Network IO & HTTP Communication

ABSTRACT

This section covers the interface between C code and the network. We explore the start_server abstraction, the use of Function Pointers to handle requests, and the specific string formatting required to communicate via the HTTP protocol.

1. The Server Entry Point

To start a web server, we use a library function that abstracts away the complex socket setup (socket, bind, listen, accept).

void start_server(void(*handler)(char*, int), int port);

Understanding the Function Pointer Argument

The first argument, void(*handler)(char*, int), is a Function Pointer. This allows start_server to execute a custom logic whenever a new request arrives.

• void: The return type of the function being pointed to.
• char*: The first argument passed to the handler—the raw string of the incoming HTTP request.
• int: The second argument—the file descriptor (socket) used to send the response back to the browser.
• handler: The parameter name used inside start_server to invoke the callback.

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2. Reading and Writing HTTP

HTTP is a text-based protocol. To send a valid response, we must use the write system call and follow a specific header format.

The HTTP Response Header

A valid response must start with a status line and headers, followed by a double newline (\r\n\r\n) to separate the metadata from the body.

char header[] = "HTTP/1.1 200 OK\r\nContent-Type: text/plain\r\n\r\n";

• \r (Carriage Return): Moves the cursor to the beginning of the line.
• \n (Newline): Moves the cursor down to the next line.
• 200 OK: The Response Code (Standard success). Other codes include
  • 404 (Not Found)
  • 500 (Server Error)
  • 403 (Forbidden).
• text/plain: The MIME type, telling the browser how to interpret the bytes.

Implementation: simple_handler

void simple_handler(char* request, int response_socket){
    // Debugging: Print the incoming request to the server terminal
    printf("---start of request---%s\n---end of request---\n", request);
    printf("response socket: %d\n", response_socket);
 
    // Protocol requirement: Headers MUST be sent before the body
    write(response_socket, header, strlen(header)); 
    
    // The response body
    write(response_socket, "Hello!", 6); 
    return;
}

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3. Execution and Memory Context

When we pass a function to start_server, we are passing its memory address.

int main(){
    int a[] = {1};
    printf("a: %p\nsimple: %p\n", &a, &simple_handler);
    start_server(&simple_handler, 8000);
}

Output:

a: 0x7ffd...34
simple: 0x6429...96

Memory Regions

As seen in the output (e.g., a: 0x7ffd... vs simple: 0x6429...), these addresses reside in very different parts of memory:

• The Stack: Where local variables like the array a are stored.
• The Code (Text) Segment: Where the compiled instructions for simple_handler reside. The &simple_handler pointer points here.

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4. Summary of HTTP Components

Component	Example	Description
Status Line	HTTP/1.1 200 OK	Version and success/error code.
MIME Type	Content-Type: text/plain	Format of the data (plain text, html, json).
Separator	\r\n\r\n	Crucial blank line between headers and body.
Body	Hello!	The actual content seen by the user.