Only parts of the screen gets updated

I am currently making my first 3D OpenGL game (with SDL). You’re supposed to find missing children in a forest, and you can place stones behind you, so you know where you’ve been.

Some times when I ‘save’ a kid, or when I place a stone, only a part of the screen gets updated/rendered (the part is a square ~1/4 of the screen).
Edit: For some reason, without doing anything to the code, the whole screen is freezing.

The only thing I can do to fix it is to make the program draw something new, or remove something from the ‘drawing-list’ (like saving a new kid or placing a new stone) when the bug is occurring.
The game itself is running, it’s only the rendering part that freezes up.

I have tried to capture the bug on video with Fraps and Bandicam, but when the bug happens, the recording doesn’t get that. It shows that it is running like normal.

I would like to add some source code, but I have no idea what portion of the code is bugged.
Maybe some of you have seen/heard of it before, and know what could be the cause?

This is how I set up the window:

buffer = SDL_SetVideoMode(width, height, myBpp, SDL_OPENGL | SDL_RESIZABLE); // buffer = SDL_Surface

My first guess is that the glViewport call didn’t get the right parameters (or you didn’t call it at all), you added a scissor test or have a stencil test? If that aren’t the causes, a screenshot might be helpful.

glViewport doesn’t clip. Yes, Scissor test does have an effect. Also, make sure you are clearing your depth buffer.

Clipping might not be the correct term, but if you set glViewport(0,0, wight/2, height/2) you end up with 75% of your window not being rendered, which sound like “the part is a square ~1/4 of the screen”.

Thanks so much for your replies! I read them all and tried out all sorts of stuff.
Finally, it looks like I needed to include the glFinish() function after the rendering.

I hope I’ve fixed it, at least. The glitching hasn’t been present for a while now, after using glFinish()

Sounds like you are using a single buffered context. Use double buffered.