SDL2 fails to create opengl window in wasm64 builds

[jspanchu]

SDL2 fails to create an opengl window with error message "Could not create GLES window surface"

Reproducible code:

#include "SDL.h"
#include <emscripten.h>
#include <webgl/webgl2.h>

#include <stdio.h>
#include <string.h>

#define WIDTH 640
#define HEIGHT 480

void onDraw(void *arg) {
  SDL_Window *window = *((SDL_Window **)arg);
  glClear(GL_COLOR_BUFFER_BIT);
  glDrawArrays(GL_TRIANGLES, 0, 3);
  SDL_GL_SwapWindow(window);
}

int main(int argc, char *argv[]) {
  int res = SDL_Init(SDL_INIT_VIDEO | SDL_INIT_TIMER);
  if (res) {
    fprintf(stderr, "Error initializing SDL %s", SDL_GetError());
  }
  SDL_GL_SetAttribute(SDL_GL_MULTISAMPLEBUFFERS, 0);

  SDL_GL_SetAttribute(SDL_GL_CONTEXT_PROFILE_MASK, SDL_GL_CONTEXT_PROFILE_ES);
  SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, 3);
  SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 0);

  SDL_Window *window = SDL_CreateWindow("OpenGL Window", SDL_WINDOWPOS_CENTERED,
                                        SDL_WINDOWPOS_CENTERED, WIDTH, HEIGHT,
                                        SDL_WINDOW_OPENGL);
  if (!window) {
    fprintf(stderr, "Couldn't create window: %s\n", SDL_GetError());
    return 0;
  }
  SDL_SetWindowResizable(window, SDL_TRUE);

  SDL_GLContext context = SDL_GL_CreateContext(window);
  if (!context) {
    fprintf(stderr, "Couldn't create context: %s\n", SDL_GetError());
    return 0;
  }

  GLuint vertex_shader;
  GLuint fragment_shader;
  GLuint program;
  {
    const GLchar *source = "attribute vec4 apos;"
                           "attribute vec4 acolor;"
                           "varying vec4 color;"
                           "void main() {"
                           "color = acolor;"
                           "gl_Position = apos;"
                           "}";
    vertex_shader = glCreateShader(GL_VERTEX_SHADER);
    glShaderSource(vertex_shader, 1, &source, NULL);
    glCompileShader(vertex_shader);
  }
  {
    const GLchar *source = "precision lowp float;"
                           "varying vec4 color;"
                           "void main() {"
                           "gl_FragColor = color;"
                           "}";
    fragment_shader = glCreateShader(GL_FRAGMENT_SHADER);
    glShaderSource(fragment_shader, 1, &source, NULL);
    glCompileShader(fragment_shader);
  }
  program = glCreateProgram();
  glAttachShader(program, vertex_shader);
  glAttachShader(program, fragment_shader);
  glLinkProgram(program);
  glUseProgram(program);
  // clang-format off
  const float pos_and_color[] = {
    //     x,     y, r, g, b
       -0.6f, -0.6f, 1, 0, 0,
        0.6f, -0.6f, 0, 1, 0,
        0.f,   0.6f, 0, 0, 1,
    };
  // clang-format on
  GLuint vbo;
  glGenBuffers(1, &vbo);
  glBindBuffer(GL_ARRAY_BUFFER, vbo);
  glBufferData(GL_ARRAY_BUFFER, sizeof(pos_and_color), pos_and_color,
               GL_STATIC_DRAW);
  glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, 20, 0);
  glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, 20, (void *)8);
  glEnableVertexAttribArray(0);
  glEnableVertexAttribArray(1);

  emscripten_set_main_loop_arg(onDraw, &window, 0, 1);
}

32-bit works.

$ emcc main.c -o index.html -sUSE_SDL=2 -sUSE_WEBGL2
$ python3 -m http.server 

64-bit doesn't work

$ emcc main.c -o index.html -sMEMORY64=1 -sMAXIMUM_MEMORY=6GB -sALLOW_MEMORY_GROWTH -sWASM_BIGINT=1 -sUSE_SDL=2 -sUSE_WEBGL2
$ python3 -m http.server 

Version of emscripten/emsdk:

emcc --version
emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 3.1.46-git (7c7321c265c47d54dacdd25d00c2c3c9ec2a8425)
Copyright (C) 2014 the Emscripten authors (see AUTHORS.txt)
This is free and open source software under the MIT license.
There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Version of browser:
Chromium Version 119.0.6011.0 (Developer Build) custom (64-bit)

[sbc100]

Thats interesting, I thought that SDL2 itself currently fails to compile with wasm64. I currently see:

/usr/local/google/home/sbc/dev/wasm/emscripten/cache/ports/sdl2/SDL-release-2.24.2/src/SDL_assert.c:263:25: error: cast to 'char *' from smaller integer type 'int' [-Werror,-Wint-to-pointer-cast]
  263 |             char *buf = (char *) EM_ASM_INT({
      |                         ^~~~~~~~~~~~~~~~~~~~~
  264 |                 var str =
      |                 ~~~~~~~~~
  265 |                     UTF8ToString($0) + '\n\n' +
      |                     ~~~~~~~~~~~~~~~~~~~~~~~~~~~
  266 |                     'Abort/Retry/Ignore/AlwaysIgnore? [ariA] :';
      |                     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  267 |                 var reply = window.prompt(str, "i");
      |                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  268 |                 if (reply === null) {
      |                 ~~~~~~~~~~~~~~~~~~~~~
  269 |                     reply = "i";
      |                     ~~~~~~~~~~~~
  270 |                 }
      |                 ~
  271 |                 return allocate(intArrayFromString(reply), 'i8', ALLOC_NORMAL);
      |                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  272 |             }, message);
      |             ~~~~~~~~~~~
1 error generated.

Did you somehow work around this error? I recently landed this change is that first step in fixing the SDL2 build: #20258

[jspanchu]

Yep, there's a work around that's not obvious from my other PR #20276. I should've mentioned it there.

1. In order for SDL2 to build I've had to change my SDL_assert.c to use EM_ASM_PTR instead. And the other change is in SDL_emscriptenmouse.c to use MAIN_THREAD_EM_ASM_PTR.
2. After that, a01c286 fixed GLES surface creation.