Re: [Mingw-w64-public] Thoughts on supporting the C++11 thread library on Windows

[Jonathan Wakely <jwakely dot gcc at gmail dot com>]

On 9 May 2012 20:06, K. Frank wrote:
> However, as noted in my previous post, I have happily done some
> (limited) windows-api threading programming with Ruben's build
> (and also did the windows-api threading programming necessary
> to implement <thread>),

If you use GCC built with --enable-threads=posix then you shouldn't
need to implement <thread>, it should be provided.

In any case, as you're not using the win32 thread model you shouldn't
are about my proposals and you're just derailing the topic ;-)


> all, I guess, with a gcc build built using
> --enable-threads=posix, so what then does --enable-threads=win32
> actually do?

The code is all open source, feel free to read it.

--enable-threads=win32 tells GCC to use the gthr-win32.h header to
implement the gthreads abstraction API.  Gthreads provides a
pthreads-like API which libstdc++ (and libobjc and other bits of GCC)
use for threading facilities.  WIth --enable-threads=posix GCC uses
the gthr-posix.h file which provides an implementation of gthreads
based on pthreads (which is a one-to-one mapping.) With
--enable-threads=win32 GCC uses the gthr-win32.h file which provides
an implementation based on Windows primitives, which as I said several
days ago means that the __gthread_mutex_t type is implemented as
semaphores.

libstdc++ uses the __gthread_mutex_t type internally when it needs a
mutex, and when gthr-posix.h is used also uses __gthread_mutex_t to
implement std::mutex.

My original email, which I now seem to be repeating over and over,
suggests changes to gthr-win32.h to allow std::mutex and C++11 other
types to be defined in terms of the Windows primitives currently used
in gthr-win32.h

That gthr-win32.h header already exists, and already defines
__gthread_mutex_t in terms of a semaphore, and it's too late to change
that.  So maybe if you re-read my original mail now it will make more
sense.  I proposed extending gthr-win32.h to allow std::thread and
std::mutex to be provided WITHOUT CHANGING THE EXISTING IMPLEMENTATION
OF gthr-win32.h, which would not allow native condition variables to
be used because std::mutex is based on a semaphore and changing
gthr-win32.h to not use a semaphore would not be backwards compatible
and so is not an option.

In order to solve that problem I proposed a completely new gthreads
implementation, for argument's sake gthr-win64.h, which being
completely new could implement std::mutex differently, e.g. as a
critical section, and std::condition_variable as a Windows condition
variable,

As you are using gthr-posix.h you shouldn't need to care, you should
already have std::mutex, std::thread, std::condition_variable etc.

So now I hope everyone's on the same page, but I don't actually think
this thread has anything new since my original email on this subject
four days ago  :-\