Emscripten: "Cannot set timing mode for main loop" & "You should use 0 for the frame rate in emscripten_set_main_loop"

[fgoujeon]

With the following program…:

#include <SDL2/SDL.h>
#include <emscripten.h>

void iterate()
{
}

int main(int, char**)
{
    auto pwindow = SDL_CreateWindow
    (
        "Test",
        SDL_WINDOWPOS_UNDEFINED,
        SDL_WINDOWPOS_UNDEFINED,
        640,
        480,
        SDL_WINDOW_RESIZABLE
    );

    auto prenderer = SDL_CreateRenderer
    (
        pwindow,
        -1,
        0
    );

    emscripten_set_main_loop(iterate, 0, 1);

    return 0;
}

… I get the following error in the web console: emscripten_set_main_loop_timing: Cannot set timing mode for main loop since a main loop does not exist! Call emscripten_set_main_loop first to set one up.

Since emscripten_set_main_loop_timing seems to be called by SDL_CreateRenderer, I try to fix my program like so:

#include <SDL2/SDL.h>
#include <emscripten.h>

void iterate()
{
    SDL_Window* pwindow;
    SDL_Renderer* prenderer;
    static auto first_call = true;

    if(first_call)
    {
        pwindow = SDL_CreateWindow
        (
            "Test",
            SDL_WINDOWPOS_UNDEFINED,
            SDL_WINDOWPOS_UNDEFINED,
            640,
            480,
            SDL_WINDOW_RESIZABLE
        );

        prenderer = SDL_CreateRenderer
        (
            pwindow,
            -1,
            0
        );

        first_call = false;
    }
}

int main(int, char**)
{
    emscripten_set_main_loop(iterate, 0, 1);

    return 0;
}

But now I get the following warning in the web console: Looks like you are rendering without using requestAnimationFrame for the main loop. You should use 0 for the frame rate in emscripten_set_main_loop in order to use requestAnimationFrame, as that can greatly improve your frame rates!

It seems like there's no way to get neither of those errors.

[kripken]

This does seem odd, I'm not sure why SDL_CreateRenderer affects the main loop. And I confirm I see those warnings on the two cases in the test suite that use those two functions (tests/sdl2_gl_frames_swap.c, tests/sdl_togglefullscreen.c).

cc @Daft-Freak

[Daft-Freak]

For the second message, try creating the renderer with the SDL_RENDERER_PRESENTVSYNC flag. I think if you don't pass that the GLES renderer does eglSwapInterval(0). The first message is probably caused by the fact that it calls eglSwapInterval as well.

[rschulman]

I'm having this same problem in porting a game to wasm. The code calls SDL_CreateRenderer long before the main loop is entered while setting up the UI for the first time. I get an error that it can't set timing because a main loop doesn't exist. I tried @Daft-Freak's suggestion to add the PRESENTVSYNC flag, but it doesn't seem to have changed anything. Any other ideas?

[julitopower]

I am experiencing the same error "Cannot set timing, ..." with the following code

#include <iostream>

#include <SDL2/SDL.h>
#include <SDL2/SDL_ttf.h>

#include <emscripten.h>

void loop_handler(void*) {

}

int main(int argc, char** argv) {

  SDL_Init(SDL_INIT_VIDEO);
  TTF_Init();

  SDL_Window* win = SDL_CreateWindow("WebAssembly test", 0, 0, 800, 600, 0);
  SDL_Renderer* renderer = SDL_CreateRenderer(win, 0, 0);


  TTF_Font* font = TTF_OpenFont("assets/DejaVuSansMono.ttf", 100);
  SDL_Color white = {255, 255, 255};
  SDL_Surface* sf = TTF_RenderText_Solid(font, "WebAssembly rocks!", white);
  SDL_Texture* tex = SDL_CreateTextureFromSurface(renderer, sf);

  emscripten_set_main_loop_arg(loop_handler, nullptr, -1, 1);
  //  SDL_RenderCopy(renderer, tex, nullptr, nullptr);
  //  SDL_RenderPresent(renderer);  


  // std::cout << std::endl << "Hello World" << std::endl;

  // SDL_DestroyTexture(tex);
  // SDL_FreeSurface(sf);
  // SDL_DestroyRenderer(renderer);
  // SDL_DestroyWindow(win);

  // TTF_CloseFont(font);
  // TTF_Quit();
  // SDL_Quit();

  return 0;
}

[Daft-Freak]

You can just ignore the error, it's only there when ASSERTIONS is enabled and the main loop still gets created. To fix this we either need to allow calling emscripten_set_main_loop_timing before creating the main loop, or change eglSwapInterval to not use it.

[royalstream]

The warning goes away if I do this, but I wonder if this could have other consequences:

#include <SDL2/SDL.h>
#include <emscripten.h>

void iterate()
{
    // Here I have some code animating a sprite
}

int main(int, char**)
{
    SDL_Init( SDL_INIT_VIDEO ); // I have this call, I guess it makes no difference

    emscripten_set_main_loop(iterate, 0, 1); // moved before creating window and renderer

    auto pwindow = SDL_CreateWindow
    (
        "Test",
        SDL_WINDOWPOS_UNDEFINED,
        SDL_WINDOWPOS_UNDEFINED,
        640,
        480,
        SDL_WINDOW_RESIZABLE
    );

    auto prenderer = SDL_CreateRenderer
    (
        pwindow,
        -1,
        0
    );

   return 0;
}

[Daft-Freak]

That shouldn't work as emscripten_set_main_loop doesn't return, so your window/renderer won't be getting created. Also, calling SDL_CreateRenderer after setting the main loop overrides the main loop timing based on the SDL_RENDERER_PRESENTVSYNC flag. (EM_TIMING_RAF if set, EM_TIMING_SETTIMEOUT otherwise.)

Where the warning comes from:
SDL_CreateRenderer(..., 0) ->
GLES2_CreateRenderer(..., 0) ->
SDL_GL_SetSwapInterval(0) ->
SDL_EGL_SetSwapInterval(..., 0) ->
eglSwapInterval(... 0) ->
emscripten_set_main_loop_timing(EM_TIMING_SETTIMEOUT, 0) ->
"Looks like you are rendering without using requestAnimationFrame for the main loop..."

[royalstream]

Never mind then. I was assuming emscripten_set_main_loop only stored a pointer to the iterate function and returned right away.

[alienzhou]

I have the same errors. However, in my practice, "Cannot set timing mode for main loop" error message wont break the application. It is more like a warning.