[Pyrex] Casting Python sequence to pointer and vice versa

[Greg Ewing]

Christian Heimes wrote:

> problem 1: The cdef extern from "stdlib.h" block isn't intuitive.
> Although I know that the C functions are declared in stdlib.h I had to
> google for the correct declarations.

You have to know how to translate C declarations
into Pyrex declarations. There's not much I can do
about that, other than improve the Pyrex documentation
if you can point out something that wasn't clear.

> 
> problem 2 (the nasty gotcha): The usual malloc(sizeof(ULong)*size)
> statement failed with a cryptic error message.

Can you show me the exact Pyrex code you were using?
The following compiles okay for me:

   cdef extern from "stdlib.h":
     ctypedef unsigned long size_t
     void *malloc(size_t size)

   ctypedef unsigned long ULong

   cdef void blarg(int size):
     cdef void *p
     p = malloc(sizeof(ULong)*size)

>      # cast Python block of unsigned longs (k) to unsigned long pointer
>      # return size or -1 in the case of an error
>      size = Py2Ptr(block, 'k', v)

I haven't tried to provide anything like this because there
are so many different ways that you might want to convert
between Python data and C data. But I'll think about your
suggestion.

Thanks,
Greg