Hot Code Reloading in C

More details about this code can be found in my blog post.

This is some example code which demos hot-code reloading using dlopen(3) and dlsym(3) of game code while the game is running.

The code should work in theory on any UNIX platform. It's tested on Mac OS.

The "game" part is not really there -- this is just a demo on hot-reloading at the moment.

The graphics part is done using Raylib.

Why?

When doing graphics programming for games or "recreational" programming, one wants to have a fast feedback loop.

Additionally, when testing / debugging a game, we often need to reload the logic of the game and test it with a given game state. Often enough we can't fully serialise and reload game state from/to disk.

How It Works

The "game" is split into two parts. A main executable and a shared library which holds the game code.

At startup, the main executable looks for a game.so and tries to load it.

In the game loop, when r is pressed, the game.so shared library is reload.

Loading and reloading is done using dlopen(3). We look for symbols in the shared object using dlsym(3) and reset the function pointers to the new symbols.

Function pointers are defined for all the functions we need/want to reload.

Game logic

Game state is allocated on the heap using init_gamestate().

In the game loop, we call update_gamestate() for each frame. This function is supposed to update the global game state. Rendering is done by calling update_frame().

At the beginning of each frame the code checks if the r key is pressed. If so, the do_reload() function is called, which in turn calls reload() with the correct parameters to reload game.so.

links

I got inspired to try this by this blog post, which does something similar but for Windows.

I also found this video by tsoding helpful.

building and running

This project uses cmake. cmake will fetch version 5.0 of raylib and build it:

$ brew install cmake
$ mkdir build
$ cd build
$ cmake ..
$ cmake -build .

This should generate the mygame main executable and the libgamecode.so shared library in the build directory.

To run the demo:

$ ./mygame

Hot Reloading

To test hot reloading, edit src/game.c while the game is running -- e.g change the cube color.

Then rebuild:

$ cd build
$ make

Now hit r with the game window active. The color of the cube should change.