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Re: Does static initialization of a local variable always happens synchronized ?

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Am 11.09.2021 um 13:47 schrieb Alf P. Steinbach:
> On 11 Sep 2021 11:06, Bonita Montero wrote:
>> Look at this code:
>>
>> #include <iostream>
>>
>> using namespace std;
>>
>> #if defined(_MSC_VER)
>>      #define NOINLINE __declspec(noinline)
>> #elif defined(__GNUC__) || defined(__clang__)
>>      #define NOINLINE __attribute((noinline))
>> #endif
>>
>> struct S
>> {
>>      S();
>> };
>>
>> NOINLINE
>> S::S()
>> {
>>      cout << "S::S()" << endl;
>> }
>>
>> NOINLINE
>> S &fn()
>> {
>>      static
>>      S s;
>>      return s;
>> }
>>
>> int main()
>> {
>>      fn();
>>      fn();
>> }
>>
>>
>> I call fn twice, but of course s is initialized only once at the first
>> call of fn. What I asked myself if this happens synchronized, i.e. if
>> multiple threads try to initialize s, it's only initiailized only once
>> in a locked context.
>> With MSVC this happens as expected: before the initialization of s, fn
>> calls _Init_thread_header and afterwards it calls _Init_thread_footer.
>> These functions each call EnterCriticalSection or LeaveCriticalSection
>> on a global mutex. But is this behaviour guaranteed for C++ in general ?
> 
> Yes.
> 
> C++ 17 §9.7/4 (next to last sentence applies):
> ❝Dynamic initialization of a block-scope variable with static storage 
> duration or thread storage duration is performed the first time control 
> passes through its declaration; such a variable is considered 
> initialized upon the completion of its initialization. If the 
> initialization exits by throwing an exception, the initialization is not 
> complete, so it will be tried again the next time control enters the 
> declaration. If control enters the declaration concurrently while the 
> variable is being initialized, the concurrent execution shall wait for 
> completion of the initialization. If control re-enters the declaration 
> recursively while the variable is being initialized, the behavior is 
> undefined.❞

It couldn't be really undefined since the flag which remembers that
the object has been created successfully is set afterwards. Otherwise
it woud be impossible that the object will be tried to be re-created
if the constructor throws an exception and the function containing
the object will be called again. So an infinite recursion is the only
possible behaviour here.

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Does static initialization of a local variable always happens synchronized ? Bonita Montero <Bonita.Montero@gmail.com> - 2021-09-11 11:06 +0200
  Re: Does static initialization of a local variable always happens synchronized ? "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2021-09-11 13:47 +0200
    Re: Does static initialization of a local variable always happens synchronized ? Bonita Montero <Bonita.Montero@gmail.com> - 2021-09-11 16:17 +0200
      Re: Does static initialization of a local variable always happens synchronized ? "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2021-09-11 17:09 +0200
        Re: Does static initialization of a local variable always happens synchronized ? Bonita Montero <Bonita.Montero@gmail.com> - 2021-09-11 17:18 +0200
        Re: Does static initialization of a local variable always happens synchronized ? David Brown <david.brown@hesbynett.no> - 2021-09-11 20:03 +0200
  Re: Does static initialization of a local variable always happens synchronized ? Paavo Helde <myfirstname@osa.pri.ee> - 2021-09-11 15:13 +0300
  Re: Does static initialization of a local variable always happens synchronized ? Bonita Montero <Bonita.Montero@gmail.com> - 2021-09-11 16:02 +0200
  Re: Does static initialization of a local variable always happens synchronized ? Bonita Montero <Bonita.Montero@gmail.com> - 2021-09-11 21:19 +0200
    Re: Does static initialization of a local variable always happens synchronized ? "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2021-09-11 22:05 +0200
      Re: Does static initialization of a local variable always happens synchronized ? Bonita Montero <Bonita.Montero@gmail.com> - 2021-09-12 08:24 +0200

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