Re: How does gdb deal with bizarre frame descriptions?

[Mike Frysinger <vapier@gentoo.org>]

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On Tuesday 18 May 2010 16:05:54 Tom Horsley wrote:
> I've been looking at the frame info for a main program
> compiled on ubuntu with g++ (4.4.3), and the frame description
> info, both for the eh_frame CFA and the DW_AT_frame_base
> attribute is totally bizarre. The frame pointer is described
> as being at *(ebp - 16) (for a certain range of addresses).
> 
> If I debug this program with gdb, it seems to get the frame
> info correct, and apparently believes the frame pointer
> is the ebp register (which seems to make sense if I
> disassemble the code). If I look in *(ebp - 16) I see
> trash.
> 
> What the devil is gdb doing to get this right? Is it
> utterly ignoring the trash generated by the compiler and
> deducing the frame description by disassembling the
> prologue code or something?
> 
> gdb version: GNU gdb (GDB) 7.1-ubuntu
> g++ version: gcc version 4.4.3 (Ubuntu 4.4.3-4ubuntu5)
> architecture: i686

look at the frames sections of the gdb internals manual as it describes the 
algorithms gdb uses (or rather, the framework gdb has that each arch/port 
extends).

you can also use 'set debug frame 1' to get more insight on the frame 
construction.
-mike

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