CODEWITHSUNDEEP
c++multithreadingvisual-c++thread-safetyc++11
Matt Fortier
Matt Fortier

Reputation: 1223

Is static init thread-safe with VC2010?

I've been looking all around SO and MSDN for an answer to this question, but cannot seem to find a clear and final answer...

I know that it's in the C++11 standard and that current GCC version behave this way, but does VC2010 currently guarantees thread-safety of a local static variable initialization?

i.e.: Is this thread-safe with VC2010?

    static S& getInstance()
    {
        static S instance;
        return instance;
    }

...And if not, what is the current best practice to get a thread-safe singleton implementation in C++ with VC2010?

EDIT: As pointed out by Chris Betti's answer, VC2010 doesn't implement thread-safeness of local static variable init.

Upvotes: 10

Views: 5063

Answers (2)

Chris Betti
Chris Betti

Reputation: 2923

From Visual Studio 2010's documentation on Static:

Assigning a value to a static local variable in a multithreaded application is not thread safe and we do not recommend it as a programming practice.

The second part of your question has some good existing answers.

Updated Nov 22, 2015:

Others have verified, specifically, that static initialization is not thread safe either (see comment and other answer).

User squelart on VS2015:

you may want to add that VS2015 finally gets it right: https://msdn.microsoft.com/en-au/library/hh567368.aspx#concurrencytable ("Magic statics")

Upvotes: 12

zhaorufei
zhaorufei

Reputation: 2193

The following code snippet shows "locally scoped static object initialisation" is NOT thread-safe:

#include <windows.h>
#include <stdio.h>
#include <process.h>
struct X {
    ~X() { puts("~X()"); }
    int i_ ;
    void print(void) {
        printf("thread id=%u, i = %d\n", GetCurrentThreadId(), i_);
    }
    X(int i) {
        puts("begin to sleep 10 seconds");
        Sleep(1000 * 10);
        i_ = i;
        printf("X(int) i = %d\n", i_);
        puts("end");
    }
};

X & getX()
{
    static X static_x(1000);
    return static_x;
}

void thread_proc(void *)
{
    X & x = getX();
    x.print();
}

int main(int argc, char *argv[])
{
    HANDLE all_threads[2] = {};
    all_threads[0] = HANDLE( _beginthread(thread_proc, 0, 0) );
    printf("First thread Id: %u\n", GetThreadId(all_threads[0]) );
    Sleep(1000);
    all_threads[1] = HANDLE( _beginthread(thread_proc, 0, 0) );
    printf("Second thread Id: %u\n", GetThreadId(all_threads[1]) );
    WaitForMultipleObjects( _countof(all_threads), all_threads, TRUE, 1000 * 20);
    puts("main exit");
    return 0;
}

The output will be(of course thread id will be different on your machine):

First thread Id: 20104
begin to sleep 10 seconds
Second thread Id: 20248
thread id=20248, i = 0
X(int) i = 4247392
end
thread id=20104, i = 1000
main exit
~X()

Before the first thread returns which means the singleton's ctor is called and returned, the second thread get the un-initialized object and call it's member method(because the static object is in BSS segment, it'll be initilized to zero after loader load the executable) and get the wrong value: 0.

Turning on assembly listing by /FAsc /Fastatic.asm will get the assembly code for function getX():

01:  ?getX@@YAAAUX@@XZ PROC                 ; getX
02:  
03:  ; 20   : {
04:  
05:    00000    55       push    ebp
06:    00001    8b ec        mov     ebp, esp
07:  
08:  ; 21   :   static X static_x(1000);
09:  
10:    00003    a1 00 00 00 00   mov     eax, DWORD PTR ?$S1@?1??getX@@YAAAUX@@XZ@4IA
11:    00008    83 e0 01     and     eax, 1
12:    0000b    75 2b        jne     SHORT $LN1@getX
13:    0000d    8b 0d 00 00 00
14:     00       mov     ecx, DWORD PTR ?$S1@?1??getX@@YAAAUX@@XZ@4IA
15:    00013    83 c9 01     or  ecx, 1
16:    00016    89 0d 00 00 00
17:     00       mov     DWORD PTR ?$S1@?1??getX@@YAAAUX@@XZ@4IA, ecx
18:    0001c    68 e8 03 00 00   push    1000           ; 000003e8H
19:    00021    b9 00 00 00 00   mov     ecx, OFFSET ?static_x@?1??getX@@YAAAUX@@XZ@4U2@A
20:    00026    e8 00 00 00 00   call    ??0X@@QAE@H@Z      ; X::X
21:    0002b    68 00 00 00 00   push    OFFSET ??__Fstatic_x@?1??getX@@YAAAUX@@XZ@YAXXZ ; `getX'::`2'::`dynamic atexit destructor for 'static_x''
22:    00030    e8 00 00 00 00   call    _atexit
23:    00035    83 c4 04     add     esp, 4
24:  $LN1@getX:
25:  
26:  ; 22   :   return static_x;
27:  
28:    00038    b8 00 00 00 00   mov     eax, OFFSET ?static_x@?1??getX@@YAAAUX@@XZ@4U2@A
29:  
30:  ; 23   : }

At line 10 the cryptic symbol [?$S1@?1??getX@@YAAAUX@@XZ@4IA] is the global indicator(also in BSS) which flags whether the singleton is ctored or not, it will be flaged as true by line 14-17, just before calling into the ctor, that's the problem, this also explains why the second thread immediately got the un-initialized singleton object and happily call it's member function. There's no thread-safety related code inserted by the compiler.

Upvotes: 6

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