Creating 1 way connection among Client and Server using c Lang.

Many of us have studied Computer Networks. It is the subject that provides us the main concepts regarding Data Communication and Networks.

Computer Networks divides the whole system into some steps which are the layers of different model.

The two main models of Computer Networks are OSI model (Open System Interconnection) and TCP/IP model (Transmission Control Protocol/Internet Protocol).

To sum up we have basic knowledge about the networks,their topologies, LAN, WAN, MAN, different devices,how these devices works on different layers, different protocols and algorithms etc.

But when it comes to socket programming many of us fear to write code.
So here I am presenting how to set up one way connection among the client and server.

Prerequistics

1. C Language.
2. Computer Network Basics.

System Requirement and Language

1. Implementation will be on C Language.
2. Linux System is required to perform the task.

Introduction to socket and socket programming

What is a Socket actually?

Socket in nothing but the combination of the IP address and the Port Number. In the transport Layer, process to process delivery is occured, and for this port number is also required along with the IP address of the system.

In order to establish connection among the client and server, both the server and the client need to create socket.

To write code for socket, we use a pre-built structure in C Language, struct sockaddr and struct sockaddr_in which comes under the header <netinet/in.h>

struct sockaddr contains,

struct sockaddr {
  unsigned short sa_family; // address family, AF_xxx
  char sa_data[14]; // 14 bytes of protocol address
}; 

and struct sockaddr_in contains,

struct sockaddr_in {
  short int sin_family; // Address family
  unsigned short int sin_port; // Port number
  struct in_addr sin_addr; // Internet address
  unsigned char sin_zero[8]; // Same size as struct sockaddr
};

In the above structure you could see different variables,
so basically sin_family stores the Address family of the IP Address like if it is IPv4 we use AF_INET and if it is IPv6 we use AF_INET6,
sin_port stores the port number,
sin_addr is a variable of another struct type in_addr whic is defines as:-

struct in_addr {
  uint32_t s_addr; // that's a 32-bit int (4 bytes)
};

Flowchart showing the connection.

Image Source: GFG

As shown in the above message first we need to create socket for both client and server side.

To create socket, socket() function is used. It comes under the header <sys/socket.h>.

Syntax of socket() is:-

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

here, domain is family of the network, for IPv4 it is AF_INET and for IPv6 it is AF_INET6,
type is the socket type, for UDP we use SOCK_DGRAM and for TCP we use SOCK_STREAM,
and the third argument protocol stores the protocol value used in the program. For default protocol we use 0.

Now in server side,

We need to bind the address, i.e we need to cast the struct sockaddr_in to struct sockaddr type or we can say that we have to associate the socket with the local address.
and for this task bind() function is used, which comes under the header <sys/socket.h>.

Syntax for bind() is:-

int bind(int s_fd,struct sockaddr *s_addr,int s_len);

where s_fd is the file descriptor returned by the socket of the server, s_addr is the pointer to the sockaddr and s_len is the length of the s_addr.If it returns -1 then that means binding process is unsuccessfull.

Then we need to listen for the clients, and for this purpose we use listen() function which also comes under the same header <sys/socket.h>.

Syntax for listen() is:-

int listen(int s_fd, int backlog);

where s_fd is the file descriptor returned by the socket, and backlog is the total client limit of the socket. If it returns -1 then that means listening process is unsuccessfull.

The next step is to start accepting the clients, and for this accept() function is used. This function also comes under the header <sys/socket.h>.

Syntax for accept():-

int accept(int s_fd, struct sockaddr *c_addr, int *c_len);

where s_fd is the file descriptor returned by the socket of the server,
c_addr is the pointer to sockaddr of the client file descriptor and c_len is the pointer to the length of c_addr. It return the socket file descriptor of the client. If it return -1 that mean the process is unsuccessfull.

After these steps one can start writing message to the client using write() function and vice-versa.

For Client side
After creating sockect,a client starting connection with the server using connect() function. It is belong to the same header <sys/socket.h>.

Syntax of connect() is:-

int connect(int cfd, const struct sockaddr *c_addr, c_len);

where cfd is the file descriptor returned by the socket of the client, c_addr is the pointer to the sockaddr and c_len is the size of c_addr. If the function returns -1 then it means that the connection is not successfull.

After this step client can connect with the server and can read data written by the server and vice-versa.

Let's begin with the code

Server Code
Let us first try to implement the required headers.

