[pypy-dev] How to convert python string to C char*?

[Yicong Huang]

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 Thursday, May 14, 2015 at 4:14:07 PM UTC+8, Armin Rigo wrote:
>
> Hi Yicong, 
>
> (CC to the cffi mailing list) 
>
> On 14 May 2015 at 05:02, Yicong Huang <hengh... at gmail.com <javascript:>> 
> 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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