EXC_BAD_ACCESS eval_iter.c:171 `if (flecs_query_run_until(...`

[ChadHartman]

Describe the bug

EXC_BAD_ACCESS eval_iter.c:171 if (flecs_query_run_until(...

To Reproduce
Steps to reproduce the behavior:

• Flecs v4.0.5
• Mac OSX 15.5 (24F74)
• Apple clang version 17.0.0 (clang-1700.0.13.3)
• Target: arm64-apple-darwin24.5.0
• C standard c11

Reproducible example:

#include <stdlib.h>
#include <flecs.h>

/* === Components === */

typedef struct Rect {
  float x, y, w, h;
} Rect;

typedef struct BackgroundColor {
  uint8_t r, g, b, a;
} BackgroundColor;

ECS_COMPONENT_DECLARE(Rect);
ECS_COMPONENT_DECLARE(BackgroundColor);

/* === Systems === */

static void App_onStart(ecs_iter_t *it) {

  ecs_entity_t viewPrefab = ecs_entity(
      it->world,
      {.name = "ViewPreFab",
       .add = ecs_ids(EcsPrefab)});
  ecs_set(it->world, viewPrefab, Rect, {0});

  // Root
  ecs_entity_t rootView = ecs_new_w_pair(it->world, EcsIsA, viewPrefab);
  ecs_set(it->world, rootView, BackgroundColor, {0});

  // Button Collection
  ecs_entity_t buttonCollectionView = ecs_new_w_pair(it->world, EcsIsA, viewPrefab);
  ecs_add_pair(it->world, buttonCollectionView, EcsChildOf, rootView);

  // Title
  ecs_entity_t titleView = ecs_new_w_pair(it->world, EcsIsA, viewPrefab);
  ecs_add_pair(it->world, titleView, EcsChildOf, buttonCollectionView);

  // NewGame Button
  ecs_entity_t newGameButtonView = ecs_new_w_pair(it->world, EcsIsA, viewPrefab);
  ecs_add_pair(it->world, newGameButtonView, EcsChildOf, buttonCollectionView);

  // LoadGame Button
  ecs_entity_t loadGameButtonView = ecs_new_w_pair(it->world, EcsIsA, viewPrefab);
  ecs_add_pair(it->world, loadGameButtonView, EcsChildOf, buttonCollectionView);

  // Exit Button
  ecs_entity_t exitButtonView = ecs_new_w_pair(it->world, EcsIsA, viewPrefab);
  ecs_add_pair(it->world, exitButtonView, EcsChildOf, buttonCollectionView);
}

static void App_onStore(ecs_iter_t *it) {

  // Loop through entities
  while (ecs_query_next(it)) {
    const Rect *rects = ecs_field(it, Rect, 0);
    if (ecs_field_is_set(it, 1)) {
      const BackgroundColor *colors = ecs_field(it, BackgroundColor, 1);
      for (int32_t i = 0; i < it->count; ++i) {
        const Rect *rect = &rects[i];
        const BackgroundColor *color = &colors[i];

        printf(
            "Render Rect{x=%.0f, y=%.0f, w=%.0f, h=%.0f}, Color={r=%02x, g=%02x, b=%02x, a=%02x}",
            rect->x,
            rect->y,
            rect->w,
            rect->h,
            color->r,
            color->g,
            color->b,
            color->a);
      }
    } else {
      for (int32_t i = 0; i < it->count; ++i) {
        const Rect *rect = &rects[i];
        printf(
            "Render Rect{x=%.0f, y=%.0f, w=%.0f, h=%.0f}",
            rect->x,
            rect->y,
            rect->w,
            rect->h);
      }
    }
  }

  // Evaluate exit condition
  static uint8_t counter = 0;
  if (++counter == 5) {
    ecs_quit(it->world);
  }
}

/* === Main === */

int main() {

  ecs_world_t *world = ecs_init();
  ECS_COMPONENT_DEFINE(world, Rect);
  ECS_COMPONENT_DEFINE(world, BackgroundColor);
  ECS_SYSTEM(world, App_onStart, EcsOnStart);
  ECS_SYSTEM(world, App_onStore, EcsOnStore, [in] Rect, [in] ?BackgroundColor);

  int res = ecs_app_run(world, &(ecs_app_desc_t){0});
  ecs_fini(world);
  return res;
}

Expected behavior

In my game I run a lua script which creates several view entities during the EcsOnStart phase. I expect that to run once and then render the view entities during the EcsOnStore phase.

Additional context

I apologize if this is not a bug but API abuse. I reread the documentation several times and wasn't sure about several things.

1. Is it appropriate to create entities in systems? If not and using ecs_app_run when can I create entities?
2. Is it ok to create Prefabs in systems?
3. Am I using the optional correctly?
4. Thank you so much!

[SanderMertens]

The issue is the while loop inside the callback. Take a look at this for an example: https://github.com/SanderMertens/flecs/blob/master/examples/c/systems/basics/src/main.c

If you want full control over the iteration inside the system, use the run callback as shown in this example: https://github.com/SanderMertens/flecs/blob/master/examples/c/systems/custom_runner/src/main.c

Is it appropriate to create entities in systems? If not and using ecs_app_run when can I create entities?

Yup that is totally fine.

Is it ok to create Prefabs in systems?

Also fine.

Am I using the optional correctly?

The code works! You can also do this:

    const Rect *rects = ecs_field(it, Rect, 0);
    const BackgroundColor *colors = ecs_field(it, BackgroundColor, 1);
    if (colors) {
      // ...
    }

Thank you so much!

np!