Networking, Part 6: Building a web server

How can you serve a simple static web page?

A web server needs to implement HTTP (Hypertext transfer protocol), which is a specification of how client machine can request resources from a server and how a server can respond to a client message.

We're not going to build a fully compliant web server. Instead we will build the simplest possible web server, using the parts of TCP server that have already been introduced.

First, we'll set up a passive server socket

 hints.ai_family = AF_INET;
 hints.ai_socktype = SOCK_STREAM;
 hints.ai_flags = AI_PASSIVE;
 getaddrinfo(NULL, "0", &hints, &result); // "0"  = use any port that is free

We don't need the timeout after we've finished using the port - it can be reused immediately,

 int optval = 1;
 setsockopt(sock_fd, SOL_SOCKET, SO_REUSEPORT, &optval, sizeof(optval));

Bind the socket to a port on the local machine and start listening for connections

 bind(sock_fd, result->ai_addr, result->ai_addrlen)
 listen(sock_fd, 10)

Now that we've bound to a port, we can find out the actual port number used. Notice we need convert it from network to host byte ordering, (ntohs)

  struct sockaddr_in sin;
  socklen_t socklen = sizeof(sin);
  getsockname(sock_fd, (struct sockaddr *)&sin, &socklen)
  printf("Listening on port %d\n", ntohs(sin.sin_port)) 
````

Now call `accept` block until we have a client request to service. For each client, wait for the web browser's request then always send a friendly message back. This starts with the response header that includes the MIMETYPE - the type of data that is represented by the bytes that follow. The response header, like the request header is terminated by two blank lines together `\r\n\r\n`/ Note this code also demonstrates use of Linux's `dprintf` which allows printf-like formatting directly to a file descriptor.
```C
  while(1) {
    int client_fd = accept(sock_fd, (struct sockaddr*) &client_info, &size);

    char *connected_ip= inet_ntoa(client_info.sin_addr);
  // ^^^^ Does this look thread-safe to you?
    int port = ntohs(client_info.sin_port);


    char buffer[1000];
    read(client_fd, buffer, sizeof(buffer)-1);
    dprintf(client_fd, "HTTP/1.0 200 OK\r\n");
    dprintf(client_fd, "Content-Type: text/html\r\n\r\n");
    dprintf(client_fd, "<html><body><h1>Hello</h1></body></html>");
    shutdown(client_fd , SHUT_RDWR);
    close(client_fd);
  }

How can you serve a simple static image file?

  FILE*file = fopen("apicture.jpg","r");
  if(file) {
    fseek(file,0, SEEK_END);
    long size = ftell(file);
    fseek(file,0,SEEK_SET);
    printf("Sending %ld bytes\n", size);

    char*buf = malloc(size);
    fread(buf,1,size,file);

    char response[2048];

    sprintf( response, "HTTP/1.0 200 OK\r\n"
             "Content-Type: image/jpeg\r\n"
             "Content-Length: %ld\r\n\r\n" , size);

    write(client_fd, response, strlen(response));

    write(client_fd, buf, size);
    fclose(file);
    free(buf);
  }     
  shutdown(client_fd , SHUT_RDWR);
  close(client_fd);