Part 2: Headers and Servers

In parts 2 and 3, you'll be writing your own simple webserver that keeps track of and lets you modify the value of a number (which should be a bit easier than writing a whole chat server).

2.0 Getting started

Task

  1. If you haven't done so already, log onto ieng6, then run the corresponding prep command for your class (cs29fa24 for Joe's, cs29fa24b for Aaron's).
  2. Fork the lab5 starter repo, then clone it. (If you're unsure how, you can refer back to how we did this in Lab 3 and Lab 4)

Header files

Looking into the lab5 repository, you'll find two source files (number-server.c, http-server.c), and one header file (http-server.h). In the previous labs and PAs for the class, our code has consisted of a single .c file, containing a main() and all associated functions. As programs get more complex, it becomes increasingly important to organize the code by seperating it into multiple files.

http-server.h should look like this:

#ifndef HTTP_SERVER_H
#define HTTP_SERVER_H

#include <stdlib.h>
// ... more #includes

void start_server(void(*handler)(char*, int), int port);
#define BUFFER_SIZE 2048

#endif

This header file contains the declaration of start_server, which includes all info needed for C to call a function, which are its arguments and return type. However the definition of the function (i.e. the actual code that it runs) are in http-server.c. To call this function, number-server.c needs the declaration, which it gets by including http-server.h.

EXTRA INFO 📝:

  • It's not necessary to fully understand the implementation of start_server, but the idea behind the function is available in the pa3 writeup.

  • Every function in a C file should have its visibility defined in one of two ways:

    • Public functions (meant to be called from other source files) need to be declared in a header file.
    • Private functions (typically helpers or utilities) should always be declared as static (e.g, static int foo() {...}) to specify that they shouldn't be visible to other .c files.

  • The ifndef/define/endif at the start and end of a header are an include guard

2.1 Compiling with multiple files

When compiling a program with multiple source files, all source (.c) files must be included as arguments to gcc. NOTE: you don't need to specify the header files to gcc; they are automatically pulled in by #include statements.

Task

  1. Try compiling the number_server using gcc. (Note: you may need to pass in the -std=c11 flag)
  2. In your notes: Write down what happens when you don't include the http-server.c file in gcc. What happens when you ONLY include the http-server.c file in gcc?
  3. Once you're successfully able to compile this program, create a Makefile so that you can compile number_server just by typing make.
    • (Note: You can refer to how we did this in the last lab)
    • You may find it useful to also include the debugging flags from last lab: -Wall -Wno-unused-variable -fsanitize=address -g
  4. Run the number-server you just compiled; it should say something like "Server started on port PPPPP".
  5. Open your browser and go to MACHINE.ucsd.edu:PPPPP where MACHINE is the ieng6 server you are on like ieng6-201, ieng6-202, or ieng6-203; PPPPP is the port number from your server like 8000. Your browser will show an error page, but you should see some output from number-server in your terminal
  6. In your notes: Add a screenshot of the terminal output from number-server

Congrats, you've got a working server running!

2.2 String Hunt

For the purposes of this lab, we'll also need a few more functions from the C <string> library in order to build our backend.

In your notes: For each of the following functions, add its signature and write a brief description of what the function does. You can look it up in the manpages by typing (man XXX) in the terminal, where XXX is a function name. For example, typing man strstr in the terminal would give you the manual for the strstr function.

  • strstr

  • sscanf

  • snprintf

2.3 HTTP 404 Response

Currently our number server isn't responding to requests at all. In part 3 of the lab, you'll make your server handle three kinds of requests, but for now lets start by responding with a '404 Page Not Found'.

HTTP Requests

When we request a given url e.g. ieng6-201.ucsd.edu:8000/post?user=joe&message=hi, our web-client (a browser, curl, or our fancy-client) does some work on our behalf.

The start of the url gives us the address (ieng6-201.ucsd.edu) and port (8000) of the server our client connects to. Once connected, it sends an HTTP request, which looks something like this:

GET /post?user=joe&message=hi HTTP/1.1
User-Agent: curl/7.29.0
... many other headers

The string after GET is the path of the request, and that's the part that will change depending on the type of request. In part 3, you'll be examining this path to determine how to respond.

HTTP Responses

Once we've decided what to do with a request, we can send a response back . The second argument to handle_response is an int that is a file descriptor (called the "client socket") for us to write a response to. To send data back to the client, we can use the write function:


write(client_sock, message, message_size);
//    int          char[]   size_t

An HTTP response for an unknown path should return a "404: Not Found" response that looks like the following:

HTTP/1.1 404 Not Found
Content-Type: text/plain

... 404 message to display to user ...

Note: A nuance of this is that the newlines in HTTP responses are not just \n; they are required to be \r\n (which, interestingly, is how line endings work on Windows). Some software will work with just a \n for line breaks, but the correct thing to do is use \r\n, so in a string literal the HTTP header above would look like

"HTTP/1.1 404 Not Found\r\nContent-Type: text/plain\r\n\r\n"

Task

  1. Use curl to access MACHINE.ucsd.edu:PPPPP with the numeric server running. How does curl respond?

  2. In your notes: Add a screenshot of your browser accessing MACHINE.ucsd.edu:PPPPP with the numeric-server still running. How is this different than if the server was not running?

  3. In number-server.c, modify handle_404 to send an HTTP response back to the client. You'll need to call write twice, once to write the response header, and once to write out the body of the response.

    IMPORTANT NOTE: To see the response header being printed, you will need to add the -v flag to the curl command, i.e. 

    curl -v MACHINE.ucsd.edu:PPPPP

  4. Once you've done this, connect to your server from a browser and make sure you can see the error message, including the path of the url you tried to connect to.

  5. In your notes: Add a screenshot of the 'Not Found' page in your browser. How is this different from the previous error page?

  6. Try connecting to different paths on your server. Does you notice anything interesting if you put in add special characters to the path? Spaces? Emoji?