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X-Authentication-Warning: slinky.cs.nyu.edu: pechtcha owned process doing -bs
Date: Thu, 3 Oct 2002 18:27:14 -0400 (EDT)
From: Igor Pechtchanski <pechtcha AT cs DOT nyu DOT edu>
Reply-To: cygwin AT cygwin DOT com
To: Allen Leung <allen DOT leung AT aleri DOT com>
cc: cygwin AT cygwin DOT com
Subject: Re: Bypassing cygwin's signal handling
In-Reply-To: <Pine.GSO.4.44.0210031746330.13573-100000@slinky.cs.nyu.edu>
Message-ID: <Pine.GSO.4.44.0210031807570.13573-100000@slinky.cs.nyu.edu>
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On Thu, 3 Oct 2002, Igor Pechtchanski wrote:

> On Thu, 3 Oct 2002, Allen Leung wrote:
>
> > Dear List,
> >
> >    In a system I'm writing I need to catch page faults and find out
> > the fault address and the fault type (read or write).  On most unices
> > I can get this information inside a sigaction handler.
> > E.g. on Linux, something like this works:
> >
> > ==========================================================
> >   void fault_handler(int sig, siginfo_t * info, void * uap)
> >   {
> >      vm_addr_t    addr;
> >      VM::access_t access;
> >
> >      /* fault address */
> >      addr = (vm_addr_t)info->si_addr;
> >      /* 1 for write; 0 for read */
> >      access = (VM::access_t)
> >                ((((ucontext_t *)uap)->uc_mcontext.gregs[13] & 0x2) != 0);
> >      ...
> >   }
> > ==========================================================
> >
> > However, cygwin's signal handling mechanism doesn't propagate these
> > information to the signal handler.  On mingw, I can use window's API
> > to get the info I need:
> >
> > ==========================================================
> > LONG WINAPI fault_handler(LPEXCEPTION_POINTERS info)
> > {
> >    vm_addr_t    addr;
> >    VM::access_t access;
> >
> >    if (info->ExceptionRecord->ExceptionCode != EXCEPTION_ACCESS_VIOLATION)
> >       return EXCEPTION_CONTINUE_SEARCH;
> >
> >    access = (VM::access_t)info->ExceptionRecord->ExceptionInformation[0];
> >    addr   = (vm_addr_t)info->ExceptionRecord->ExceptionInformation[1];
> >
> >    ...
> > }
> >
> >  SetUnhandledExceptionFilter(fault_handler);
> > ==========================================================
> >
> > However, this doesn't work on cygwin because apparently cygwin is
> > overriding my own fault handler.  So I'm wondering: how can I bypass
> > cygwin's signal handling mechanism so that I get hold of the page fault
> > information?
> >
> > Thanks,
> > Allen
>
> Hey, Allen! (Of all places to meet here :-D)
>
> Cygwin currently does not support SA_SIGINFO signal handling.  It's on the
> TODO list ( http://cygwin.com//cgi-bin/cygwin-todo.cgi?20020722.130725 ),
> but I didn't have the time to work on it yet...
>
> As far as I know, there is currently no way to retrieve the fault
> information from a Unix-style signal handler in Cygwin.  You might try to
> patch the Cygwin DLL to ignore a particular signal (in
> winsup/cygwin/exceptions.cc), and then write your own Windows-style
> handler...  It'll probably be quite a bit of work.  Another idea would be
> to include a callback to your own function in handle_exceptions().  In
> either case, you'll have to build a custom cygwin1.dll...
>         Igor

Allen,

Actually, the first thing to try should probably be

#include <exceptions.h>
#include <sys/cygwin.h>

...
	exception_list el;
	cygwin_internal(CW_INIT_EXCEPTIONS, &el);
	el.handler = your_handler;
...

I'm not sure how to clean this up afterwards, but theoretically this
should work...  Chris Faylor will hopefully correct me if I'm wrong.
	Igor
-- 
				http://cs.nyu.edu/~pechtcha/
      |\      _,,,---,,_		pechtcha AT cs DOT nyu DOT edu
ZZZzz /,`.-'`'    -.  ;-;;,_		igor AT watson DOT ibm DOT com
     |,4-  ) )-,_. ,\ (  `'-'		Igor Pechtchanski
    '---''(_/--'  `-'\_) fL	a.k.a JaguaR-R-R-r-r-r-.-.-.  Meow!

"Water molecules expand as they grow warmer" (C) Popular Science, Oct'02, p.51


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