Reading a static variable in Python

Reading a static variable in Python

[Chris Johns]

Hello,

I am adding pretty printers for RTEMS to gdb. In the RTEMS operating 
system elements such as a semaphore are given an id. I would like to 
print the actual semaphore data given a semaphore id. To do this I need 
to read a kernel structure from a table indexed via a bit field in the 
id. As an example the semaphore's table is declared in RTEMS as:

RTEMS_SEM_EXTERN Objects_Information  _Semaphore_Information;

How would I read the 10 element of the _Semaphore_Information in Python ?

I am stuck on how to create a new gdb.Value variable in Python.

Thanks
Chris

Re: Reading a static variable in Python

[Tom Tromey]

>>>>> "Chris" == Chris Johns <chris@contemporary.net.au> writes:

Chris> I am adding pretty printers for RTEMS to gdb. In the RTEMS operating
Chris> system elements such as a semaphore are given an id. I would like to
Chris> print the actual semaphore data given a semaphore id. To do this I
Chris> need to read a kernel structure from a table indexed via a bit field
Chris> in the id. As an example the semaphore's table is declared in RTEMS
Chris> as:

Chris> RTEMS_SEM_EXTERN Objects_Information  _Semaphore_Information;

This isn't really enough information for us to help you.
What does Objects_Information look like?

Chris> I am stuck on how to create a new gdb.Value variable in Python.

Use gdb.parse_and_eval.  This wasn't added until after 7.0.

Tom

Re: Reading a static variable in Python

[Chris Johns]

On 11/03/2010 4:05 AM, Tom Tromey wrote:
>>>>>> "Chris" == Chris Johns<chris@contemporary.net.au>  writes:
>
> Chris>  I am adding pretty printers for RTEMS to gdb. In the RTEMS operating
> Chris>  system elements such as a semaphore are given an id. I would like to
> Chris>  print the actual semaphore data given a semaphore id. To do this I
> Chris>  need to read a kernel structure from a table indexed via a bit field
> Chris>  in the id. As an example the semaphore's table is declared in RTEMS
> Chris>  as:
>
> Chris>  RTEMS_SEM_EXTERN Objects_Information  _Semaphore_Information;
>
> This isn't really enough information for us to help you.
> What does Objects_Information look like?
>

Yes sorry about that. It is a C struct [1].

> Chris>  I am stuck on how to create a new gdb.Value variable in Python.
>
> Use gdb.parse_and_eval.

Nice. This is an excellent solution. I suppose being an expression it 
can do the pointer maths as well. I was looking at ways to construct a 
gdb.Value with a gdb.Symbol as an argument.

>  This wasn't added until after 7.0.

I am using a 7.1 snapshot as it has the 'source' command to load a 
Python file. This is also an excellent addition.

Chris

[1] 
http://www.rtems.org/onlinedocs/doxygen/cpukit/html/structObjects__Information.html

Re: Reading a static variable in Python

[Tom Tromey]

>>>>> "Chris" == Chris Johns <chris@contemporary.net.au> writes:

Tom> Use gdb.parse_and_eval.

Chris> Nice. This is an excellent solution. I suppose being an expression it
Chris> can do the pointer maths as well. I was looking at ways to construct a
Chris> gdb.Value with a gdb.Symbol as an argument.

Yes, strangely there doesn't seem to be a way to do that.
If you want this, feel free to file a bug report for it...

Tom

Re: Reading a static variable in Python

[Phil Muldoon]

On 03/11/2010 04:53 PM, Tom Tromey wrote:
>>>>>> "Chris" == Chris Johns <chris@contemporary.net.au> writes:
> 
> Tom> Use gdb.parse_and_eval.
> 
> Chris> Nice. This is an excellent solution. I suppose being an expression it
> Chris> can do the pointer maths as well. I was looking at ways to construct a
> Chris> gdb.Value with a gdb.Symbol as an argument.
> 
> Yes, strangely there doesn't seem to be a way to do that.
> If you want this, feel free to file a bug report for it...

Frame's read_var function creates a gdb.Value from a given gdb.Symbol,
but obviously one needs the frame and the symbol for that.  There was
a stub in the gdb.Symbol code to do this without a frame, but it was
very limited so I removed it. Frame's function read_var uses
read_var_value which needs a frame (well, for some reads -
symbol_read_needs_frame returns a mixed bag of results).  Not sure if
it is possible to construct a value from a symbol without the frame in
all cases?

Cheers,

Phil

Re: Reading a static variable in Python

[Tom Tromey]

>>>>> "Phil" == Phil Muldoon <pmuldoon@redhat.com> writes:

Phil> Not sure if it is possible to construct a value from a symbol
Phil> without the frame in all cases?

Not in all cases, but we could throw an exception if a frame is required.

Tom

Re: Reading a static variable in Python

[Chris Johns]

On 3/12/2010 3:53 AM, Tom Tromey wrote:
>>>>>> "Chris" == Chris Johns<chris@contemporary.net.au>  writes:
>
> Tom>  Use gdb.parse_and_eval.
>
> Chris>  Nice. This is an excellent solution. I suppose being an expression it
> Chris>  can do the pointer maths as well. I was looking at ways to construct a
> Chris>  gdb.Value with a gdb.Symbol as an argument.
>
> Yes, strangely there doesn't seem to be a way to do that.
> If you want this, feel free to file a bug report for it...
>

I am happy with the gdb.parse_and_eval solution. If a need arises I will 
create a bug report.

Thanks
Chris