How to use sockets in Linux

2 minutes, 23 seconds
How to use sockets in Linux

linux

Linux sockets are a way to do inter-process communication (IPC), allowing processes to communicate with each other locally or over a network. They support several communication protocols, the most common being TCP and UDP.

Here’s a basic tutorial to get you started with Linux sockets using TCP, as it's the most widely used protocol for connection-oriented communications.

Prerequisites

  • Basic understanding of C programming.
  • A Linux environment to compile and run your code.

Step 1: Include Necessary Headers

First, include the necessary headers in your C program:

C
#include <stdio.h>      // Standard input/output definitions
#include <stdlib.h>     // Standard library
#include <string.h>     // String handling functions
#include <unistd.h>     // UNIX standard function definitions
#include <sys/socket.h> // Main sockets header
#include <netinet/in.h> // Internet address family

Step 2: Create a Socket

You'll need to create a socket using the socket() function. This function returns an integer that acts as a socket descriptor.

C
int sockfd = socket(AF_INET, SOCK_STREAM, 0);
if (sockfd < 0) {
   perror("ERROR opening socket");
   exit(1);
}
  • AF_INET: Address family (IPv4).
  • SOCK_STREAM: Socket type (TCP).
  • 0: Protocol (default protocol for the chosen type, TCP in this case).

Step 3: Bind the Socket

Bind the socket to an address and port on your system using the bind() function.

C
struct sockaddr_in serv_addr;
memset(&serv_addr, 0, sizeof(serv_addr));
serv_addr.sin_family = AF_INET;
serv_addr.sin_addr.s_addr = INADDR_ANY; // Listen on any IP address
serv_addr.sin_port = htons(12345); // Port number
if (bind(sockfd, (struct sockaddr *)&serv_addr, sizeof(serv_addr)) < 0) {
   perror("ERROR on binding");
   exit(2);
}

Step 4: Listen for Connections

After binding, listen for incoming connections:

C
listen(sockfd, 5);

The 5 here specifies the maximum size of the queue for pending connections.

Step 5: Accept Connections

Accept a connection with the accept() function. This function will block the process until a client connects to the server.

C
struct sockaddr_in cli_addr;
socklen_t clilen = sizeof(cli_addr);
int newsockfd = accept(sockfd, (struct sockaddr *)&cli_addr, &clilen);
if (newsockfd < 0) {
   perror("ERROR on accept");
   exit(3);
}

Step 6: Read and Write to the Socket

Use read() and write() functions to receive and send data:

C
char buffer[256];
memset(buffer, 0, 256);
int n = read(newsockfd, buffer, 255);
if (n < 0) perror("ERROR reading from socket");
printf("Here is the message: %s\n", buffer);
n = write(newsockfd, "I got your message", 18);
if (n < 0) perror("ERROR writing to socket");

Step 7: Close the Socket

Finally, close the socket once you're done with it:

C
close(newsockfd);
close(sockfd);
Compiling and Running

To compile your C program, use gcc:

TXT
gcc -o server yourprogram.c

Run your server program:

./server

You'll need a client to connect to this server. You can write a similar program that uses connect() instead of bind() and listen(), or you can use tools like telnet or nc (netcat) to test your server.

This is a very basic introduction to sockets in Linux. Real-world applications may need to handle errors more gracefully, use non-blocking sockets or select/poll for handling multiple connections simultaneously.