ctypes and reading value under pointer passed as param of a callback

Mark Tolonen M8R-yfto6h at mailinator.com
Fri Jul 25 17:07:25 CEST 2008


"waldek" <waldemar.rymarkiewicz at gmail.com> wrote in message 
news:deaeba30-c160-4e55-9049-43af27e96217 at u12g2000prd.googlegroups.com...
> On Jul 24, 5:01 pm, Thomas Heller <thel... at python.net> wrote:
>> waldekschrieb:
>>
>> > Hi,
>>
>> > I'm using C dll with py module and wanna read value (buffer of bytes)
>> > returned in py callback as parameter passed to dll function.
>>
>> The callback receives a pointer instance.  You can dereference the 
>> pointer
>> to read individual bytes in this way:
>>    print data[0], data[5], data[42]
>> or use slicing to read a bunch of bytes:
>>    print data[0:42]
>>
>> So, you probably want something like this:
>>
>> > --------------------------------------------------
>> > def  mycallback(data, size):
>> >     # how to read data buffer here ?
>>
>>       print data[:size]
>>
>> >     return 0
>>
>> > cbfunc = CFUNCTYPE(c_int, POINTER(c_uint8), c_int)
>>
>> > mydll = cdll.somedll
>> > mdll.foo(cbfunct)
>> > ---------------------------------------------------------------
>>
>> > Question: How to get bytes from the buffer passed to mycallback ???
>> > Let's assume that the buffer consist of 4 bytes 4 bytes 8 bytes.
>>
>> > I tried unpack("ii8s", data[0]) and nothing. I tried different ctypes
>> > passed to callback and nothing.
>>
>> Thomas
>
> Cool, works fine now.
>
> ... and how to convert a list of bytes to py string or int ??
>
> i.e
> data[:4] -> py int
> data[4:8] -> py int
> data[8:] -> py strng
>
> I tried unpack and unpack_from but the buff should be an string or
> buffer not a list of bytes as data is.
>
> any suggestions?

Try:

cbfunc = CFUNCTYPE(c_int, c_char_p, c_int)

ctypes understands c_char_p as a null-terminated character string and will 
pass a string to your callback instead of a list of ints.

--Mark




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