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Re: Request/question from RMS
- From: "Eli Zaretskii" <eliz at is dot elta dot co dot il>
- To: ac131313 at cygnus dot com
- Cc: gdb at sources dot redhat dot com
- Date: Thu, 10 Jan 2002 17:46:16 +0200
- Subject: Re: Request/question from RMS
- References: <3C3DA906.firstname.lastname@example.org>
- Reply-to: Eli Zaretskii <eliz at is dot elta dot co dot il>
> Date: Thu, 10 Jan 2002 09:45:26 -0500
> From: Andrew Cagney <email@example.com>
> > In Emacs .gdbinit I found this:
> > define xreload
> > set $valmask = ((long)1 << gdb_valbits) - 1
> > set $nonvalbits = gdb_emacs_intbits - gdb_valbits
> > end
> > I think GDB should avoid making this necessary--that it should
> > reread .gdbinit when it reloads the executable. Or there should
> > be a certain user-defined command that will be run after GDB
> > reloads the executable.
> I know there are now things like posthook-run commands. However,
> without knowing how EMACS uses the above (I've no desire to debug emacs,
> debugging mozilla was scary enough :-^) it is hard to suggest a replacement.
Here's an example of how this is used:
print (($ & $valmask) << $nonvalbits) >> $nonvalbits
Print $, assuming it is an Emacs Lisp integer. This gets the sign right.
print (struct Lisp_Symbol *) ((((int) $) & $valmask) | gdb_data_seg_bits)
Print the name and address of the symbol $.
This command assumes that $ is an Emacs Lisp symbol value.
In other words, Emacs Lisp objects are represented by C int's, whereby
a few high bits are used for the tag that distinguishes between the
types, while the rest of the int is normally a pointer to a place
where the object is stored (an Emacs integer is an exception: it
represents itself). When you debug Emacs, you use commands like xint
and xsymbol to display the values of the Lisp object (here, an integer
and a symbol, respectively) in human-readable format. Here's an
example of usage:
(gdb) p Qnil
$1 = 270900228
$2 = (struct Lisp_Symbol *) 0x259c04
(`Qnil' is the C variable which holds the Lisp symbol `nil'.)