ctypes and unsigned char*

[oliver.andrich]

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.

Best regards,
Oliver

[Diez B. Roggisch]

ol************@ gmail.com 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

[Gabriel Genellina]

On 19 ago, 14:44, "oliver.andr... @gmail.com"
<oliver.andr... @gmail.comwrote :

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.r estype = c_ubyte_p

--
Gabriel Genellina

[oliver.andrich]

On 19 Aug., 20:40, Gabriel Genellina <[e-mail].a rwrote:

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.r estype = c_ubyte_p

I will have to look into that. My function I want to wrap has the
following signature

unsigned char *MagickGetImage Profile(MagickW and *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.MagickGetIm ageProfile.rest ype = c_char_p
result = dll.MagickGetIm ageProfile(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.MagickGetIm ageProfile.rest ype = POINTER(c_ubyte )

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

Best regards,
Oliver

[Gabriel Genellina]

En Mon, 20 Aug 2007 08:30:50 -0300, ol************@ gmail.com
<ol************ @gmail.comescri bi�:

On 19 Aug., 20:40, Gabriel Genellina <[e-mail].a rwrote:

>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 *MagickGetImage Profile(MagickW and *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.MagickGetIm ageProfile.rest ype = c_char_p
result = dll.MagickGetIm ageProfile(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(resul t, length)

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

--
Gabriel Genellina

[oliver.andrich]

On 23 Aug., 05:46, "Gabriel Genellina" <[e-mail].a rwrote:

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(resul t, length)

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

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

Best regards,
Oliver