working with ctypes and complex data structures

[Michael Felt]

On 05-Oct-16 22:29, Emile van Sebille wrote:
Thanks for the reply!

After a shirt coffeebreak - back into the fray - and I found the following:

> +76  class cpu_total:
>    +77      def __init__(self):
>    +78          __perfstat__ = CDLL("libperfstat.a(shr_64.o)")
>    +79          prototype = CFUNCTYPE(c_int, c_void_p, c_void_p, c_int,
> c_int)
error #2 - see below
> +80          args = (1, "name", None), (2, "buff", None), (1, "size",
> 0), (1, "count", 1)
error #1. paramater type 2 (the buffer might be where data is being put, 
but for the call, the pointer is INPUT)
> +81          cpu_total = prototype(("perfstat_cpu_total",
> __perfstat__), args)
>    +82
>    +83          buff = perfstat_cpu_total_t()
>    +84          cpu_total(buff=buff, size=sizeof(buff))
>    +85 
The error #2 is #2, because only after I corrected the inputs did I get 
Type Exceptions. I had hoped that c_void_p was a "typeless" type, mainly 
the right size, but the ctypes interface is more particular when you use 
the CFUNCTYPE() - so now I have my boilerplate for what I hope
can be a tutorial for interfacing - by hand - with complex functions, 
i.e., using one class as the data aka typedef defintion, and a second 
class to work
on it.

In template form:

class perfstat_xxx_t:
     _fields_ = ( ... )

class perfstat_xxx:
     def init(self):
         _perflib = ctypes.CDLL("libperfstat.a(shr_64.o)") # AIX member 
in an archive
         _fn_xxx = _perflib.xxx
         # ALL AIX perfstat routines have 4 arguments: (name, buff, 
sizeof_buff, count)
         # the buff is a pointer to where the information will be stored
         _fn = CFUNCTYPE(c_int, c_void_p, POINTER(xxx_t), c_int, c_int)
         _args = (1, "name", None), (1, "buff", None), (1, "size", 0), 
(1, "count", 1)
         _xxx = _fn(("xxx", _fn_xxx), _args)

         _buff = perfstat_cpu_total_t()
         _xxx(buff=_buff, size=sizeof(_buff))
         self._v = _buff

So, the next question/test will be to move the _fields_ into the 
"second" class definition. I just need to find what "self" attribute 
that is.