Detect whether currently running on main thread

Platform
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1

Hello.
My UI involves lots of threads, and often call gdk_threads_idle_add to apply updates to UI.
Some of the calls need the result to be returned back,
so I let the calling thread listen to the mutex to be written by the idle task.

Problem is, main loop will deadlock if I accidentally call such function from the main thread. The calling main thread is waiting for main loop inside the main loop! I would like to detect such occasions so I could debug these cases easily.
In short, is there a way to detect if current thread is the main thread?

Thanks in advance.

1 Like
2

You may call g_thread_self() from either main or your GtkApplication::startup signal handler and store the returned id:

static GThread *main_thread_id;

gboolean
is_main_thread (void)
{
  assert (main_thread_id != NULL);

  return g_thread_self () == main_thread_id;
}


int
main ()
{
  main_thread_id = g_thread_self ();

  /* ... */
}
3

Is there any way to do it without global variable? It makes me feel iffy…

4

You could check if the current thread is the owner of the global default main context.

5

If you are passing messages between a worker thread and the main thread, you should probably have a GMainContext handle for each. For the main thread, this will be NULL. There should be a 1:1 correspondence between GMainContext instances and threads: each thread should have exactly one GMainContext. Any operation you’re running should probably have a pointer to the GMainContext it’s meant to run in or return its results to.

As such, you can check which GMainContext a function is running in by calling g_main_context_is_owner().

There’s an example of this in gnome-software (more here).

If you haven’t already, you should read these guides:

3 Likes
6

No matter what you do, you’ll come down to this, either directly by your application or a level below because there is no concept of “main thread” at the operating system level in Linux.

Your other option is to check if gettid() == getpid(), but this is Linux only (and there is no syscall wrapper for gettid().

GNOME Builder sets a global to g_thread_self() at startup and asserts everywhere we expect to be on the main thread to catch threading issues early and often.

1 Like
7

Thank you! In my case, I guess I can check if current thread is owner of global main context.

I see, I simply prefer global variable-involving stuffs to be handled by the layer below. I had bitten by globals, a lot…

8

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