Thanks, the method did work.
To my understanding, "ALL_RESULTS" is used to prevent pypy GC the pointing
buffer.
And thus, it is the C callback function's responsibility to free the buffer.
Am I right?
On Thu, May 14, 2015 at 4:13 PM, Armin Rigo <ar...@tunes.org> wrote:
> Hi Yicong,
>
> (CC to the cffi mailing list)
>
> On 14 May 2015 at 05:02, Yicong Huang <hengha....@gmail.com> wrote:
> > We had a python function that return a string value. The function will
> > callback in C code.
> > The below is an example of the code:
> >
> > @ffi.callback("char *(char *, char *)")
> > def strconcat(x, y):
> > x1 = ffi.string(x)
> > y1 = ffi.string(y)
> > print x1 + y1
> > return x1 + y1
> >
> > The error messages are:
> > Trying to convert the result back to C:
> > TypeError: initializer for ctype 'char *' must be a cdata pointer, not
> str
> >
> > How could we convert the return python string value to C char*?
>
> This is not obvious, because you have problems writing it in C too:
> returning a "char *" is only possible when you return some constant
> string literal (only in C, can't do that with cffi), or when you
> allocate the resulting string and keep it alive for as long as the
> caller needs it (and you don't really know how long that is).
>
> If you have a callback to a C library that you didn't write, it must
> be written in its documentation. If it is not, and you really have no
> choice, then you have to guess. The following would work in all cases
> but leak the strings by never releasing them:
>
> ALL_RESULTS = []
> @ffi.callback("char *(char *, char *)")
> def strconcat(x, y):
> x1 = ffi.string(x)
> y1 = ffi.string(y)
> print x1 + y1
> p = ffi.new("char[]", x1 + y1)
> ALL_RESULTS.append(p)
> return p
>
> There are chances that it works as expected if you only keep alive the
> last returned result; try it out, but no guarantee.
>
>
> A bientôt,
>
> Armin.
>
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