Is __thread guaranteed to be zero initalized?

Question

I have two variables. One is __thread the other is static __thread. If I had a constructor all I'd do is zero all the member variables. I also happen to use this class only outside of function bodies. Will my variables be zero initalized skipping the need for me to write a constructor?

To clarify. I have a class that is defined as __thread Foo foo and static __thread Foo foo. I don't mean I have a variable literally named __thread that I somehow tricked the compiler to let me use without naming a type. I'm positive static is zero initialized but IDK if __thread undoes it or if __thread without static will be zero initalized

`__thread` is not a thing in C++. This is a C keyword which has no meaning in C++. — Nicol Bolas, Dec 29 '20 at 06:04
@NicolBolas Some compilers support that, see https://stackoverflow.com/a/7047301/2527795 — VLL, Dec 29 '20 at 06:10
If you're using C++, why don't you use the actual C++ language feature for creating thread-local variables? — Nicol Bolas, Dec 29 '20 at 06:20
@NicolBolas TBH I should have tagged this differently. It is C++ at the moment but I may downgrade to C. I hear thread_local does more work and supports non trivial constructors which I don't need — Eric Stotch, Dec 29 '20 at 06:25

score 1 · Accepted Answer · answered Dec 29 '20 at 06:05

Thread-local variables are zero initialized before any other constructors. If you have no constructors, then only the zero initialization is done. In case of a class type, this means all members are set to zero.

Zero initialization is performed in the following situations:

For every named variable with static or thread-local storage duration that is not subject to constant initialization, before any other initialization.

If T is an non-union class type, all base classes and non-static data members are zero-initialized, and all padding is initialized to zero bits. The constructors, if any, are ignored.

https://en.cppreference.com/w/cpp/language/zero_initialization

Is __thread guaranteed to be zero initalized?

1 Answers1