#include<stdio.h>
#include<unistd.h>
#include<netinet/in.h>
#include<sys/socket.h>
#include<string.h>
#include<stdlib.h>

Here,<unistd.h> is used to make use of functions like read() and write().
<stdlib.h> is for exit() and
<string.h> is for strcmp() and other string functions.

Now let's implement the socket() function.

struct sockaddr_in s_addr;    

int s_fd,s_len;

if((s_fd = socket(AF_INET,SOCK_STREAM,0))==-1){             
    printf("[-]Error in Socket\n");
        exit(0);
}
printf("[+]Server Socket created\n");


s_addr.sin_family = AF_INET;                                  
s_addr.sin_port = 3452;                                       
s_len=sizeof(s_addr); 

Let's implement the other functions i.e.
bind(),listen() and accept()

if(bind(s_fd,(struct sockaddr*)& s_addr,s_len)==-1){           
        printf("[-]Error in binding\n");
        exit(0);
    }
    printf("[+]Binding successfull\n");

    if(listen(s_fd,5)==-1){                                     
        printf("[-]Error in listen\n");
        exit(0);
    }
    printf("[+]Listening... \n");


    c_len=sizeof(c_addr);                                   
    if((c_fd=accept(s_fd,(struct sockaddr*)&c_addr,&c_len))==-1){
        printf("\n[-]Error in accepting\n");
        exit(0);
    }
    printf("[+]New Client Connected!!!: \n\n");

The final code will look like :-
server.c

#include<stdio.h>
#include<unistd.h>
#include<netinet/in.h>
#include<sys/socket.h>
#include<sys/types.h>
#include<stdlib.h>

int main()
{
    struct sockaddr_in s_addr,c_addr;    

    int s_fd,c_fd,s_len,c_len;



    if((s_fd=socket(AF_INET,SOCK_STREAM,0))==-1){               
        printf("[-]Error in Socket\n");
        exit(0);
    }
    printf("[+]Server Socket created\n");

    s_addr.sin_family = AF_INET;                                  
    s_addr.sin_port = 4000;                                       
    s_len=sizeof(s_addr); 

    if(bind(s_fd,(struct sockaddr*)& s_addr,s_len)==-1){           
        printf("[-]Error in binding\n");
        exit(0);
    }
    printf("[+]Binding successfull\n");

    if(listen(s_fd,5)==-1){                                     
        printf("[-]Error in listen\n");
        exit(0);
    }
    printf("[+]Listening... \n");


    c_len=sizeof(c_addr);                                   
    if((c_fd=accept(s_fd,(struct sockaddr*)&c_addr,&c_len))==-1){
        printf("\n[-]Error in accepting\n");
        exit(0);
    }
    printf("[+]New Client Connected!!!: \n\n");

    char buff[100];

    while(1)
    {   
        read(c_fd,buff,100);                                      
        printf("From Client: %s\n",buff);           
        if (strcmp(buff, "Disconnected\n") == 0){
                printf("[-]Client Disconnected");
                break;
            }
    }
    close(c_fd);
    return 0;
}

Similarly client code will be:-
client.c

#include<stdio.h>
#include<unistd.h>
#include<netinet/in.h>
#include<sys/socket.h>
#include<string.h>
#include<stdlib.h>

int main()
{
        struct sockaddr_in c_addr;                                 
        int c_fd,c_len;
        char buff[100];
        char buff1[2]; 

        if((c_fd=socket(AF_INET,SOCK_STREAM,0))==-1){            
            printf("[-]Error in Socket\n");
            exit(0);
        }
        printf("[+]Client Socket created\n");

        c_addr.sin_family=AF_INET;                                    
        c_addr.sin_addr.s_addr=INADDR_ANY;                       
        c_addr.sin_port=4000;                                     
        c_len=sizeof(c_addr); 

        if(connect(c_fd,(struct sockaddr*)& c_addr,c_len)==-1) { 
            printf("[-]Error in Connect\n");
            exit(0);
        }
        printf("[+]Connected to the Server: \n\n");
        while(1)
        {
            printf("Enter your Message Mr.Client: ");
            fgets(buff,sizeof(buff),stdin);

            if (strcmp(buff, "Exit\n") == 0){
                printf("[-]Disconneted from Server");
                write(c_fd,"Disconnected\n",100);
                break;
            }
            else{
                write(c_fd,buff,100);
            }
        }       
        close(c_fd);                                              
        return 0;
}

Thanks for going through this little blog. If you find any thing wrong please comment below. Don't hesitate to give feedbacks and suggestions.