-fexceptions? -fasync-unwind-tables?

Ulrich Weigand weigand at immd1.informatik.uni-erlangen.de
Tue May 13 15:53:20 CDT 2003

Gregory Turner wrote:

> The second, -fasync-unwind-tables, provides "dwarf2 unwind tables".  I've 
> looked through the dwarf2 spec and some other sources, but I have yet to find 
> solid documentation for this... perhaps it's a "use the source, Luke" 
> situation.  But this second option promises "instruction-granularity" unwind 
> tables of some kind.  IOW, presumably, this would be enough to implement 
> __try and friends without looking beyond the stack pointer.

By "unwind tables" the GCC manual means here what the DWARF-2 standard
calls "Call Frame Information"; they are described in chapter 6.4.

Bascially, the call frame information for a function denotes all points
within the routine where 'something interesting' related to the call
stack happens: when the function modifies the stack pointer, when the
function uses some other register as frame pointer, when and where
call-saved registers are stored in the call frame.

Using this information, it is possible for the exception handling runtime
to 'virtually unwind' the call stack, which allows to retrieve the frame
address, return address, and saved register values for every frame in the
call chain.

> So... anybody know where these options are actually documented?  I mean, I see 
> the blurb in the info pages, which is enticing, but surely there is some API 
> or datastructure to look at somewhere....

Most of the details are internal to the exception handling routines
(in libgcc).  The exported interface is documented in the unwind.h
header (which comes with gcc and/or glibc).  To actually trigger
stack unwinding, you'd need to use _Unwind_RaiseException or
_Unwind_ForcedUnwind.  However, those routines are also integrated
into the language-specific exception processing, which goes beyond
pure unwind tables, so I'm not sure whether you can use them for
your purposes.

What exactly do you want to achieve?


  Dr. Ulrich Weigand
  weigand at informatik.uni-erlangen.de

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