C in the Real World: Networking & Security

BSD Sockets

socket() creates a network endpoint. bind() assigns an address. listen() marks it passive. accept() waits for connections. connect() initiates a connection. send()/recv() transfer data. Close with close(). The socket API is identical across Linux, macOS, and BSD — it has not changed fundamentally since 4.2 BSD (1983). Every network program in any language ultimately calls these same kernel interfaces.

Secure Coding Practices

The most common C vulnerabilities: format string bugs (printf(user_input) instead of printf('%s', user_input) — attacker controls format specifiers), integer overflow (int a = INT_MAX; a++ is undefined behavior), off-by-one errors (<=n should be

Modern C: C11 and C23

C11 added: _Atomic types for lock-free programming, _Static_assert for compile-time checks, thread_local storage, anonymous structs/unions, and improved Unicode support. C23 (ratified 2023) adds: typeof(), #embed for binary file inclusion, nullptr literal (replacing NULL for pointers), bool/true/false as keywords, constexpr for constants. Use -std=c23 or -std=c11 with recent gcc/clang.

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

#define PORT 8080
#define BUFSIZE 4096

int main(void) {
    int server = socket(AF_INET, SOCK_STREAM, 0);
    int opt = 1;
    setsockopt(server, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof(opt));

    struct sockaddr_in addr = {
        .sin_family = AF_INET,
        .sin_port   = htons(PORT),
        .sin_addr   = {INADDR_ANY}
    };
    bind(server, (struct sockaddr*)&addr, sizeof(addr));
    listen(server, 10);
    printf("Listening on :%d\n", PORT);

    while (1) {
        int client = accept(server, NULL, NULL);
        char buf[BUFSIZE];
        ssize_t n = recv(client, buf, sizeof(buf)-1, 0);
        if (n > 0) {
            buf[n] = '\0';
            char resp[] = "HTTP/1.1 200 OK\r\nContent-Length: 13\r\n\r\nHello, World!";
            send(client, resp, sizeof(resp)-1, 0);
        }
        close(client);
    }
}