what does __gthread_active_p() do/mean?

what does __gthread_active_p() do/mean?

[Adam Megacz]

What is this function supposed to do? (gthr-*.h)

I'm investigating some breakage involving mingw and sjlj-exceptions's
need for thread-local storage; I believe that it's returning the wrong
value on mingw, but I'd like to know what the meaning of the return
value is.

  - a

Re: what does __gthread_active_p() do/mean?

[Jeff Sturm]

On 14 Jan 2002, Adam Megacz wrote:
> What is this function supposed to do? (gthr-*.h)

From gthr.h:

/* If this file is compiled with threads support, it must
       #define __GTHREADS 1
   to indicate that threads support is present.  Also it has define
   function
     int __gthread_active_p ()
   that returns 1 if thread system is active, 0 if not.

Depending on how a program is linked, thread functions may or may not be
available.  Calling __gthread_active_p() will tell.

> I'm investigating some breakage involving mingw and sjlj-exceptions's
> need for thread-local storage; I believe that it's returning the wrong
> value on mingw, but I'd like to know what the meaning of the return
> value is.

Judging by the comments, if you compile/link without -mthreads,
gthr-win32.h assumes your program won't use threads.  Could that be what
is happening?

Jeff

Re: what does __gthread_active_p() do/mean?

[Adam Megacz]

Jeff Sturm <jsturm@one-point.com> writes:
> > I'm investigating some breakage involving mingw and sjlj-exceptions's
> > need for thread-local storage; I believe that it's returning the wrong
> > value on mingw, but I'd like to know what the meaning of the return
> > value is.

> Judging by the comments, if you compile/link without -mthreads,
> gthr-win32.h assumes your program won't use threads.  Could that be what
> is happening?

Yes, that's quite possibly what it is.

However, this isn't always a safe assumption. For various reasons, I
am willing to give up TLS garbage collection in order to not have to
distribute mingwthr.dll (or whatever it's called). This is a pretty
big deal for me.

So the assumption that "no -mthreads == no threads" isn't always true.

Would anybody object to having this always return 1 on mingw
platforms? The TlsGet() and TlsSet() functions which it enables don't
require mingwthr.dll.

  - a

Re: what does __gthread_active_p() do/mean?

[Jeff Sturm]

On 17 Jan 2002, Adam Megacz wrote:
> So the assumption that "no -mthreads == no threads" isn't always true.

True.  Most platforms have some way to explicitly request a threaded
runtime, e.g. -lpthread.  For those it's critical that they can disable
the gthr-* functionality.

But for mingw, win32 thread functions are always available with or without
-mthreads.  So you could probably make your proposed modification without
breaking anything (in a single-threaded program, freeing the TLS isn't an
issue).

> Would anybody object to having this always return 1 on mingw
> platforms? The TlsGet() and TlsSet() functions which it enables don't
> require mingwthr.dll.

I'm not exactly objecting, but realize that this will add some overhead to
mingw programs that use EH without threads.  There ought to be a way to
declare to the compiler that you don't intend to use threads, and
ostensibly this is done now by omitting -mthreads.

Alternatively, could you somehow eliminate libmingwthrd from the link
spec?  Perhaps handle threads in the STARTFILE_SPEC instead?

Jeff