working with ctypes and complex data structures
Emile van Sebille
emile at fenx.com
Wed Oct 5 16:29:38 EDT 2016
On 10/05/2016 01:06 PM, Michael Felt wrote:
>
>
> On 02-Oct-16 19:50, Michael Felt wrote:
>> I am trying to understand the documentation re: ctypes and interfacing
>> with existing libraries.
>>
>> I am reading the documentation, and also other sites that have largely
>> just copied the documentation - as well as the "free chapter" at
>> O'Reilly (Python Cookbook).
>>
> Using O'Reilly and the standard documentation I now have these two
> excerpts. The second def works, the first does not.
>
> Someone helping me understand what I am not reading properly in this bit
> of documentation - is much appreciated.
>
>>>> from ctypes import c_int, WINFUNCTYPE, windll
>>>> from ctypes.wintypes import HWND, LPCSTR, UINT
>>>> prototype = WINFUNCTYPE(c_int, HWND, LPCSTR, LPCSTR, UINT)
>>>> paramflags = (1, "hwnd", 0), (1, "text", "Hi"), (1,
>>>> "caption", None), (1, "flags", 0)
>>>> MessageBox = prototype(("MessageBoxA", windll.user32), paramflags)
>>>>
>
> The MessageBox foreign function can now be called in these ways:
>
>>>>
>
>>>> MessageBox()
>>>> MessageBox(text="Spam, spam, spam")
>>>> MessageBox(flags=2, text="foo bar")
>>>>
>
> Note that MessageBox() may give two arguments.
>
> My code: note both def init() are meant to do the same thing, just
> different call syntax.
>
> +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)
> +80 args = (1, "name", None), (2, "buff", None), (1, "size",
> 0), (1, "count", 1)
> +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
> +86 class perfstat_cpu_total:
> +87 def __init__(self):
> +88 __perfstat__ = CDLL("libperfstat.a(shr_64.o)")
> +89 cpu_total = __perfstat__.perfstat_cpu_total
> +90
> +91 _perfstat_cpu_total = perfstat_cpu_total_t()
> +92 ret = cpu_total(None,
> +93 byref(_perfstat_cpu_total),
> +94 sizeof(perfstat_cpu_total_t), 1)
> +95 self.boot = _perfstat_cpu_total.lbolt.v / 100
> +96 self.ncpus = _perfstat_cpu_total.ncpus
> +97 self._v = _perfstat_cpu_total
>
>
> When I call the second I can print values and even call functions,
> reload the _v, etc..
>
> The first class fails with this message:
>
> a = cpu_total()
> Traceback (most recent call last):
> File "<stdin>", line 135, in <module>
> File "<stdin>", line 84, in __init__
> TypeError: call takes exactly 1 arguments (2 given)
>
> Thanks for your attention.
the line it's objecting to is:
> +84 cpu_total(buff=buff, size=sizeof(buff))
because the expectation is defined as:
> +77 def __init__(self):
HTH,
Emile
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