Pythons scripting API question

Pythons scripting API question

[Evan Driscoll]

Hi,

I'd like to get a list of the local variables in a frame, or at least 
the parameters. I'm looking through the documentation of the Python API, 
and not seeing how to do it.

In some sense, I'm trying to write a command that behaves a bit like 
'info locals', but outputs in a different format. (And, of course, I'd 
prefer not to parse that. There's also some other stuff I want to do as 
well.)

I'm using GDB 7.4.1 with Python 2.7.1.

Evan

Re: Pythons scripting API question

[Keith Seitz]

On 05/30/2012 11:10 AM, Evan Driscoll wrote:

> I'd like to get a list of the local variables in a frame, or at least
> the parameters. I'm looking through the documentation of the Python API,
> and not seeing how to do it.

Frames contain blocks, blocks contain variables. Blocks in python can be 
iterated, so:

$ ./gdb -nx -q gdb -ex "break main" -ex "run"
(gdb) python import gdb
(gdb) python for n in gdb.selected_frame().block(): print n,
argc argv args
(gdb) python print n.type
struct captured_main_args
(gdb) python print n.name
args
(gdb) python print n.is_argument
False
(gdb) python print n.value(gdb.selected_frame())
{argc = 0, argv = 0x488f80 <_start>, use_windows = -8032, interpreter_p 
= 0x0}

This will give you both parameters and locals. There are methods you can 
use to determine which is which (e.g., Symbol.is_argument).

See the relevant sections in the Gdb Users Manual (23.2.2.16, 23.2.2.18).

Keith

Re: Pythons scripting API question

[Evan Driscoll]

On 05/30/2012 04:23 PM, Keith Seitz wrote:
> Frames contain blocks, blocks contain variables. Blocks in python can be
> iterated

Great, it's that information that I was missing.

However, if I do this:

> $ ./gdb -nx -q gdb -ex "break main" -ex "run"
> (gdb) python import gdb
> (gdb) python for n in gdb.selected_frame().block(): print n,
> argc argv args
> (gdb) python print n.type
> struct captured_main_args
> (gdb) python print n.name
> args
> (gdb) python print n.is_argument
> False
> (gdb) python print n.value(gdb.selected_frame())
> {argc = 0, argv = 0x488f80 <_start>, use_windows = -8032, interpreter_p
> = 0x0}

then on the last step I get this instead:

(gdb) python print n.value(gdb.selected_frame())
Traceback (most recent call last):
   File "<string>", line 1, in <module>
AttributeError: 'gdb.Symbol' object has no attribute 'value'
Error while executing Python code.

Instead, I have to use

(gdb) python print gdb.selected_frame().read_var(n)
{argc = 6742752, argv = 0x0, use_windows = -7696, interpreter_p = 0x0}


Is this just a version difference?

Evan

Re: Pythons scripting API question

[Keith Seitz]

On 05/30/2012 03:42 PM, Evan Driscoll wrote:
> Instead, I have to use
>
> (gdb) python print gdb.selected_frame().read_var(n)
> {argc = 6742752, argv = 0x0, use_windows = -7696, interpreter_p = 0x0}
>
>
> Is this just a version difference?

It appears that Symbol.value was added on 2012-02-07 to address 
python/12027. That bug shows the target milestone to be 7.5, so I'm 
guessing that although this was fixed before the 7.4.1 release, it was 
not back-ported to that release.

In other words, yes, it does appear to be a difference between your 
version of gdb (7.4.1) and mine (7.4.50, aka CVS HEAD).   :-)

Keith

Re: Pythons scripting API question

[Joachim Protze]

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On 30.05.2012 23:23, Keith Seitz wrote:
> Frames contain blocks, blocks contain variables. Blocks in python can
> be iterated, so:
>
> $ ./gdb -nx -q gdb -ex "break main" -ex "run"
> (gdb) python import gdb
> (gdb) python for n in gdb.selected_frame().block(): print n,
> argc argv args
> (gdb) python print n.type
> struct captured_main_args
> (gdb) python print n.name
> args
> (gdb) python print n.is_argument
> False
> (gdb) python print n.value(gdb.selected_frame())
> {argc = 0, argv = 0x488f80 <_start>, use_windows = -8032,
> interpreter_p = 0x0}
>
Thank you for this example!
I never figured out this feature of Blocks :(

> See the relevant sections in the Gdb Users Manual (23.2.2.16, 23.2.2.18).
I totally miss the hint that Blocks drop Value objects on iteration in
the documentation.
How to access the "hierarchically organized" sub-blocks? Or did I
missunderstand the concept of Blocks?

main()
{
    int i;
    for(...)
    {
        int a;
    }
    {
        int i;
    }
}

I would expect a Block for the main function with two children Blocks -
but how to get these children?

- Joachim


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Re: Pythons scripting API question

[Tom Tromey]

>>>>> "Joachim" == Joachim Protze <joachim.protze@tu-dresden.de> writes:

Joachim> Thank you for this example!
Joachim> I never figured out this feature of Blocks :(

Keith> See the relevant sections in the Gdb Users Manual (23.2.2.16, 23.2.2.18).

Joachim> I totally miss the hint that Blocks drop Value objects on iteration in
Joachim> the documentation.

It was patched to be more clear:

2012-02-22  Tom Tromey  <tromey@redhat.com>

	* gdb.texinfo (Blocks In Python): Clarify block iteration.

The new text says:

A @code{gdb.Block} is iterable.  The iterator returns the symbols
(@pxref{Symbols In Python}) local to the block.  [...]

If this is insufficient, suggest more...

Joachim> How to access the "hierarchically organized" sub-blocks?

Unfortunately we are missing API for this.
Could you file a bug report for it?

Or alternatively, try to write it?  I don't think gdb has a direct way
to iterate over sub-blocks; but at the very least you can iterate over
the blockvector and look for blocks with a certain parent.

Tom