This C program implements a simple server using TCP (Transmission...

September 1, 2025 at 08:34 AM

/* A simple server in the internet domain using TCP
   The port number is passed as an argument */
#include <stdio.h>
#include <sys/types.h> 
#include <sys/socket.h>
#include <netinet/in.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

void error(char *msg)
{
    perror(msg);
    exit(1);
}

int main(int argc, char *argv[])
{
     int sockfd, newsockfd, portno, clilen;
     char buffer[256];
     struct sockaddr_in serv_addr, cli_addr;
     int n;
     if (argc < 2) {
         fprintf(stderr,"ERROR, no port provided\n");
         exit(1);
     }

     /* create socket */

     sockfd = socket(AF_INET, SOCK_STREAM, 0);
     if (sockfd < 0) 
        error("ERROR opening socket");

     /* fill in port number to listen on. IP address can be anything (INADDR_ANY) */

     bzero((char *) &serv_addr, sizeof(serv_addr));
     portno = atoi(argv[1]);
     serv_addr.sin_family = AF_INET;
     serv_addr.sin_addr.s_addr = INADDR_ANY;
     serv_addr.sin_port = htons(portno);

     /* bind socket to this port number on this machine */

     if (bind(sockfd, (struct sockaddr *) &serv_addr,
              sizeof(serv_addr)) < 0) 
              error("ERROR on binding");
     
     /* listen for incoming connection requests */

     listen(sockfd,5);
     clilen = sizeof(cli_addr);

     /* accept a new request, create a newsockfd */

     newsockfd = accept(sockfd, (struct sockaddr *) &cli_addr, &clilen);
     if (newsockfd < 0) 
          error("ERROR on accept");

     /* read message from client */

     bzero(buffer,256);
     n = read(newsockfd,buffer,255);
     if (n < 0) error("ERROR reading from socket");
     printf("Here is the message: %s\n",buffer);

     /* send reply to client */

     n = write(newsockfd,"I got your message",18);
     if (n < 0) error("ERROR writing to socket");
     return 0; 
}

This C program implements a simple server using TCP (Transmission Control Protocol). Here's a line-by-line explanation of what the code does:

Setup and Error Handling:
- The error() function is defined to print error messages and terminate the program.
Main Function:
- Argument Check:
  - The program checks if a port number is provided as a command-line argument. If not, it prints an error message and exits.
- Socket Creation:
  - A socket is created using the socket() function with these parameters:
    - AF_INET: Indicates an IPv4 internet domain.
    - SOCK_STREAM: Specifies a TCP (stream-based) connection.
  - If the socket() call fails, an error is reported, and the program exits.
- Bind to Port:
  - The serv_addr structure is initialized using the provided port number:
    - sin_family is set to AF_INET for IPv4.
    - sin_addr is set to INADDR_ANY, allowing the server to listen on all network interfaces.
    - sin_port is set to the specified port number (converted to network byte order using htons()).
  - The bind() function associates the socket with the specified IP address and port number.
  - If binding fails, an error is reported, and the program exits.
- Listen for Connections:
  - The listen() function is called to mark the socket as a passive socket, ready to accept incoming connection requests.
  - A backlog of 5 is specified, meaning up to 5 pending connections are queued.
- Accept Incoming Connections:
  - The accept() function waits for a client to connect. Once a client connects, it creates a new socket (newsockfd) specific to the connection with that client.
  - If accept() fails, an error is reported, and the program exits.
- Communication with the Client:
  - Read From Client:
    - The read() function reads data sent by the client into a buffer.
    - If reading fails, an error is reported, and the program exits.
    - After a successful read, the received message is printed to the server's standard output.
  - Send a Reply:
    - The write() function sends a response back to the client: "I got your message".
    - If writing fails, an error is reported, and the program exits.
- Return:
  - The program completes successfully and exits.

Summary:

This program is a basic example of a TCP server. The server:

Accepts a port number as an argument and binds to it.
Waits for a client to connect.
Reads a message from the client and prints it to the server terminal.
Sends a response ("I got your message") to the client.

It serves as an introductory implementation of socket programming in C to demonstrate TCP communication.

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