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Re: [c++-pthreads] Initialization of local static mutex

  • To: Roland Schwarz <roland.schwarz@xxxxxxxxx>
  • Subject: Re: [c++-pthreads] Initialization of local static mutex
  • From: Mark Mitchell <mark@xxxxxxxxxxxxxxxx>
  • Date: Mon, 09 Oct 2006 15:09:51 -0700
Roland Schwarz wrote:


However, in practice, your code is safe.


This is what I was suspecting. But you will admit, that this is a
different thing than prove.
Yes. For the reasons I stated, I don't think this is a provable thing; the ISO C++ standard doesn't contemplate threads, so it's hard to reason about them. The multi-platform C++ ABI used by G++ (and many other compliers) does contemplate threads, but doesn't specifically address the particular case that you posted. 


But if the initialization would have been done as

foo()
{
  static int bar;
}

bar would happen to be zero initialized on any conforming compiler.
(before any other initializations take place)
So zero initialization is more conservative with respect to the
standard. Isn't it?
Yes. It would certainly be nice if PTHREAD_MUTEX_INIT were uniformly zero, but on some hardware that's not feasible.
There are other issues around thread-safe initialization of statics with a constant initializers which are addressed by some C++ ABIs, but you have the simpler case of a constant initializer. 

Now that you have brought up this issue I am curious which "other
issues" you are refering to.


void f() {
  static int i = g();
}
Here, you cannot do the initialization statically; it must be done the first time that f() is called. So, the key question is whether the initialization is thread-safe. Does the programmer have to change that initialization to be thread-safe, using locks in f()? Or does the compiler take care of it?
The multi-platform C++ ABI defines library functions that compilers may use to ensure thread-safe initialization of local statics. However, using these functions is optional. So, some compilers do it, some don't, and some do it only in certain modes. 

--
Mark Mitchell
CodeSourcery
mark@xxxxxxxxxxxxxxxx
(650) 331-3385 x713