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

[Armin Rigo]

Hi,

On 23 September 2014 14:54, Eleytherios Stamatogiannakis
<estama at gmail.com> wrote:
>> 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.

You cannot have a C function returning a 'char[]'.  That's why you
need to declare it returning a 'char *', and then you don't know the
length.  Sorry, it's the way C works; there is nothing I can do about
that :-)

Occasionally, we see C functions with this kind of signature:

   size_t getString(xxx, char **result);

This would return the length, and use 'result' as an output parameter,
to store into '*result' a pointer to the string.  If you really care
about performance, then you might want to change the C library you're
binding to in order to do that.


A bientôt,

Armin.