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X-Authentication-Warning:	slinky.cs.nyu.edu: pechtcha owned process doing -bs
Date:	Thu, 3 Oct 2002 17:59:06 -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:	<20021003174509.A24847@aleri.com>
Message-ID:	<Pine.GSO.4.44.0210031746330.13573-100000@slinky.cs.nyu.edu>
Importance:	Normal
MIME-Version:	1.0

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
-- 
				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!

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