Networking System Calls

Networking System Calls | Generated by AI

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In operating systems like Linux/Unix, the lowest-level system calls for networking, particularly for establishing a TCP connection, come from the Berkeley Sockets API (often just called “sockets”). These system calls interact directly with the kernel’s networking stack. The most fundamental ones for TCP connections are:

socket() - Creates a new socket.
bind() - Assigns an address to the socket (optional for clients).
connect() - Initiates a connection to a remote server (for clients).
listen() - Marks the socket as accepting connections (for servers).
accept() - Accepts an incoming connection (for servers).
send() / recv() - Sends or receives data over the connection.
close() - Closes the socket.

These are the raw system calls you’d use at the lowest level, below any higher-level libraries like libcurl or frameworks. Here’s a simple example in C showing both a TCP client and server establishing a connection.

TCP Server Example

This code sets up a server that listens for a connection on port 8080, accepts it, and sends a message.

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

int main() {
    int server_fd, client_fd;
    struct sockaddr_in server_addr, client_addr;
    socklen_t client_len = sizeof(client_addr);
    char *message = "Hello from server!\n";

    // Create socket
    server_fd = socket(AF_INET, SOCK_STREAM, 0);
    if (server_fd == -1) {
        perror("Socket creation failed");
        exit(EXIT_FAILURE);
    }

    // Set up server address structure
    server_addr.sin_family = AF_INET;
    server_addr.sin_addr.s_addr = INADDR_ANY; // Bind to any address
    server_addr.sin_port = htons(8080);       // Port 8080

    // Bind socket
    if (bind(server_fd, (struct sockaddr *)&server_addr, sizeof(server_addr)) < 0) {
        perror("Bind failed");
        exit(EXIT_FAILURE);
    }

    // Listen for connections
    if (listen(server_fd, 3) < 0) { // Allow up to 3 queued connections
        perror("Listen failed");
        exit(EXIT_FAILURE);
    }
    printf("Server listening on port 8080...\n");

    // Accept incoming connection
    client_fd = accept(server_fd, (struct sockaddr *)&client_addr, &client_len);
    if (client_fd < 0) {
        perror("Accept failed");
        exit(EXIT_FAILURE);
    }
    printf("Connection accepted\n");

    // Send message to client
    send(client_fd, message, strlen(message), 0);

    // Clean up
    close(client_fd);
    close(server_fd);
    return 0;
}

TCP Client Example

This code connects to the server at 127.0.0.1:8080 and reads the message.

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

int main() {
    int sock_fd;
    struct sockaddr_in server_addr;
    char buffer[1024] = {0};

    // Create socket
    sock_fd = socket(AF_INET, SOCK_STREAM, 0);
    if (sock_fd == -1) {
        perror("Socket creation failed");
        exit(EXIT_FAILURE);
    }

    // Set up server address structure
    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(8080);              // Port 8080
    inet_pton(AF_INET, "127.0.0.1", &server_addr.sin_addr); // Localhost

    // Connect to server
    if (connect(sock_fd, (struct sockaddr *)&server_addr, sizeof(server_addr)) < 0) {
        perror("Connection failed");
        exit(EXIT_FAILURE);
    }
    printf("Connected to server\n");

    // Receive message from server
    recv(sock_fd, buffer, sizeof(buffer), 0);
    printf("Server message: %s", buffer);

    // Clean up
    close(sock_fd);
    return 0;
}

Explanation of Key System Calls

socket(AF_INET, SOCK_STREAM, 0): Creates a TCP socket (AF_INET for IPv4, SOCK_STREAM for TCP).
bind(): Associates the socket with a local address and port (server-side).
listen(): Prepares the socket to accept connections (server-side).
accept(): Blocks until a client connects, then returns a new socket for communication.
connect(): Initiates a TCP handshake with the server (client-side).
send() / recv(): Handle data transfer over the established connection.
close(): Terminates the socket.

These are as low-level as it gets for TCP networking in user space. Below this, you’d be dealing with kernel internals (e.g., the TCP/IP stack itself), which isn’t directly accessible via system calls without writing kernel code.

To compile and run:

Save the server code as server.c and client code as client.c.
Compile: gcc server.c -o server and gcc client.c -o client.
Run the server first: ./server.
In another terminal, run the client: ./client.

Let me know if you want more details or a deeper dive!

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