Sets the OpenGL attribute attr to value. The attributes you set don't take effect until after a call to SDL_SetVideoMode. You should use SDL_GL_GetAttribute to check the values after a SDL_SetVideoMode call.
SDL_GL_SetAttribute( SDL_GL_RED_SIZE, 5 );
SDL_GL_SetAttribute( SDL_GL_GREEN_SIZE, 5 );
SDL_GL_SetAttribute( SDL_GL_BLUE_SIZE, 5 );
SDL_GL_SetAttribute( SDL_GL_DEPTH_SIZE, 16 );
SDL_GL_SetAttribute( SDL_GL_DOUBLEBUFFER, 1 );
if ( (screen=SDL_SetVideoMode( 640, 480, 16, SDL_OPENGL )) == NULL ) {
fprintf(stderr, "Couldn't set GL mode: %s\n", SDL_GetError());
SDL_Quit();
return;
}Note: The SDL_DOUBLEBUF flag is not required to enable double buffering when setting an OpenGL video mode. Double buffering is enabled or disabled using the SDL_GL_DOUBLEBUFFER attribute.