Re: [boost] Strings and thread safety

From: Jeff Garland (jeff_at_[hidden])
Date: 2006-09-08 13:24:24

Felipe Magno de Almeida wrote:
> On 9/8/06, Jeff Garland <jeff_at_[hidden]> wrote:
> 
>>> [snipped]
> 
>>> There's something I want to clear out, lexical_cast is reentrant and only
>>> uses "stacked" variables no ?
>> So what?  Those stack types are free to reference global data. And in the case
>> of stringstream, in combination with operators<< and operator>>, they most
>> certainly do access global data in the form of locales and facets.
> 
> But I believe almost every STL implementation has thread-safe
> iostreams, with locked access to global locales and etc. I know that
> VC 7.1 and STLPort has. Without it, it is almost impossible to write
> thread correct C++ programs. Or am I missing something?

It's not portable because the standard says nothing about threading or thread 
safety in I/O.  And as I recall I've seen at least one implementation that 
supports compiling out all thread safety, because it is a huge performance hit 
  to make I/O threadsafe.  So, again it may mostly work, but it isn't assured 
by anything in the standard.  So in the context of lexical_cast, nothing is 
assured...

>> [snipped]
> 
>> The real problem here, is that lexical_cast *will work* most of the time in an
>> MT environment without difficulty. But that's not good enough to declare it
>> 'thread-safe'...which is the entire assumption in this thread.  Actually, as
>> this whole discussion illustrates, it's probably enough to make it
>> 'thread-dangerous' ;-)
> 
> It is only thread-safe if what it uses has the basic thread-safety
> guarantees. Which, for all I know, it is true in every implementation
> that cares about writing thread-safe programs.

If it were me, I'd guard any use of lexical cast I thought was to be used in 
an MT context because it's a time-bomb waiting to fail.  I'm sorry to keep 
going on about this, but the whole thread that spawned this one had the 
embedded presumption that lexical_cast is thread-safe and I wanted to dispel 
that myth -- it's obviously deeply entrenched.

Jeff