Network Programming - Part 1

Hi Aspirants,

We'll learn about network programming today. As a core concept of Networks, it goes deep and requires a good amount of time to cover. We'll try to limit it to the scope of specialist officer's exams. If anyone interested in deep learning, raise it in comments, I'll share the sources.

==>> This article is a part of PK Series

What is Inter Process Communication (IPC)?

IPC is a mechanism that allows exchange of data between processes, by using a set of programming interfaces. The IPC mechanism can be classified into Pipes (FIFO) and Shared memory (Semaphores). In pipes, data flow is unidirectional. A pipe is generally created by invoking a pipe system call which generates a pair of file descriptors. We'll learn about shared memory concepts and semaphores in Operating Systems.

 What is a file descriptor?

In simple words, when you open a file, OS creates an entry to represent that file and stores information about that file (somewhere in kernel). These entries are integers and are called file descriptors. So if your process opens 10 files, your process table will 10 entries for file descriptors. Similarly, when you open a network socket, it is represented by an integer which is called socket descriptor. 

What is a socket?

The socket is a BSD method for accomplishing IPC. It allows one process to speak to another and works very similar to files i.e read/write on a socket is similar to that of files. A socket is created using the socket( ) system call. 

Types of Sockets

Two most common types are:

• SOCK_STREAM: Stream sockets, which provide reliable two way connection oriented communication streams. It uses TCP.
• SOCK_DGRAM: Datagram sockets, which provide connectionless, unreliable service, used for packet by packet transfer of information. It uses User Datagram Protocol (UDP)

System Calls using Sockets

1. socket( ) - returns socket descriptor, -1 on error.
Syntax:
#include <sys/types.h>
#include <sys/socket.h>
int socket (int domain, int type, int protocol);

domain should be set to AF_INET (uses ip address)
type - SOCK_STREAM or SOCK_DGRAM
protocol- set to zero typically.

2. bind( ) - associates the socket with an address, -1 on error.
Syntax:
#include <sys/types.h>
#include <sys/socket.h>
int socket (int sockfd, struct sockaddr *my_addr, int addrlen);

sockfd- socket file descriptor returned by socket( ).
my_addr- pointer to a structure that contains information about local IP address and port number.
addrlen- set to sizeof (struct sockaddr)

3. connect( ) - connect to a remote socket, -1 on error
Syntax:
#include <sys/types.h>
#include <sys/socket.h>
int socket (int sockfd, struct sockaddr *serv_addr, int addrlen);

serv_addr- pointer to a structure that contains the destination IP address and port number

4. listen( ) - wait for incoming connections, -1 on error
Syntax:
int listen (int sockfd, int backlog);

backlog- set max no of requests that will be queued before requests are denied.

5. accept( )- returns a new socket file descriptor for every single connection, -1 on error
Syntax:
#include <sys/socket.h>
int accept (int sockfd, void *addr, int *addrlen);

addr- pointer to a local structure sockaddr_in, here information about incoming connection will be stored
addrlen- should be set to sizeof(struct sockaddr_in) before accept( ) is called.

Other systems calls along with rest of the topic will be covered in next article.

 Quote of the day 

The secret of getting ahead is getting started. -Mark Twain

                                                                                                                                             Deepak A 

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