Python: should gdb.Value array indexing check bounds?

Python: should gdb.Value array indexing check bounds?

[Paul_Koning]

Currently, if a gdb.Value object corresponds to an array, array indexing is done C-style: no bounds checking, just pointer arithmetic.  That's somewhat unnatural to Python.  Should it do a range check instead, and raise IndexError for out of range index values?

That wouldn't affect indexing of pointers, since those don't have a range so the only possible approach is the C one.

                paul 

Re: Python: should gdb.Value array indexing check bounds?

[Matt Rice]

On Tue, Sep 20, 2011 at 9:04 AM,  <Paul_Koning@dell.com> wrote:
> Currently, if a gdb.Value object corresponds to an array, array indexing is done C-style: no bounds checking, just pointer arithmetic.  That's somewhat unnatural to Python.  Should it do a range check instead, and raise IndexError for out of range index values?
>
> That wouldn't affect indexing of pointers, since those don't have a range so the only possible approach is the C one.

I'd think no because of things like:
http://gcc.gnu.org/onlinedocs/gcc-4.1.2/gcc/Zero-Length.html
(which is possible with non-zero length arrays also).

Re: Python: should gdb.Value array indexing check bounds?

[Paul Koning]

> From: Matt Rice [mailto:ratmice@gmail.com] 
> 
> On Tue, Sep 20, 2011 at 9:04 AM,  <Paul_Koning@dell.com> wrote:
>> Currently, if a gdb.Value object corresponds to an array, array indexing is done C-style: no bounds checking, just pointer arithmetic.  That's somewhat unnatural to Python.  Should it do a range check instead, and raise IndexError for out of range index values?
>> 
>> That wouldn't affect indexing of pointers, since those don't have a range so the only possible approach is the C one.
> 
> I'd think no because of things like:
> http://gcc.gnu.org/onlinedocs/gcc-4.1.2/gcc/Zero-Length.html
> (which is possible with non-zero length arrays also).

I suppose that could be handled by making the check not apply in that case.  Or the bounds could be made writable?  Right now the bounds go with the type, not the value; for them to be writable they would have to go with the value.

Part of my reason for asking is that I'm looking at how to do iterators.  In Python you can iterate over a list (array), and I'd like to be able to do that for a gdb.Value which is an array.  And come to think of it, that can certainly be done even if the indexing operation itself doesn't honor bounds, but it would be more Pythonically consistent if the range of indexes over which the iterator iterates matches the range of indexes that indexing allows.

	paul

Re: Python: should gdb.Value array indexing check bounds?

[Tom Tromey]

>>>>> "Paul" == Paul Koning <paulkoning@comcast.net> writes:

Paul> Currently, if a gdb.Value object corresponds to an array, array
Paul> indexing is done C-style: no bounds checking, just pointer
Paul> arithmetic.  That's somewhat unnatural to Python.  Should it do a
Paul> range check instead, and raise IndexError for out of range index
Paul> values?

I think it would be good if the bad cases could somehow be avoided.

Matt> I'd think no because of things like:
Matt> http://gcc.gnu.org/onlinedocs/gcc-4.1.2/gcc/Zero-Length.html
Matt> (which is possible with non-zero length arrays also).

Paul> I suppose that could be handled by making the check not apply in that
Paul> case.

I think there are other bad cases -- e.g., the classic C struct hack
uses an array with 1 element.

Paul> Or the bounds could be made writable?  Right now the bounds go
Paul> with the type, not the value; for them to be writable they would have
Paul> to go with the value.

This sounds like Jan's VLA patchset.

I'm not sure how rewriting the bounds would work in practice.
I guess you could just cast the array to a pointer today.

Tom