Battlesnake Challenge # C

In this series I'll share my progress with my self-imposed programming challenge: build a Battlesnake in as many different programming languages as possible.

Check the first post for a short intro to this series.

You can also follow my progress on GitHub.

C

After Python and JavaScript, it's time to roll up the sleeves and pick a language that requires a bit more serious development effort: C.

C is the language of Operating Systems and embedded software. The language has only basic data types, and its standard library offers no convenient high-level functionalities such as JSON parsers or web servers.
To make things even more challenging: in C memory management is the responsibility of the developer, not the language. It has been a while since I needed to do that.

All-in-all, not a great match for building a Battlesnake, but I guess not all implementations can be done within 100 lines of code.

Hello world Setup

This is how snake.c looks:

#include <stdio.h>

int main(int argc, char *argv[]) {
    printf("Hello world!\n");
}
</stdio.h>

This is how the Dockerfile looks:

FROM gcc:14
RUN mkdir /app
WORKDIR /app
COPY snake.c .
RUN gcc -o snake snake.c
CMD ["./snake"]

And here's the development setup in action:

A basic web server

There's no simple web server available in the standard C library.
Writing a web server from scratch, even a simple one, is a huge amount of work and code.
Fortunately, because there are only 4 API endpoints that need to be implemented, and the input is known beforehand, I could cut a lot of corners.
The main connection handling loop looks like this:

while (1) {
    int client_socket = accept(server_socket, &client_address,
        &client_address_len);
    int buffer_size = 32768;
    char *buffer = (char *)malloc(buffer_size * sizeof(char));
    int bytes = (int)recv(client_socket, buffer, buffer_size, 0);
    const char *get_meta_data = "GET /";
    const char *post_start = "POST /start";
    const char *post_move = "POST /move";
    const char *post_end = "POST /end";
    if (strncmp(buffer, get_meta_data, strlen(get_meta_data)) == 0) {
        handle_get_meta_data(client_socket);
    } else if (strncmp(buffer, post_start, strlen(post_start)) == 0) {
        char *body = get_body(client_socket, buffer, buffer_size, bytes);
        handle_start(client_socket, body);
    } else if (strncmp(buffer, post_move, strlen(post_move)) == 0) {
        char *body = get_body(client_socket, buffer, buffer_size, bytes);
        handle_move(client_socket, body);
    } else if (strncmp(buffer, post_end, strlen(post_end)) == 0) {
        handle_end(client_socket);
    }
    close(client_socket);
    free(buffer);
}

A thing that gave me a real headache was handling the HTTP body properly, as the HTTP request can be received chunked in smaller parts:

char *get_body(int client_socket, char *buffer, int buffer_size, int bytes) {
    int content_length = get_content_length(buffer);
    char *result = strstr(buffer, "\r\n\r\n") + 4;
    while (strlen(result) 



<h2>
  
  
  Game logic
</h2>

<p>A big part of the game logic code is there to parse the incoming JSON data. The standard C library does not contain a JSON parser, and a typical parser library contains thousands of lines of code. With a lot of hacks I was able to parse the Battlesnake JSON, <em>and only that JSON</em>, in less than 50 lines of code.</p>

<p>Below are two of the five functions in the code related to JSON parsing:<br>
</p>

<pre class="brush:php;toolbar:false">char * get_field(const char *json, const char *name) {
    char *needle = (char *)malloc((strlen(name) + 3) * sizeof(char));
    sprintf(needle, "\"%s\"", name);
    char *ptr = strstr(json, needle);
    ptr += strlen(needle) + 1;
    free(needle);
    return ptr;
}

char * get_object(const char *json, const char *name, char *buffer) {
    char *ptr = get_field(json, name);
    int idx = 0, indent = 0;
    do {
        if (*ptr == '{')
            indent++;
        else if (*ptr == '}')
            indent--;
        buffer[idx++] = *ptr++;
    } while (indent > 0);
    buffer[idx] = '\0';
    return buffer;
}

The remainder of the game logic is quite mundane, and very simplistic:

void preferred_directions(char *board, int head_x, int head_y, int dirs[]) {
    char *buffer = (char *)malloc(strlen(board) * sizeof(char));
    char *food = get_array(board, "food", buffer);
    int food_x = 256;
    int food_y = 256;
    nearest_food(food, head_x, head_y, &food_x, &food_y);
    if (head_x != food_x) {
        if (head_x 



<p>I'm sure it can be improved a lot by anyone who spend a bit more time on it. But for my challenge this is fine, although admittedly the C snake is not the sharpest tool in the shed.</p>

<p>And this is the complete code in action:</p>

<p><img src="/static/imghwm/default1.png" data-src="https://img.php.cn/upload/article/000/000/000/172111956159191.gif?x-oss-process=image/resize,p_40" class="lazy" alt="C Battlesnake" loading="lazy"    style="max-width:90%"  style="max-width:90%" data-animated="true"></p>

<p>The full code for the C Battlesnake can be found here on GitHub.</p>

<h2>
  
  
  Feedback appreciated!
</h2>

<p>I hope you like reading along with my coding adventures.</p>

<p>Let me know in the comments below what you think about the code above, or what programming languages you are looking forward to in this series.</p>

<p>Until the next language!</p>


          

            
  

            
        

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