how to add some log in mutter?

[Halley Zhao via gnome-shell-list]

Hi experts:
I want to debug somthing in mutter, the question is how to add log in
mutter?

the background is:
1.
on my platform(fc30), weston-simple-dmabuf-drm fail to run, it fails after
call zwp_linux_buffer_params_v1_create().
after export WAYLAND_DEBUG=1, I don't get clue.
"
(create_dmabuf_buffer, 711)
(create_dmabuf_buffer,723)format: XR24, width: 256, height: 256, flags: 0x0
Error: zwp_linux_buffer_params.create failed.
Error: zwp_linux_buffer_params.create failed.
Error: zwp_linux_buffer_params.create failed.
"
2.
then I decide to add some log in mutter to see where is the fail.
since libmutter-2.0.so is loaded by gnome-shell in the background, I try to
write my debug log to disk file. but when I add some log in dma_buf_bind(),
it doesn't take effect. why?
what's the correct way to add log in mutter?
thanks

here are code piece to add log:
static void _my_print_test_log(const char* file_name)
{
    FILE* _tmp_fp = NULL;
    _tmp_fp = fopen(file_name, "w+");
    if (_tmp_fp) {
      fprintf(_tmp_fp, "#### my test ####");
      fclose(_tmp_fp);
    }
}

static void
dma_buf_bind (struct wl_client *client,
              void             *data,
              uint32_t          version,
              uint32_t          id)
{
  MetaWaylandCompositor *compositor = data;
  struct wl_resource *resource;

  PRINT_MARK;
  resource = wl_resource_create (client, &zwp_linux_dmabuf_v1_interface,
                                 version, id);
  wl_resource_set_implementation (resource, &dma_buf_implementation,
                                  compositor, NULL);

  _my_print_test_log("/tmp/my_mutter_001.txt");

  send_modifiers (resource, DRM_FORMAT_ARGB8888);
  send_modifiers (resource, DRM_FORMAT_XRGB8888);
  send_modifiers (resource, DRM_FORMAT_ARGB2101010);
  send_modifiers (resource, DRM_FORMAT_RGB565);
}

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