Modify address of a gdb.Value

Modify address of a gdb.Value

[Niko Sams]

Hi,

I'm trying to write a pretty-printer for Qt QMap using python.

QMap uses the following code:
static inline Node *concrete(QMapData::Node *node) {
    return reinterpret_cast<Node *>(reinterpret_cast<char *>(node) - payload());
}

How can I do such an operation with gdb.Value?

thanks,
Niko

Re: Modify address of a gdb.Value

[Tom Tromey]

>>>>> "Niko" == Niko Sams <niko.sams@gmail.com> writes:

Niko> I'm trying to write a pretty-printer for Qt QMap using python.

Niko> QMap uses the following code:
Niko> static inline Node *concrete(QMapData::Node *node) {
Niko>     return reinterpret_cast<Node *>(reinterpret_cast<char *>(node) - payload());
Niko> }

Niko> How can I do such an operation with gdb.Value?

Nobody has implemented inferior function calls using Value yet.
So the payload() part is not implementable.

If you know what it returns, or you can compute it in Python, then
this is no big deal.  You can use Value.cast to cast to some other
type, then do pointer math.

  newval = (val.cast(Type('char').pointer()) - payload).cast(val.type)

libstdc++ uses a similar implementation trick in some places, those
printers already do this kind of thing.

Tom

Re: Modify address of a gdb.Value

[Vladimir Prus]

Tom Tromey wrote:

>>>>>> "Niko" == Niko Sams <niko.sams@gmail.com> writes:
> 
> Niko> I'm trying to write a pretty-printer for Qt QMap using python.
> 
> Niko> QMap uses the following code:
> Niko> static inline Node *concrete(QMapData::Node *node) {
> Niko>     return reinterpret_cast<Node *>(reinterpret_cast<char *>(node) - payload());
> Niko> }
> 
> Niko> How can I do such an operation with gdb.Value?
> 
> Nobody has implemented inferior function calls using Value yet.

Boo. Is this something hard, or is expected soon?

- Volodya

Re: Modify address of a gdb.Value

[Paul Pluzhnikov]

On Mon, Jun 15, 2009 at 10:05 AM, Vladimir
Prus<vladimir@codesourcery.com> wrote:

> Tom Tromey wrote:
>> Nobody has implemented inferior function calls using Value yet.

But (I believe) an equivalent could be achieved by constructing a string
representation, and then calling gdb.parse_and_eval() on it [1].

However, I've been burned by doing so: in multi-threaded program
parse_and_eval (and I think inferior function call) resumes inferior,
and that causes no end of grief (including possible recursive entry back
into python, which triggers GDB assertions :-(

Tom, you may want to mention this gotcha when you expand parse_and_eval
documentation.

[1] parse_and_eval is not yet in mainline; the current patch for it is here:
http://sourceware.org/ml/gdb-patches/2009-06/msg00326.html

Cheers,
-- 
Paul Pluzhnikov

Re: Modify address of a gdb.Value

[Tom Tromey]

>>>>> "Vladimir" == Vladimir Prus <vladimir@codesourcery.com> writes:

Tom> Nobody has implemented inferior function calls using Value yet.

Vladimir> Boo. Is this something hard, or is expected soon?

I don't know.  As far as I know nobody has looked into it.

Tom

Re: Modify address of a gdb.Value

[Niko Sams]

>  newval = (val.cast(Type('char').pointer()) - payload).cast(val.type)
that is working, thanks.


> Nobody has implemented inferior function calls using Value yet.
> So the payload() part is not implementable.
>
> If you know what it returns, or you can compute it in Python, then
> this is no big deal.  You can use Value.cast to cast to some other
> type, then do pointer math.

I tried to implement this payload method in python, the c++ code is
the following:
template <class Key, class T>
struct QMapPayloadNode
{
    Key key;
    T value;
    QMapData::Node *backward;
};
static inline int payload() {
    return sizeof(PayloadNode) - sizeof(QMapData::Node *);
}

the obvious solution would be gdb.lookup_type('QMapPayloadNode<%s,
%s>' % (self.ktype, self.vtype)).sizeof
but that doesn't work, i get this error:
RuntimeError: No type named QMapPayloadNode<int, QString *>
This is because that QMapPayloadNode is not instanciated, it's only
used for this sizeof.

So any idea how i can compute the payload?

Niko

Re: Modify address of a gdb.Value

[Niko Sams]

On Wed, Jun 17, 2009 at 07:51, Niko Sams<niko.sams@gmail.com> wrote:
>>  newval = (val.cast(Type('char').pointer()) - payload).cast(val.type)
> that is working, thanks.
>
>
>> Nobody has implemented inferior function calls using Value yet.
>> So the payload() part is not implementable.
>>
>> If you know what it returns, or you can compute it in Python, then
>> this is no big deal.  You can use Value.cast to cast to some other
>> type, then do pointer math.
>
> I tried to implement this payload method in python, the c++ code is
> the following:
> template <class Key, class T>
> struct QMapPayloadNode
> {
>    Key key;
>    T value;
>    QMapData::Node *backward;
> };
> static inline int payload() {
>    return sizeof(PayloadNode) - sizeof(QMapData::Node *);
> }
>
> the obvious solution would be gdb.lookup_type('QMapPayloadNode<%s,
> %s>' % (self.ktype, self.vtype)).sizeof
> but that doesn't work, i get this error:
> RuntimeError: No type named QMapPayloadNode<int, QString *>
> This is because that QMapPayloadNode is not instanciated, it's only
> used for this sizeof.
>
> So any idea how i can compute the payload?

Ok, here comes my hacky workaround. Tested on amd64 only.

        def payload (self):

            #we can't use QMapPayloadNode as it's never instanciated and so gcc
            #doesn't include it in debug information.
            #as a workaround take the sum of sizeof(members)
            ret = self.ktype.sizeof
            ret += self.vtype.sizeof
            ret += gdb.lookup_type('void').pointer().sizeof

            #but because of data alignment the value can be higher
            #so guess it's aliged by sizeof(void*)
            #TODO: find a real solution for this problem
            ret += ret % gdb.lookup_type('void').pointer().sizeof

            ret -= gdb.lookup_type('void').pointer().sizeof
            return ret

Any better idea?

Re: Modify address of a gdb.Value

[Tom Tromey]

>>>>> "Niko" == Niko Sams <niko.sams@gmail.com> writes:

Niko> I tried to implement this payload method in python
[...]
Niko> the obvious solution would be gdb.lookup_type('QMapPayloadNode<%s,
Niko> %s>' % (self.ktype, self.vtype)).sizeof
Niko> but that doesn't work, i get this error:
Niko> RuntimeError: No type named QMapPayloadNode<int, QString *>
Niko> This is because that QMapPayloadNode is not instanciated, it's only
Niko> used for this sizeof.

Niko> So any idea how i can compute the payload?

There's no easy way.  gdb only knows what the compiler tells it, and
in this case, the information is omitted.

You could try recreating the ABI struct layout rules in Python.
This is a pain but not insanely hard.

Tom