Final code for the "BSD socket API explained" video

bsd.c

// Source for the https://www.youtube.com/watch?v=pp9AM5A1mDs
// Written & tested on MacOS
// To discuss or report issues, join https://discord.gg/KfR8E6wSds

#include <arpa/inet.h>
#include <errno.h>
#include <fcntl.h>
#include <netinet/in.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <time.h>
#include <unistd.h>
#include <string.h>

enum { EV_OPEN, EV_CLOSE, EV_READ, EV_HTTP_REQUEST };

struct conn {
  struct conn *next;  // Next connection in the linked list
  int fd;             // Socket (file descriptor)
  char buf[512];      // HTTP Request data
  int len;            // Number of bytes read so far
  int is_listening;   // Connection is a listening connection
  int is_closing;     // Connection must be closed
  int is_readable;    // Connection is readable
  void (*fn)(struct conn *, int event);
};

struct server {
  struct conn *conns;  // List of all active connections
};

void add_connection(struct server *server, int fd, int is_listening,
                    void (*fn)(struct conn *, int)) {
  struct conn *c = calloc(1, sizeof(*c));
  c->next = server->conns;
  server->conns = c;
  c->fd = fd;
  c->is_listening = is_listening;
  fcntl(fd, F_SETFL, fcntl(fd, F_GETFL, 0) | O_NONBLOCK);
  c->fn = fn;
  fn(c, EV_OPEN);
}


void server_poll(struct server *server) {
  fd_set rset;
  FD_ZERO(&rset);
  int max = 0;

  // First iteration: add all sockets to the read set
  for (struct conn *c = server->conns; c != NULL; c = c->next) {
    FD_SET(c->fd, &rset);
    c->is_readable = 0;
    if (c->fd > max) max = c->fd;
  }

  // Tell kernel: wait until any of the sockets is readable
  if (select(max + 1, &rset, NULL, NULL, NULL) < 0) return;

  // Now, some of the sockets are readable, mark those
  for (struct conn *c = server->conns; c != NULL; c = c->next) {
    c->is_readable = FD_ISSET(c->fd, &rset);
  }

  // Iterate over all connection, accept/read/write
  for (struct conn *tmp, *c = server->conns; c != NULL; c = tmp) {
    tmp = c->next;
    if (c->is_readable == 0) continue;  // Not readable, skip it

    if (c->is_listening) {
      // Listening connection: accept new connection
      struct sockaddr_in sin2;
      socklen_t slen = sizeof(sin2);
      int sock = accept(c->fd, (struct sockaddr *) &sin2, &slen);
      if (sock < 0) continue;
      add_connection(server, sock, 0, c->fn);
      printf("Accepted new connection from %s:%hu\n", inet_ntoa(sin2.sin_addr),
             ntohs(sin2.sin_port));
    } else {
      // Accepted connection: read requests, write responses
      int n = read(c->fd, c->buf + c->len, sizeof(c->buf) - c->len);
      if (n < 0 && errno == EAGAIN) {
        // No data yet, do nothing
      } else if (n <= 0) {
        c->is_closing = 1;
      } else {
        printf("Read %d bytes from %d\n", n, c->fd);
        c->len += n;
        c->fn(c, EV_READ);
        // Did we get an empty line?
        if (memmem(c->buf, c->len, "\r\n\r\n", 4) != NULL ||
            memmem(c->buf, c->len, "\n\n", 2) != NULL) {
          // Yes! Send a response
          c->fn(c, EV_HTTP_REQUEST);
          c->is_closing = 1;
        }
      }
    }

    if (c->is_closing) {
      // Remove from the linked list
      struct conn **head = &server->conns;
      while (*head != c) head = &(*head)->next;
      *head = c->next;
      printf("Closing %p %d\n", c, c->fd);
      c->fn(c, EV_CLOSE);
      close(c->fd);
      free(c);
    }
  }
}

static void ev_handler(struct conn *c, int event) {
  if (event == EV_OPEN) {
    printf("Connection %p opened, fd %d\n", c, c->fd);
  }

  if (event == EV_CLOSE) {
    printf("Connection %p closed, fd %d\n", c, c->fd);
  }

  if (event == EV_READ) {
    printf("Connection %p read data, fd %d, len %d\n", c, c->fd, c->len);
  }

  if (event == EV_HTTP_REQUEST) {
    printf("Connection %p HTTP request!, fd %d, len %d\n", c, c->fd, c->len);
    char resp[128];
    time_t now = time(NULL);
    printf("Sending response\n");
    snprintf(resp, sizeof(resp),
             "HTTP/1.0 200 OK\n"
             "Content-Length: 25\n\n%s",
             ctime(&now));
    write(c->fd, resp, strlen(resp));
  }
}

int main(void) {
  uint16_t port = 9000;
  struct sockaddr_in sin = {.sin_port = htons(port), .sin_family = AF_INET};
  int on = 1, lsn;

  lsn = socket(PF_INET, SOCK_STREAM, 0);
  setsockopt(lsn, SOL_SOCKET, SO_REUSEADDR, &on, sizeof(on));
  bind(lsn, (struct sockaddr *) &sin, sizeof(sin));
  listen(lsn, 128);

  struct server server = {.conns = NULL};
  add_connection(&server, lsn, 1, ev_handler);

  printf("Listening on port %hu\n", port);

  // Infinite loop accepting new connections
  for (;;) {
    server_poll(&server);
  }

  return 0;
}