Is static initialization by function call thread-safe in C?

Question

Imagin this:

void *ImCalledByThreads (/*...*/)
{
    //some stuff
    static typePlaceholder AmIThreadSafe = QuestionTag();
    //other stuff
}

Is this initialisation threadsafe?

Even if QuestionTag() is threadsafe, What happens if the first thread runs the function and another invokes this line. Would there be a detection that the function will only be executed once? or is this simply just a bad idea for multithreading?

That's not legal C code. But if you're talking about C++ instead of C, it's almost certainly not thread-safe. I'm not sure what C++11 says, but C++03 and earlier don't mention threads at all. And from the generated code I've looked at from various compilers, they don't add any synchronization primitives around the variable initialization. — Adam Rosenfield, Dec 18 '13 at 22:49
@AdamRosenfield It's thread safe in gcc(c++) for a while, C++11 mandates it to be — nos, Dec 18 '13 at 22:53
Stupid MSVC.... I'm talking about pure C. But I just tested this on windows and no errors... But I forgot that this is not reliable to C standard what MSVC expects as C code. Thanks anyway. — dhein, Dec 18 '13 at 22:56
@Zaibis are you certain MSVC compiles your code as C, not C++ ? At any rate, C or C++, it's not threadsafe in msvc until VS 2013. — nos, Dec 18 '13 at 22:59
@nos Yeah I'm pretty sure that its set to C code, as I try to avoid c++ when ever its possible. But I'm using MSVC 2010 and jsut used it for compilation check. Mainly I'm using gcc on FreeBSD 9.2 — dhein, Dec 18 '13 at 23:00
@nos: Good point, my memory is a little fuzzy. My current versions of G++ and Clang both do insert the appropriate locks around static initializers even when operating in C++03 mode. — Adam Rosenfield, Dec 18 '13 at 23:01
@Zaibis: Sorry, got the language confused. In C, static initializers can only be constant expressions, so there's nothing to be thread-safe. — Kerrek SB, Dec 18 '13 at 23:44

score 3 · Accepted Answer · edited May 23 '17 at 10:32

It's not legal C code, since C requires initializers of static variables to be compile-time constants. Hence the initialization can happen at program load time, before any threads have a chance to be started, so there can be no race conditions there.

From C99 §6.7.8/4:

All the expressions in an initializer for an object that has static storage duration shall be constant expressions or string literals.

Visual Studio may allow non-constants as a non-standard extension, but for that, all bets are off. Check its documentation and/or its generated assembly code to see if it's thread-safe or not.

For C++, where non-constants initializers are allowed, see the question Is initialization of local static function-object thread-safe?. Short answer: Yes, in C++11, no in C++03 and earlier (which don't mention threads in the standard), though compilers may still make it thread-safe if they choose.

Is static initialization by function call thread-safe in C?

1 Answers1