Networking

Networking#

In C, networking is the BSD sockets API plus a handful of well-known libraries layered on top.

Sockets#

The system call interface is the foundation. A minimal TCP client.

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

int main(void) {
    struct addrinfo hints = {0}, *res;
    hints.ai_family   = AF_UNSPEC;       // IPv4 or IPv6
    hints.ai_socktype = SOCK_STREAM;

    if (getaddrinfo("example.com", "80", &hints, &res) != 0) return 1;

    int fd = socket(res->ai_family, res->ai_socktype, res->ai_protocol);
    if (connect(fd, res->ai_addr, res->ai_addrlen) < 0) return 1;

    const char *req = "GET / HTTP/1.0\r\nHost: example.com\r\n\r\n";
    send(fd, req, strlen(req), 0);

    char buf[4096];
    ssize_t n;
    while ((n = recv(fd, buf, sizeof buf, 0)) > 0) {
        fwrite(buf, 1, (size_t)n, stdout);
    }

    close(fd);
    freeaddrinfo(res);
}

A minimal TCP server uses socket → bind → listen → accept.

Address Resolution#

Always use getaddrinfo / getnameinfo; the older gethostbyname is deprecated and IPv4-only.

I/O Multiplexing#

For more than a handful of connections, don’t thread per connection; use the kernel’s event interface.

  • select / poll, portable but O(n) per call.

  • epoll (Linux), kqueue (BSD/macOS), IOCP (Windows), scalable.

  • io_uring (Linux), async system calls; the modern high-perf path.

TLS#

Don’t roll your own. The standard libraries.

  • OpenSSL , everywhere; large API surface.

  • BoringSSL, Google’s fork; used in Chrome / Android.

  • mbedTLS, embedded-friendly.

  • wolfSSL , compact alternative.

  • LibreSSL , OpenBSD’s OpenSSL fork.

The structure of an OpenSSL client.

SSL_CTX *ctx = SSL_CTX_new(TLS_client_method());
SSL_CTX_set_min_proto_version(ctx, TLS1_2_VERSION);
SSL_CTX_set_default_verify_paths(ctx);

SSL *ssl = SSL_new(ctx);
SSL_set_fd(ssl, fd);
SSL_set_tlsext_host_name(ssl, "example.com");
SSL_connect(ssl);

SSL_write(ssl, req, strlen(req));
SSL_read(ssl, buf, sizeof buf);

Always verify certificates: SSL_get_verify_result and matching the hostname.

HTTP Clients#

HTTP Servers (Embeddable)#

Async / Event Loops#

  • libuv , async I/O; powers Node.js.

  • libevent , mature event-notification library.

  • libev , minimal event loop.

Higher-Level Protocols#

Pitfalls#

  • Partial reads / writes, always loop until the byte count you wanted, or use send/recv flags.

  • Endianness, use htons/htonl and ntohs/ntohl for all multi-byte values on the wire.

  • Buffer sizes, never trust user-supplied lengths; validate.

  • TLS verification, the default is often “accept anything” until you wire up a CA bundle. Always verify.

  • Blocking writes can deadlock, non-blocking sockets + event loop, or per-call timeouts, for anything serious.

Contents