FAQ: how to vary the byte offset of a field of a ctypes.Structure

[Thomas Heller]

p.lavarre at ieee.org schrieb:
> How do I vary the byte offset of a field of a ctypes.Structure?
> 
> How do I "use the dynamic nature of Python, and (re-)define the data
> type after the required size is already known, on a case by case
> basis"?
> 
> \\\
> 
> For example, suppose sometimes I receive the value '\x03hi' + \x04bye'
> for the struct:
> 
>         class Struct34(ctypes.Structure):
>                 _pack_ = 1
>                 _fields_ = [('first', 3 * ctypes.c_ubyte),
>                         ('second', 4 * ctypes.c_ubyte)]
> 
> but then sometimes instead I receive the value '\x05left' + \x06right'
> for the struct:
> 
>         class Struct56(ctypes.Structure):
>                 _pack_ = 1
>                 _fields_ = [('first', 5 * ctypes.c_ubyte),
>                         ('second', 6 * ctypes.c_ubyte)]
> 
> Thus in general I receive (0xFF ** 2) possible combinations of field
> lengths.
> 
> ///
> 
> How do I declare all those hugely many simply regular combinations as
> one CTypes.structure?
> 
> I also need to do series of 3 or 4 or 5 strings, not just 2 strings.
> But always the byte offsets of the subsequent fields vary when the
> byte sizes of the preceding fields vary. The byte size of the
> enclosing packed struct varies as the length of the packed bytes it
> contains.

Often it helps to ask yourself the question: How would I do this in C?

IMO, the answer to this question, applied to your problem, would be:
*Not* by using a structure. A structure is fine if the definition is fixed,
or at most has *one* variable sized field at the very end.  Nothing
is true for your problem.

Thomas