4000+ threads when app run from Qt Creator, but only ~128 threads when run by itself?

[Adam Wilt]

I have an app that can create a lot of threads:

        for (auto i = 0; i < count; i++) {
            QThread *t = new QThread(this);
            Compute *c = new Compute(i+1, nullptr);
            
            if (t && c) {
                threads_.append(t);
                computes_.append(c);
                c->moveToThread(t);
                ...
                t->start(); 
                ...

When I build for release, packaging the app with macdeployqt, and run the app from Qt Creator, I can create over 4000 threads (on macOS, where kern.num_taskthreads=4096). If I exceed that limit, I start getting error messages in the Application Output panel about threads not starting, but the app doesn't crash.

But when I then run the app by double-clicking it in Finder, the app crashes trying to start the Nth thread, where N is 126 to 129:

Thread 129 Crashed:
0   ???                           	    0x7ff8a7b12940 ???
1   libsystem_kernel.dylib        	    0x7ff81764300e __pthread_kill + 10
2   libsystem_pthread.dylib       	    0x7ff8176791ff pthread_kill + 263
3   libsystem_c.dylib             	    0x7ff8175c4d24 abort + 123
4   QtCore                        	       0x1132d5ec9 qAbort() + 9 (qglobal.cpp:3397)
5   QtCore                        	       0x1132d9c79 qt_message_fatal(QtMsgType, QMessageLogContext const&, QString const&) + 9 (qlogging.cpp:1878)
...
16  QtCore                        	       0x1134c7b69 void (anonymous namespace)::terminate_on_exception<QThreadPrivate::start(void*)::$_0>(QThreadPrivate::start(void*)::$_0&&) + 8 (qthread_unix.cpp:292) [inlined]
17  QtCore                        	       0x1134c7b69 QThreadPrivate::start(void*) + 361 (qthread_unix.cpp:315)
18  libsystem_pthread.dylib       	    0x7ff8176794e1 _pthread_start + 125
19  libsystem_pthread.dylib       	    0x7ff817674f6b thread_start + 15

Why is the behavior so different when launching the app by itself, vs. starting it from within Qt Creator? Is there some way to get past the ~128 thread limit I'm seeing when the app is run by itself?

(Note: I don't need to run 4000 threads, though that works surprisingly well on an M1 Mac. I'm just trying to understand why I'm seeing such different behavior!)

[DerReisende]

@Adam-Wilt Qt increases the max number of files limit. Maybe this is the reason for the difference:

#ifdef Q_OS_MACOS
    // increase the number of file that can be opened in Qt Creator.
    struct rlimit rl;
    getrlimit(RLIMIT_NOFILE, &rl);

    rl.rlim_cur = qMin((rlim_t)OPEN_MAX, rl.rlim_max);
    setrlimit(RLIMIT_NOFILE, &rl);
#endif

[Christian Ehrlicher]

Maybe Qtcreator sets some ulimits different? I know there is a max ulimit you can set for threads on linux so maybe there's something similar on mac.

[DerReisende]

@Adam-Wilt Qt increases the max number of files limit. Maybe this is the reason for the difference:

#ifdef Q_OS_MACOS
    // increase the number of file that can be opened in Qt Creator.
    struct rlimit rl;
    getrlimit(RLIMIT_NOFILE, &rl);

    rl.rlim_cur = qMin((rlim_t)OPEN_MAX, rl.rlim_max);
    setrlimit(RLIMIT_NOFILE, &rl);
#endif

[Adam Wilt]

@DerReisende that was it: I put that in main.cpp, and now I can create thousands of threads. And now I know about getrlimit & setrlimit, which I had never seen before. THANK YOU!