Re: [ctypes-users] Dereferencing a slice of pointer

[Thomas H. <th...@py...>]

Holger Duerer <H.D...@gm...> writes:

> I am currently trying to wrap a function which allocates a buffer for
> me; something like
> ,----
> | int foo(char **out_buffer, size_t *out_buffer_length);
> `----
>
> The data in the buffer can be binary, i.e. it may contain a 0-byte,
> which is why we get a length returned as well.
>
> Because of the 0-byte, I cannot use something like 
>
> ,----
> | out_buffer = c_char_p()
> | out_buffer_length = c_int()
> | success = the_lib.foo(byref(out_buffer), byref(out_buffer_length))
> `----
>
> What I currently have is:
> ,----
> | out_buffer = POINTER(c_char)()
> | out_buffer_length = c_int()
> | success = the_lib.foo(pointer(out_buffer), byref(out_buffer_length))
> | out_buffer = "".join([out_buffer[i] for i in range(out_buffer_length)])
> `----
> and this works.
>
> Two questions:
> a) Could this be done better?

No, it's the 'correct' way to do it, but let me make two remarks.
You could use byref(out_buffer) in the the_lib.foo call instead of
pointer(out_buffer), this will be slightly faster.
And it seems in your code the pointer can never be freed because you
overwrite the out_buffer variable. Maybe you were just simplifying...

> b) Why doesn't the pointer metaclass support slicing?  I.e. this could
>    have simplified my last line to
>    ,----
>    | out_buffer = "".join(out_buffer[:out_buffer_length])
>    `----
>    I had a quick glance at the code and it is just missing -- is this
>    intentional or just not yet done?

I probably had a mental block (Brett vor'm Kopf) because I always
thought about slicing support for arrays instead of pointers first.
And I couldn't decide if it would be better to return a new ctypes array
instance as the slice, or a list of array elements.
The array instance would be much more complicated, because this would
have to be an instance of a probably not yet existing class.

For pointer slicing, it is simpler because an array would not make any
sense.  So I think the list of elements is what needs to be returned,
for pointers *and* for arrays.  Same for slice assignment, of course.

Oh, and for arrays or pointers of c_chars it could be specialized to
return a string instead of a list of characters instead, so that you
could write

   ,----
   | out_buffer = out_buffer[:out_buffer_length]
   `----

instead of the join above.
Sounds good?

Thanks,

Thomas