Is it safe to use `std::shared_ptr` and `std::weak_ptr` in a signal handler?

Question

I know it is not safe for malloc or free to be called, directly or indirectly, from a signal handler.

But, if I can guarantee that at least one shared reference will remain alive, is it then safe to copy-construct and destruct additional shared or weak references, or do I have to roll my own refcounting?

(Yes, I know signal handlers usually shouldn't do much. But I have a good reason this time.)

Answer 1

The C++ standard defines the concept of a "plain old function" as follows:

A POF (“plain old function”) is a function that uses only features from [the C/C++] common subset, and that does not directly or indirectly use any function that is not a POF, except that it may use plain lock-free atomic operations.

Furthermore:

The behavior of any function other than a POF used as a signal handler in a C++ program is implementation-defined.

Obviously, C++ class objects are not part of the C/C++ common subset, and therefore using them in a signal handler yields implementation-defined behavior.

Now, many implementations will allow you limited usage of C++ features. If your implementation doesn't permit memory allocations or exceptions, then here's what we know about those smart pointer types.

Then all weak_ptr constructors are explicitly declared noexcept . So they can't throw exceptions. This also means that they're not allowed to allocate memory (since failing to allocate memory would throw an exception). Yes, they could allocate memory and std::terminate if it fails, but that would be exceedingly rude for an implementation to do.

The copy&move constructors of shared_ptr similarly is noexcept , so the same applies. This is also true for the aliasing constructor for shared_ptr .

If you are absolutely certain that at least one shared_ptr will still exist, then destroying a shared_ptr is explicitly stated to not have side effects. Which probably includes memory de-allocations.

Those are the guarantees that the standard library gives you.