ctypes and unsigned char* - Python

oliver.andrich schrieb:
> Hi,
>
> can anybody with ctypes experience tell me, how to handle a C function
> that returns an unsigned char*? Obviously it is not a restype of
> c_char_p.

From the docs:

c_ubyte

Represents the C unsigned char datatype, it interprets the value as
small integer. The constructor accepts an optional integer initializer;
no overflow checking is done.



Diez

On 19 ago, 14:44, "oliver.andr..."
<oliver.andr...> wrote:
> Hi,
>
> can anybody with ctypes experience tell me, how to handle a C function
> that returns an unsigned char*? Obviously it is not a restype of
> c_char_p.

Being signed or unsigned is not important here. But you have to
disambiguate "returns an unsigned char*"
- the function returns a string of bytes (unsigned char), null-
terminated.
- the function returns a pointer to a single byte.

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

--
Gabriel Genellina

On 19 Aug., 20:40, Gabriel Genellina <gagsl-...@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

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

dll.MagickGetImageProfile.restype = POINTER(c_ubyte)

with the same result. But maybe I should do some further debugging....

Best regards,
Oliver

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

> On 19 Aug., 20:40, Gabriel Genellina <gagsl-...@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

On 23 Aug., 05:46, "Gabriel Genellina" <gagsl-...@yahoo.com.ar> wrote:
> 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...)

Thank a lot, that does exactly what I expected it to do.

Best regards,
Oliver