It's been a long time since I did a project for the 3DS and it's cool to see people are still writing stuff for it! You're probably getting a crash due to accessing an invalid pointer. Your framebuffer looks very suspicious! You've just set it to a constant (where did you get it?)! It might be the correct value, but you should get the pointer from gfxGetFramebuffer as that will certainly be correct.

Here's how my project uses the aforementioned function: https://github.com/Gallagator/3ds-Synth/blob/master/source/waveThread.h#L63C20-L63C37

The whole repo is not a great example of how you should write C but it might serve as a guide for how to interact with 3ds.h