Re: How to get binary data to callback function

"Nikolaus Rath" <[email protected]> wrote in message 
news:[email protected]...
> Nikolaus Rath <[email protected]> writes:
>> "Martin (gzlist)" <[email protected]> writes:
>>> On 22/12/2009, Nikolaus Rath <[email protected]> wrote:
>>>>
>>>> When I define a callback function with a prototype of
>>>>
>>>>    ctypes.CFUNCTYPE(None, ctypes.c_char_p, ctypes.c_int)
>>>
>>> You have the prototype wrong, c_char_p is for null-terminated strings:
>>>
>>> <http://docs.python.org/library/ctypes.html#ctypes.c_char_p>
>>>
>>> You want to use `ctypes.POINTER(ctypes.c_char)` instead for the 
>>> definition.
>>
>> Ah, tricky. Thanks for helping out. I have just submitted a doc patch to 
>> make
>> this more clear (http://bugs.python.org/issue7569).
>
> This actually leads to a new question. How can I construct a
> POINTER(c_char)? c_char_p('string') works nicely, but
> POINTER(c_char)('string') raises a TypeError.
>
> What's the trick that I need here?

POINTER(c_char)(c_char('a')), but that only gives you character.  Take a 
look at c_buffer:

>>> b=c_buffer('abcdef\0defghi',20)
>>> b
<ctypes.c_char_Array_20 object at 0x01361A80>
>>> b.raw
'abcdef\x00defghi\x00\x00\x00\x00\x00\x00\x00'
>>> b.value
'abcdef'
>>> b[6]
'\x00'

Note .value returns the first null-terminated string in the buffer.  Use 
.raw if you want the whole thing.

-Mark



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