When exactly is a thread_local variable declared at global scope initialized?

Question

For example:

#include <thread>

thread_local int n = 1;

void f()
{
    ++n; // is n initialized here for each thread or prior to entering f()?
}

int main()
{
    std::thread ta(f);
    std::thread tb(f);

    ta.join();
    tb.join();
}

It's still not entirely clear from here when is n initialized.

Related: http://stackoverflow.com/questions/24253584/when-is-a-thread-local-global-variable-initialized — Revolver_Ocelot, Feb 19 '16 at 15:08
It does look confusing a bit. What's _local_ is the _storage_ for n for each thread. But the scope of n (i.e. its _visibility_ in code) is global. — Leo Heinsaar, Feb 19 '16 at 16:04
The initialization itself takes place as the answers (here and in the linked question) describe: before its use. When exactly? Not specified by the standard. — Leo Heinsaar, Feb 19 '16 at 16:10

score 6 · Accepted Answer · answered Feb 19 '16 at 14:58

6

Simple enough, and all according to specification. n is going to be initialized whenever the new thread is run - before you enter any thread-specific functions.

To be exact, it is going to be initialized three times.

answered Feb 19 '16 at 14:58

SergeyA

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When exactly is a thread_local variable declared at global scope initialized?

1 Answers1

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