CODEWITHSUNDEEP
pythonpython-2.7structctypes
vriesdemichael
vriesdemichael

Reputation: 1340

Ctypes struct finalization

I am trying to make a dynamic struct class. The problem with a ctypes struct is that there is a finalization process that takes place when the struct is initialized or when fields is set.

In the test scripts of the ctypes source I have found this:

Structure/Union classes must get 'finalized' sooner or later, when one of these things happen:

  1. _fields_ is set.
  2. An instance is created.
  3. The type is used as field of another Structure/Union.
  4. The type is subclassed

When they are finalized, assigning fields is no longer allowed.

The structs are used for function pointers. In the subclass of Structure i have defined helper functions that fill a temporary array (not _fields_) with the values that should end up in the _fields_ variable.

This will allow users of my script to make a struct without subclassing it with something like:

class Sub (Structure)
    _fields_ = [
        (<func_name>, CFUNCTYPE(<res>,<*args>)),
        (...),
        ]

Sub(CFUNCTYPE(<res>, <*args>)(<func_impl>), ...)

But instead use:

class Sub(function_struct):
    pass

Sub.addFunction(...)
Sub(<func_impl>)

In the init function of my subclass I do set the _fields_ attribute, which should finalize the type. However, the fields added in the __init__ function do not appear as attributes. Giving me errors like 'Sub' object has no attribute '<func_name>'

I suspect that the subclass is already finalized before the __init__ call, either through #2 or #4 of my list.

Is there any way to work around the finalization or delay it enough for me to dynamically set the _fields_ attribute?

Upvotes: 0

Views: 269

Answers (1)

vriesdemichael
vriesdemichael

Reputation: 1340

I have worked around my issue by making a factory class.

factory = structFactory()
factory.addFunction(<name>, <rtype>)

# Also redefines the __init__ function before returning the type
structType = factory.getType() 

struct = structType(<impl>)

The struct can also be created from a memory address with ctypes.cast and structType.from_address.

Upvotes: 0

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