Unix socket I/O primitives

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>

#define PORT 8080
#define BUFFER_SIZE 1024

int main() {
    int server_fd, new_socket, valread;
    struct sockaddr_in address;
    int opt = 1;
    int addrlen = sizeof(address);
    char buffer[BUFFER_SIZE] = {0};
    char *hello = "Hello from server";

    // Create socket file descriptor
    if ((server_fd = socket(AF_INET, SOCK_STREAM, 0)) == 0) {
        perror("socket failed");
        exit(EXIT_FAILURE);
    }

    // Forcefully attaching socket to the port 8080
    if (setsockopt(server_fd, SOL_SOCKET, SO_REUSEADDR | SO_REUSEPORT, &opt, sizeof(opt))) {
        perror("setsockopt");
        exit(EXIT_FAILURE);
    }
    address.sin_family = AF_INET;
    address.sin_addr.s_addr = INADDR_ANY;
    address.sin_port = htons(PORT);

    // Forcefully attaching socket to the port 8080
    if (bind(server_fd, (struct sockaddr *)&address, sizeof(address)) < 0) {
        perror("bind failed");
        exit(EXIT_FAILURE);
    }
    if (listen(server_fd, 3) < 0) {
        perror("listen");
        exit(EXIT_FAILURE);
    }
    if ((new_socket = accept(server_fd, (struct sockaddr )&address, (socklen_t)&addrlen)) < 0) {
        perror("accept");
        exit(EXIT_FAILURE);
    }

    valread = read(new_socket, buffer, BUFFER_SIZE);
    printf("%s\n", buffer);
    send(new_socket, hello, strlen(hello), 0);
    printf("Hello message sent\n");

    return 0;
}

This C program demonstrates basic Unix socket I/O primitives for a server. It creates a socket, binds it to a specific port, listens for incoming connections, accepts a connection, receives data from the client, sends a response, and closes the socket.

Explanation of steps:

Include Headers: Include necessary headers for socket programming (stdio.h, stdlib.h, string.h, unistd.h, arpa/inet.h).
Define Constants: Define constants for the port number (PORT) and buffer size (BUFFER_SIZE).
Declare Variables: Declare variables for the server socket file descriptor (server_fd), client socket file descriptor (new_socket), socket options (opt), address structure (address), address length (addrlen), and a buffer for data exchange (buffer).
Create Socket: Create a socket using the socket system call. Check for errors.
Set Socket Options: Set socket options to allow reuse of the address and port using setsockopt. Check for errors.
Configure Address Structure: Configure the server address structure (address) with the specified port and any local IP address.
Bind Socket: Bind the socket to the specified address using the bind system call. Check for errors.
Listen for Connections: Listen for incoming connections with a queue size of 3 using the listen system call. Check for errors.
Accept Connection: Accept a connection from the client using the accept system call. Check for errors.
Read Data from Client: Read data from the client into the buffer using the read system call.
Print Received Data: Print the received data from the client.
Send Response to Client: Send a "Hello from server" message to the client using the send system call.
Print Sent Message: Print a confirmation message indicating that the response has been sent.
Close Sockets: Close the server and client sockets using the close system call.

Note: Error checking is included to handle potential failures in socket creation, binding, listening, and accepting connections.