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Re: guile scripting for gdb

From: Ludovic Courtès
Subject: Re: guile scripting for gdb
Date: Mon, 11 Nov 2013 12:47:51 +0100
User-agent: Gnus/5.130007 (Ma Gnus v0.7) Emacs/24.3 (gnu/linux)

Doug Evans <address@hidden> skribis:

> On Sun, Nov 10, 2013 at 4:19 PM, Ludovic Courtès <address@hidden> wrote:
>> 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.
>> Example:
>> --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)))
>> #f
>> --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
>> different.
>> Fixing it would require maintaining a C->SMOB mapping.
> I'm pretty sure we have a sufficiently similar idea in mind.
> I mention the use of multiple tables because the lifetimes of
> different kinds of objects are different, and it may be easier to
> delete the table than iterate over each element looking for entries
> that need to be deleted.
> For reference sake, gdb doesn't guarantee that in the above example
> the underlying pointers are equal.
> While the arch provides a definition of "int" it can also come from
> the debug info in the program (actually there can and typically will
> be several copies, one from each .o in the program).
> eq-ness will be problematic even with the C->SMOB mapping.

Ah, OK.  Indeed, eq?-ness only would only matter in cases where gdb
guarantees pointer equality in the first place.

Thanks for the explanation,

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