Example:
int close(sockfd);

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