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Re: guile scripting for gdb

From: Ludovic Courtès
Subject: Re: guile scripting for gdb
Date: Mon, 11 Nov 2013 01:19:11 +0100
User-agent: Gnus/5.130007 (Ma Gnus v0.7) Emacs/24.3 (gnu/linux)

Doug Evans <address@hidden> skribis:

> On Thu, Nov 7, 2013 at 3:39 PM, Ludovic Courtès <address@hidden> wrote:


>> As discussed on IRC, one possible issue is eq?-ness of SMOBs: one would
>> usually expects pointer equality to be preserved at the Scheme level.
> Yeah.
> That'll require gdb maintaining its own table(s) for each kind of smob
> we want to intern.
> Definitely doable, though there are some issues.
> E.g., while std::vector<int> may be the same type in two different programs,

What I had in mind was something simpler: suppose you have the very same
C struct pointer reaches the Scheme level, at two different points in
time, or via two different paths; currently gdb may end up allocating
two different SMOBs (i.e., two SMOBs that are not eq?), whereas I would
suggest making sure there’s only one SMOB.


--8<---------------cut here---------------start------------->8---
(gdb) guile (lookup-type "int")
#<gdb:type int>
(gdb) guile (arch-int-type (current-arch))
#<gdb:type int>
(gdb) guile (eq? (lookup-type "int") (arch-int-type (current-arch)))
--8<---------------cut here---------------end--------------->8---

Here I bet the underlying ‘struct type’ pointer return by ‘lookup-type’
is the same as that returned by ‘arch-int-type’, yet the SMOBs are

Fixing it would require maintaining a C->SMOB mapping.

> if we want eq?-ness to survive across the lifetime of the underlying gdb 
> object
> then that would take extra effort to make that work.
> Would it be ok to punt on eq?-ness until there's a compelling reason
> to make it work?

Yes, a lot can already be done with the current semantics, but at some
point it may break the user’s expectations.  It’s natural to compare
presumably-pointer-identical objects with eq?, or to use eq? hash


>> An interesting exercise would be to write pretty-printers for SCM values
>> and tools to walk Guile’s VM stack (like Guile’s gdbinit attempts to do).
> Agreed, excellent exercises.
> gdb has a "frame filter" interface that's intended to be used to
> implement multi-language backtraces.

That sounds cool.  If gdb could show trace mixing both stacks, that’d be

> Need to add a gdb/guile interface.
> I'm not sure how Guile's new VM changes things - someone may want to
> write one for 2.0 and one for 2.2.

Yeah, the VM in 2.2 is completely different.


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