non_mainthread_gl.c

#include <SDL.h>
#include <SDL_opengl.h>

typedef void (APIENTRYP PFNGLCLEARCOLORPROC) (GLclampf red, GLclampf green, GLclampf blue, GLclampf alpha);
typedef void (APIENTRYP PFNGLCLEARPROC) (GLbitfield mask);

struct
{
	PFNGLCLEARCOLORPROC ClearColor;
	PFNGLCLEARPROC Clear;
} gl;

struct ThreadData
{
	SDL_Window *win;
	volatile int done;
};

int thread_fun(void *_data)
{
	struct ThreadData *data = _data;

	SDL_GLContext ctx = SDL_GL_CreateContext(data->win);

	gl.ClearColor = (PFNGLCLEARCOLORPROC) SDL_GL_GetProcAddress("glClearColor");
	gl.Clear = (PFNGLCLEARPROC) SDL_GL_GetProcAddress("glClear");

	float prog = 0.0;
	while (!data->done)
	{
		if (prog < 1.0)
			prog += 0.002;

		gl.ClearColor(1.0-prog, 0, prog, 1.0);
		gl.Clear(GL_COLOR_BUFFER_BIT);

		SDL_GL_SwapWindow(data->win);
		SDL_Delay(16);
	}

	SDL_GL_DeleteContext(ctx);

	return 0;
}

int main(int argc, char **argv)
{
	SDL_Init(SDL_INIT_VIDEO);

	SDL_Window *win = SDL_CreateWindow("Subthread GL", SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED,
	640, 480, SDL_WINDOW_OPENGL | SDL_WINDOW_SHOWN | SDL_WINDOW_RESIZABLE);

	struct ThreadData thread_data = { win, 0 };

	SDL_Thread *thread = SDL_CreateThread(thread_fun, "", &thread_data);

	SDL_Event e;
	int done = 0;

	while (!done)
	{
		if (!SDL_WaitEvent(&e))
			break;

		if (e.type == SDL_QUIT)
			break;
	}

	thread_data.done = 1;
	SDL_WaitThread(thread, 0);

	SDL_DestroyWindow(win);

	SDL_Quit();
}