Recently Cygwin has changed the way that it internally checks arguments for bad
pointers.  Where before it used IsBad{Read,Write}Ptr() from the win32 API, now
it installs its own temporary SEH fault handler.  However, gdb is not aware of
this and so it leads to the program stopping on a SIGSEGV on perfectly
legitimate code -- when not run under the debugger, Cygwin's fault handler
catches the attempt and returns the appropriate error.  Here is a minimal
example:

$ cat ss.cc
#include <string>

int main()
{
  std::string foo;
  
  return 0;
}

$ g++ -g ss.cc -o ss

$ gdb --quiet ./ss
(gdb) r
Starting program: /tmp/false_sigsegv/ss.exe 

Program received signal SIGSEGV, Segmentation fault.
0x610af8b8 in pthread_key_create (key=0x482c08, destructor=0) at
/usr/src/sourceware/winsup/cygwin/thread.cc:129
129       if ((*object)->magic != magic)
(gdb) c
Continuing.

Program exited normally.
(gdb) 

Here pthread_key_create() was simply checking to see if 'key' happened to be a
previously-initiaized object, which in most cases is not true, and so the fault
is expected and handled.  However, it's very inconvenient and unintuitive that
gdb still stops the program, and it makes the user think there is something
wrong with his program and/or the Cygwin library.

The attached patch adds a very simple mechanism by which the Cygwin program
shall signal to gdb that it has temporarily installed its own fault handler for
EXCEPTION_ACCESS_VIOLATION, so that gdb can deal with this more gracefully by
ignoring these faults.  This uses the existing WriteDebugString() mechanism that
Cygwin already uses to communicate to the debugger, but just extends it by
defining two new "magic" string values.

I've split the patch into two parts since it requires changes to both cygwin and
gdb.

Brian

winsup/cygwin:
2006-02-02  Brian Dessent  <brian@dessent.net>

	* cygtls.h: Include sys/cygwin.h.
	(myfault::~myfault): If a debugger is present, inform it that our fault
	handler has been removed.
	(myfault::faulted): Likewise for when the fault handler is installed.
	* include/sys/cygwin.h (_CYGWIN_FAULT_IGNORE_STRING): Add define.
	(_CYGWIN_FAULT_NOIGNORE_STRING): Ditto.


gdb:
2006-02-02  Brian Dessent  <brian@dessent.net>

	* win32-nat.c (_CYGWIN_SIGNAL_STRING): Remove duplicated definition.
	(ignoring_faults): Define new static global.
	(handle_output_debug_string): Check for fault ignore/noignore signals
	from the inferior.
	(handle_exception): Do not process the fault if 'ignoring_faults' is
	set.
	(do_initial_win32_stuff): Initialize 'ignoring_faults'.