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

p.lavarre@xxxxxxxx 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

.

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