Python API iterate through the arguments of a frame

Python API iterate through the arguments of a frame

[Cristian Zamfir]

Hi, 
I would like to write a Python script that iterates through the arguments of a frame. I was hoping I can retrieve these arguments from a Frame object,  but I did not find a way to do this, unless I know the name of the variables. Similarly, I would like to iterate through all the locals of the frame.

Is this possible with the current Python API? If not, can you please point me to where I could add additional functions to the Python API? 

Thank you, 
Cristi

Re: Python API iterate through the arguments of a frame

[Tom Tromey]

>>>>> "Cristian" == Cristian Zamfir <cristian.zamfir@epfl.ch> writes:

Cristian> I would like to write a Python script that iterates through the
Cristian> arguments of a frame. I was hoping I can retrieve these arguments from
Cristian> a Frame object, but I did not find a way to do this, unless I know the
Cristian> name of the variables. Similarly, I would like to iterate through all
Cristian> the locals of the frame.

Cristian> Is this possible with the current Python API? If not, can you please
Cristian> point me to where I could add additional functions to the Python API?

You can find the locals and arguments via Frame.block.

I see that gdb.Block is under-documented in this area.  Sorry about
that, I will write a patch.

Here's a quick example:

(top-gdb) start
[...]
Temporary breakpoint 3, main (argc=
During symbol reading, incomplete CFI data; unspecified registers (e.g., rax) at 0x488914.
1, argv=0x7fffffffe558) at ../../archer/gdb/gdb.c:26
(top-gdb) python
>for sym in gdb.newest_frame().block():
>  print sym
>end
argc
argv
args


In order to find all the locals in scope, and the arguments, you may
have to iterate upwards over blocks via Block.superblock.  The block
with a non-None 'function' attribute will hold the arguments.

Tom

Re: Python API iterate through the arguments of a frame

[Cristian Zamfir]

On Feb 21, 2012, at 7:14 PM, Tom Tromey wrote:

>>>>>> "Cristian" == Cristian Zamfir <cristian.zamfir@epfl.ch> writes:
> 
> Cristian> I would like to write a Python script that iterates through the
> Cristian> arguments of a frame. I was hoping I can retrieve these arguments from
> Cristian> a Frame object, but I did not find a way to do this, unless I know the
> Cristian> name of the variables. Similarly, I would like to iterate through all
> Cristian> the locals of the frame.
> 
> Cristian> Is this possible with the current Python API? If not, can you please
> Cristian> point me to where I could add additional functions to the Python API?
> 
> You can find the locals and arguments via Frame.block.

Great, this worked.

> 
> I see that gdb.Block is under-documented in this area.  Sorry about
> that, I will write a patch.

Thank you, that would be very useful.  Is there a quick way to see the names of all available API functions, just in case there are more undocumented ones, other than looking into the  gdb/python/ directory and infer from the Python bindings (e.g., py-block.c)?

Somehow unrelated to this, I just noticed that I can retrieve the type of a symbol while running a program inside gdb, but not when loading a dumped core. In the latter case I get this error: 
AttributeError: 'gdb.Symbol' object has no attribute 'type'. 

I can however still retrieve the name and the value of the symbol when loading a core. Is it possible to also get the type?

I should mention that I also see these two warnings when loading the core:
warning: core file may not match specified executable file.
warning: Can't read pathname for load map: Input/output error.

Thanks, 
Cristi

> 
> Here's a quick example:
> 
> (top-gdb) start
> [...]
> Temporary breakpoint 3, main (argc=
> During symbol reading, incomplete CFI data; unspecified registers (e.g., rax) at 0x488914.
> 1, argv=0x7fffffffe558) at ../../archer/gdb/gdb.c:26
> (top-gdb) python
>> for sym in gdb.newest_frame().block():
>> print sym
>> end
> argc
> argv
> args
> 
> 
> In order to find all the locals in scope, and the arguments, you may
> have to iterate upwards over blocks via Block.superblock.  The block
> with a non-None 'function' attribute will hold the arguments.
> 
> Tom

RE: Python API iterate through the arguments of a frame

[Paul_Koning]

>Thank you, that would be very useful.  Is there a quick way to see the names of all >available API functions, just in case there are more undocumented ones, other than looking >into the  gdb/python/ directory and infer from the Python bindings (e.g., py-block.c)?

Standard Python approach: help(xyz) gives you the documentation for xyz.  If xyz is a
class or module, it does that recursively.  So:
  (gdb)  python help(gdb)
will give you all the documentation for everything inside the gdb module.

	paul

Re: Python API iterate through the arguments of a frame

[Joachim Protze]

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On 21.02.2012 23:52, Cristian Zamfir wrote:
> Thank you, that would be very useful.  Is there a quick way to see the names of all available API functions, just in case there are more undocumented ones, other than looking into the  gdb/python/ directory and infer from the Python bindings (e.g., py-block.c)?

You may find the python function dir(<class>) very handy to find
undocumented functions; this lists you all methods and attributes of the
class.

- Joachim


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Re: Python API iterate through the arguments of a frame

[Tom Tromey]

>>>>> "Cristian" == Cristian Zamfir <cristian.zamfir@epfl.ch> writes:

Cristian> Thank you, that would be very useful.  Is there a quick way to see the
Cristian> names of all available API functions, just in case there are more
Cristian> undocumented ones, other than looking into the gdb/python/ directory
Cristian> and infer from the Python bindings (e.g., py-block.c)?

In addition to the other responses, I'd like to add that we want the
documentation to be complete and clear; so please file bugs for any
holes or problems you notice.

Cristian> I can however still retrieve the name and the value of the symbol when
Cristian> loading a core. Is it possible to also get the type?

Yes, this information is orthogonal to where the inferior memory comes
from.

Cristian> warning: core file may not match specified executable file.
Cristian> warning: Can't read pathname for load map: Input/output error.

These could be a problem, but offhand I can't say what is going on.

I'm surprised about the AttributeError.  I'm not sure how that could
happen.

Tom

Re: Python API iterate through the arguments of a frame

[Cristian Zamfir]

On Feb 22, 2012, at 4:06 PM, Tom Tromey wrote:

>>>>>> "Cristian" == Cristian Zamfir <cristian.zamfir@epfl.ch> writes:
> 
> Cristian> Thank you, that would be very useful.  Is there a quick way to see the
> Cristian> names of all available API functions, just in case there are more
> Cristian> undocumented ones, other than looking into the gdb/python/ directory
> Cristian> and infer from the Python bindings (e.g., py-block.c)?
> 
> In addition to the other responses, I'd like to add that we want the
> documentation to be complete and clear; so please file bugs for any
> holes or problems you notice.

Sure, I will. Thanks everyone for the help. 

> 
> Cristian> I can however still retrieve the name and the value of the symbol when
> Cristian> loading a core. Is it possible to also get the type?
> 
> Yes, this information is orthogonal to where the inferior memory comes
> from.
> 
> Cristian> warning: core file may not match specified executable file.
> Cristian> warning: Can't read pathname for load map: Input/output error.
> 
> These could be a problem, but offhand I can't say what is going on.
> 
> I'm surprised about the AttributeError.  I'm not sure how that could
> happen.

It turned out that when I was loading the core I used an older version of gdb (7.3), in which gdb.Symbol did not have the type attribute. Everything works with the latest version. 

Thanks, 
Cristi

> 
> Tom