Re: fix-n-continue

[Mike Stump <mrs at apple dot com>]

On Friday, July 30, 2004, at 05:33  PM, Zack Weinberg wrote:
> Mike Stump <mrs@apple.com> writes:
>
> > Here are some patches to support fast turn around debugging for
> > darwin...
>
> Ya gonna hafta explain what it does in considerably more detail than
> that.

What would you like to know?  A google reveals  
http://developer.apple.com/documentation/DeveloperTools/Conceptual/ 
XcodeQuickTour/qt_fix/chapter_5_section_1.html which has the user  
perspective.  Microsoft has a corresponding feature, if you want to go  
see what they do.

-ffix-and-continue arranges for functions to be able to overridden at  
run time by gdb, and -findirect-data arranges for data to be be  
indirected; there two items are enough to gain fairly standardized  
debugging functionality whereby one can recompile units and load those  
recompilations into their running program with gdb and it all just  
works as expected.

The indirection for data is a preexisting darwin technique, we just use  
it more often.  Did you want me to explain the existing technique?  One  
layer of indirection that is exposed to the dynamic linker to ensure  
that there is just one instance would be a quick explanation.

We arrange for functions to be overridden by reserving 4 nops worth of  
space at the front of the function so that gdb can use this as a patch  
area to forward to the overriding function.  This allows forwarding  
even for preexisting callbacks, function pointers, virtual methods and  
so on...