ctypes and unsigned char*

Gabriel Genellina gagsl-py2 at yahoo.com.ar
Thu Aug 23 05:46:46 CEST 2007

En Mon, 20 Aug 2007 08:30:50 -0300, oliver.andrich at gmail.com  
<oliver.andrich at gmail.com> escribi�:

> On 19 Aug., 20:40, Gabriel Genellina <gagsl-... at yahoo.com.ar> wrote:
>> In the first case, use a plain c_char_p - the individual "chars" are
>> already unsigned in Python (that is, ord(xxx[i]) is always positive)
>> In the second case, first define the pointer type:
>> c_ubyte_p = POINTER(c_ubyte)
>> your_function.restype = c_ubyte_p
> I will have to look into that. My function I want to wrap has the
> following signature
> unsigned char *MagickGetImageProfile(MagickWand *wand,const char
> *name,
>     size_t *length)
> Well, the first argument and the second are straight forward, and work
> as expected. The last argument also works and it should take up the
> length of the string (unsigned char *). If I now do

So, if I understand correctly, after the call, *length will contain the  
length of the buffer pointed by the function result.

> dll.MagickGetImageProfile.restype = c_char_p
> result = dll.MagickGetImageProfile(wand, "8bim", byref(length))
> result is either an empty string or a 3 bytes long string, while
> length containes the correct length of 1086 bytes for example. I also
> tried

If the result can contain nul bytes, you have to specify its size  
explicitely (else it will be truncated at the first zero). Use the  
string_at utility function:

result_str = string_at(result, length)

(That should work fine, I presume...)

Gabriel Genellina

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