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Re: Request/question from RMS

> Date: Thu, 10 Jan 2002 09:45:26 -0500
> From: Andrew Cagney <>
> > 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:

    define xint
    print (($ & $valmask) << $nonvalbits) >> $nonvalbits
    document xint
    Print $, assuming it is an Emacs Lisp integer.  This gets the sign right.
    define xsymbol
    print (struct Lisp_Symbol *) ((((int) $) & $valmask) | gdb_data_seg_bits)
    output (char*)$->name->data
    echo \n
    document xsymbol
    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
    (gdb) xsymbol
    $2 = (struct Lisp_Symbol *) 0x259c04
    0x129928 "nil"

(`Qnil' is the C variable which holds the Lisp symbol `nil'.)

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