[pypy-dev] Declaring a function that returns a string in CFFI

[Yichao Yu]

On Tue, Sep 23, 2014 at 8:54 AM, Eleytherios Stamatogiannakis <
estama at gmail.com> wrote:

> On 23/09/14 09:52, Armin Rigo wrote:
>
>> Hi Lefteris,
>>
>> On 22 September 2014 19:37, Eleytherios Stamatogiannakis
>> <estama at gmail.com> wrote:
>>
>>> b = unicode( ffi.buffer( clib.getString(...) ) ,'utf-8')
>>>
>>> because it'll only return the first character of getString, due to being
>>> declared as a 'char*'.
>>>
>>
>> The issue is only that ffi.buffer() tries to guess how long a buffer
>> you're giving it, and with "char *" the guess is one (only
>> ffi.string() has logic to look for the final null character in the
>> array).
>>
>
> If only ffi.string has logic to look for the final null character, then
> how can below work?
>
> >> teststr=ffi.new('char[]', 'asdfasdfasdfasdfasdfasdf')
> >> unicode(ffi.buffer(teststr), 'utf-8')
> u'asdfasdfasdfasdfasdfasdf\x00'
>
> Above doesn't explicitly set the length in ffi.buffer. There is still one
> problem with ffi.buffer and the last "\x00" in input, but otherwise it
> works with only 1 copy to go from a char* to a Python unicode string.
>

The first line you have returns an object that owns the memory and
therefore knows how long it is, which is later used by ffi.buffer to figure
out how long the buffer is.

Also notice that the result of the second line has '\x00' at the end. This
also work even if the string has null bytes in the middle

In [9]: unicode(ffi.buffer(ffi.new('char[]', '\0a'))) Out[9]: u'\x00a\x00'


> The problem is that i cannot declare a C function as returning a char[] so
> that ffi.buffer will have the same behaviour on its results as it has with
> above "teststr".
>
>  You need to get its length explicitly, for example like this:
>>
>> p = clib.getString(...)        # a "char *"
>> length = clib.strlen(p)     # the standard strlen() function from C
>> b = unicode(ffi.buffer(p, length), 'utf-8')
>>
>
> I've tried that, and the overhead of the second call is more or less equal
> to the cost of the copy when using ffi.string.
>
> Kind regards,
>
>
> l.
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