G++ guru's please comment - Re: FW: pthread_create problem in Cygwin 1.1.8-2]

[Robert Collins]

----- Original Message -----
From: "Christopher Faylor" <cgf@redhat.com>


> On Mon, Apr 09, 2001 at 02:34:04PM +1000, Robert Collins wrote:
> >
> >> -----Original Message-----
> >> From: Christopher Faylor [ mailto:cgf@redhat.com ]
> >> On Mon, Apr 09, 2001 at 11:31:57AM +1000, Robert Collins wrote:
> >> >> -----Original Message-----
> >>
> >> I wouldn't expect the stack pointer to be screwed up.  I'd expect
that
> >> the *stack* is screwed up.  I don't believe that the ecx
> >> register should
> >> be preserved across function calls so I don't know why this
> >> would mater.
> >
> >Doh! Ask me how many years since I wrote assembly.... please :]
> >
> >Well, the exception occurs on the last line here (Intel format cause
I'm
> >reading intel doco).
> >
> >call   0x404c98 <sched_yield>
> >mov    %eax,DWORD PTR [%ebp-100]
> >add    %eax,4
> >mov    %edx,DWORD PTR [%eax]
> >mov    %ecx,DWORD PTR [%edx]
> >mov    DWORD PTR [%eax],%ecx
> >
> >As you can see, no stack references :[. And the crash doesn't occur
if
> >the thread doesn't lose the processor.
>
> I'm confused by this snippet of assembly.  It looks like gcc is using
> the result of sched_yield to index into a table.  But, %eax should be
> either 0 or 1 in this case.  So, maybe you're right.  This is a gcc
> problem.  And, it may be fixed by configuring gcc
with --enable-threads.

this is Intel format disasembly not att. so paraphrasing it
eax = *(ebp-100)
eax +=4
edx = *(eax)
ecx = *(edx)
*eax = ecx

Don't ask me why it loops the data around, it just does ;].

(ebp-100) is a pointer to the exception handler context. (call it
eh_handle)
I don't know what eh_handle+4 points to. But that gets trashed before
the function completes it's exit from the try{} block.

>
> This is one of those situations where it looks like gcc can't be
> released until a version of cygwin is released with the requisite
> functionality.

What's missing? I have a list of over 20 functions in front of me that
I've just coded stubs for...

Rob

>
> cgf
> >Now the registers should be saved when context switch's occur, so I
> >don't believe the corruption occurs during that section of code, but
> >rather:
> >
> >first thread setsup the try{}, returning an exception handler
context.
> >yields.
> >main runs, starts second thread
> >second thread is started, it sets up it's try{}, and recieves the
_sames
> >context_, yields.
> >first thread completes, exits the try{}, trashing the exception
handler
> >context.
> >
> >I'm not going to look into this any alonger. IMO it's the
> >__get_eh_context function in g++ which I know nothing about, and
don't
> >have a debug version built. I hopped on this initially to see if it
was
> >a pthreads issue...
> >
> >If it does turn out to be cygwin threads, rather than a non-reentrant
> >library function, I'm happy to look at it again.
> >
> >>
> >> Yep.  Probably.  Somebody should try that.
> >
> >Chuckle.
> >
> >> cgf
> >>
> >
> >Rob.
> >
> >--
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>
> --
> cgf@cygnus.com                        Red Hat, Inc.
> http://sources.redhat.com/            http://www.redhat.com/
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