Re: Counter-intuitive behavior when using string_at with a c_char_p type.

heh, okay, documentation reading fail on my part.

thanks
-jt

On Mon, Aug 6, 2012 at 9:55 AM, Diez Roggisch <[email protected]> wrote:
> On 8/2/12 7:07 AM, "JT Olds" <[email protected]> wrote:
>
>>Hey all,
>>
>>I can't decide if this is a bug or what, but this is very definitely
>>surprising behavior to me. Basically, if the restype of a C method is
>>c_char_p instead of c_void_p, even if you don't want null-terminated
>>strings you get it. Yay!
>>
>>Given the following program:
>>
>>    import ctypes
>>    import ctypes.util
>>
>>    libc = ctypes.CDLL(ctypes.util.find_library('c'))
>>    libc.memcpy.argtypes = [ctypes.c_void_p, ctypes.c_void_p,
>>ctypes.c_size_t]
>>    libc.memcpy.restype = ctypes.c_char_p
>>
>>    value = "\x01\x00\x01\x02\x03"
>>    out = ctypes.create_string_buffer(len(value))
>>    out = libc.memcpy(out, value, len(value))
>>    out = ctypes.string_at(out, len(value))
>>    assert len(out) == len(value)
>>    assert out == value
>>
>>Note that only the second assertion fails. Both assertions will
>>succeed if you change libc.memcpy.restype to ctypes.c_void_p.
>>
>>So what's going on here? I'm not completely sure, but everything after
>>the null byte gets replaced with garbage if libc.memcpy.restype is
>>ctypes.c_char_p.
>>
>>Is this intentional? Is this a bug? It made me tear my hair out for a
>>few hours today. Note that both of these work:
>
> I guess it's intentional, because for the attribute "value" to be
> meaningful for the 99.99%-case, it needs to treat the memory area pointed
> to as zero-terminated string. It can't do otherwise, as a lot of C-APIs
> rely on this.
>
> And it's even documented:
>
>  http://docs.python.org/library/ctypes#ctypes.c_char_p
>
> """
> For a general character pointer that may also point to binary data,
> POINTER(c_char) must be used.
> """
>
> So while I understand your frustration, there won't be a cure.
>
>
> Diez
>
>

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