Are thread_local objects initialized to 0 in C?

Question

Are non-local thread storage duration objects initialized to 0 in C?

static thread_local int x; // is x initialized to 0?

The C17 standard says the below (in 6.2.4.4).

An object whose identifier is declared with the storage-class specifier _Thread_local has thread storage duration. Its lifetime is the entire execution of the thread for which it is created, and its stored value is initialized when the thread is started.

It does not explicitly say it is initialized to 0. But the C++ standard makes it explicit and says: (in 3.6.2.2)

Variables with static storage duration ([basic.stc.static]) or thread storage duration ([basic.stc.thread]) shall be zero-initialized ([dcl.init]) before any other initialization takes place

So, does C initialize non-local thread_local objects to 0 or not?

user_298225 · Answer

Thread local objects without explicit initialization are initialized to “zero.”

C 2018 7.26.1 3 says thread_local expands to _Thread_local if is included. That is not shown in the question, but presumably it is.

C 2018 6.2.4 4 says “An object whose identifier is declared with the storage-class specifier _Thread_local has thread storage duration…

C 2018 6.7.9 10 says:

… If an object that has static or thread storage duration is not initialized explicitly, then:

— if it has pointer type, it is initialized to a null pointer;

— if it has arithmetic type, it is initialized to (positive or unsigned) zero;

— if it is an aggregate, every member is initialized (recursively) according to these rules, and any padding is initialized to zero bits;

— if it is a union, the first named member is initialized (recursively) according to these rules, and any padding is initialized to zero bits;

The paragraph ends there; the trailing “;” instead of a period is a typographical error.

Are thread_local objects initialized to 0 in C?

1 Answers