How to implement a new Python command that takes values from tty?

How to implement a new Python command that takes values from tty?

[Yifei Zheng]

Hi there,

I have written a few gdb pretty-printers using Python but this time I'm 
trying to write a new command in Python. I want it to take arguments 
from gdb and somewhat works like gdb's print. (It's a special printer 
for some kind of std::vector).

I know I should start by inheriting `gdb.Command` and call `__init__`, 
but I'm definitely stuck on `invoke`. The arguments passed in are in 
string, which, I would like to convert them to gdb.Value, by looking 
them up somehow. I have a partially working version that uses 
"gdb.lookup_symbol", but it does not work for expressions (of course, 
expected).

For example, say the command was called `ppack` which accepts an 
argument just as native `print` do. Desired syntax would be like "pprint 
some_var" or "pprint some_other_var.field1" or "pprint *some_ptr".

How can I let gdb evaluates the expression for me so that I don't have 
to parse it, and then give me the value instead of the name?

Thanks,
Yifei

Re: How to implement a new Python command that takes values from tty?

[Tom Tromey]

>>>>> "Yifei" == Yifei Zheng <zheng432@purdue.edu> writes:

Yifei> I have written a few gdb pretty-printers using Python but this time
Yifei> I'm trying to write a new command in Python. I want it to take
Yifei> arguments from gdb and somewhat works like gdb's print. (It's a
Yifei> special printer for some kind of std::vector).

I'm curious why not write a pretty-printer for this case as well?

Yifei> How can I let gdb evaluates the expression for me so that I don't have
Yifei> to parse it, and then give me the value instead of the name?

You want gdb.parse_and_eval.

Tom

Re: How to implement a new Python command that takes values from tty?

[Yifei Zheng]

Hi Tom,

Thank you for pointing me out "parse_and_eval".

I didn't write an pretty printer because it'll be a little ambiguous on 
type matching.

Say the type I wanted to print is "std::vector<Var*>" (which I call a 
pack) where Var is a class and has its own printer. What I wanted to do 
is to dereference all pointers in the vector and pretty-print each one. 
However, some of the "std::vector<Var*>"s are packs while some are not. 
It's at debugger users' discretion. I don't know if that's generally a 
good practice or not. Any better idea?

Yifei

On 04/10/2017 5:19 PM, Tom Tromey wrote:
>>>>>> "Yifei" == Yifei Zheng <zheng432@purdue.edu> writes:
> 
> Yifei> I have written a few gdb pretty-printers using Python but this time
> Yifei> I'm trying to write a new command in Python. I want it to take
> Yifei> arguments from gdb and somewhat works like gdb's print. (It's a
> Yifei> special printer for some kind of std::vector).
> 
> I'm curious why not write a pretty-printer for this case as well?
> 
> Yifei> How can I let gdb evaluates the expression for me so that I don't have
> Yifei> to parse it, and then give me the value instead of the name?
> 
> You want gdb.parse_and_eval.
> 
> Tom
>