std::unique_lock and std::shared_lock in same thread - shouldn't work, but does?

Question

Why is it possible to acquire two different locks on a std::shared_mutex?

The following code

#include <shared_mutex> #include <iostream> int main(int /*argc*/, char * /*argv*/[]) { std::shared_mutex mut; std::unique_lock<std::shared_mutex> ulock{mut}; if(ulock) std::cout << "uniqe_lock is bool true\n"; if(ulock.owns_lock()) std::cout << "uniqe_lock owns lock\n"; std::shared_lock<std::shared_mutex> slock{mut}; if(slock) std::cout << "shared_lock is bool true\n"; if(slock.owns_lock()) std::cout << "shared_lock owns lock\n"; }

when compiled with a g++ 9.4.0 (current available on Ubuntu 20.04.1)

g++ -std=c++17 mutex.cpp

outputs all cout lines. I.e. the thread acquires both the unique_lock and the shared_lock. As I understand cppreference on the shared_mutex, acquiring the second lock should fail while the first one is still alive.

"Within one thread, only one lock (shared or exclusive) can be acquired at the same time."

What do I miss here? Why can the shared_lock be acquired even with the previous unique_lock still active?

@kaba A thread locking a std::shared_mutex that it already has a lock on is Undefined Behavior. See here : "If lock is called by a thread that already owns the mutex in any mode (shared or exclusive), the behavior is undefined." — François Andrieux
– François Andrieux, Commented Jun 27, 2022 at 15:26
@FrançoisAndrieux Al right, thanks, that was the part I missed (so the sentence I quoted now seems a bit misleading to me). The fact you hinted on, also holds the other way around (i.e. is stated for lock_shared as well). — kaba
– kaba, Commented Jun 27, 2022 at 15:32

ildjarn · Accepted Answer · 2022-06-27 15:27:14Z

Also from cppreference: "If lock_shared is called by a thread that already owns the mutex in any mode (exclusive or shared), the behavior is undefined."

You have UB; the toolchain is not required to diagnose this, though it might if you enabled your stdlib's debug mode (_GLIBCXX_DEBUG for libstdc++).

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