I Built a Simple HTTP Server from Scratch using C
You might wonder how web servers “serve” files, and how they deliver files that piece together a website that shows on your screen. Well, knowing how a web server is implemented is certainly a good way to “learn by building”.
Although implementing a web server in C sounds very basic and detailed at a ground level, but it might help you better understand how HTTP works, and how servers actually interacts with clients. Let’s get started!
How Socket Works
Before building the web server, you need to know how a “socket” works.
If a program (or process) running on a host is like a house, a socket is like a door that lets mails go in and out. When a person in the house receives or delivers a letter, he or she can be agnostic to how the mail is being delivered in the outside world.
Using the socket network interface, our web server can use a set of functions from C standard package <sys/socket.h> and let our server “talk to” clients over the internet. Those clients are also using sockets to talk to us, so socket is basically like a consesus on how to talk with each other over the internet.
Set up Server Socket
To kick things off, I started by creating and configuring a socket for the server. I configured a few things:
AF_INET: use IPv4 (vs IPv6)SOCK_STREAM: use TCP (vs UDP)INADDR_ANY: the server accepts connections from any network interface
bind() binds the socket to a port (I’m using 8080 here) means the socket will listen to any clients trying to connect to the port 8080. listen() takes the maximum number of pending connections (I set it to 10).
Now, my server is all set up and ready to accept incoming client connections.
Handle Client Connections
With the server up and running, the next step is to handle incoming client connections. I used an infinite loop to continuously listen for new clients.
When a client connects, the server accepts the connection and creates a new thread to handle the client’s HTTP request. This way, the server can handle multiple clients concurrently.
I defined a functionhandle_client()that handles an incoming request from a client. I only implemented GET here since it shows a good case on how server “serve” files to clients.
handle_client() receives the request data, extracts the requested file name, and decodes the URL (e.g. convert %20 to space). Then, it identifies the file extension, builds an HTTP response containing the requested file, and sends it back to the client.
Build HTTP Response
Inside handle_client() , I defined another function build_http_response() that constructs an HTTP response, containing a header and the actual file.
The function starts by building an HTTP header with the appropriate MIME type based on the file extension (e.g. if requesting.jpg, MIME type is image/jpeg). If the file doesn’t exist, the function creates a 404 Not Found response. Otherwise, it retrieves the file’s size and appends the 200 OK header to the response buffer.
Next, it reads the file content and appends it to the response buffer. The response buffer returned back to handle_client() is sent back to the client .I set BUFFER_SIZE to 1MB, meaning the server can handle any HTTP response with size up to 1MB.
A Working HTTP Server!
To wrap up, I hope this tutorial on building a simple HTTP server using C help you better understand how servers actually interacts with clients.
Here is the github repo for the full code.
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