I think you won't be able to use with-out-parameters which seems to work
really well with value types, but maybe not so well for your example. Part
of the problem might be the way c-string automatically encodes and decodes
using your encoding (utf8 by default I think). Factor strings are not the
same as char*, so they have have to be converted into and out of "raw
memory" using an encoding when calling functions that take c-strings.
Here's an example using strcpy for convenience, and raw memory (using char*
instead of c-string). We will try and preserve the dst input pointer
instead of using the return value.
FUNCTION: char* strcpy ( char* dst, char* src ) ;
We will make a word that takes a Factor string, encodes it into a utf8
byte-array, allocates a new byte-array destination, then calls strcpy,
drops the return value, and uses the destination byte-array to decode back
into a Factor string.
: do-strcpy ( src -- dst )
utf8 string>alien
[ length <byte-array> dup ] [ strcpy drop ] bi
utf8 alien>string ;
IN: scratchpad "hello" do-strcpy .
"hello"
You can use byte-arrays, or raw memory allocated with malloc (make sure to
&free with destructors, or free manually).
On Fri, Nov 28, 2014 at 8:14 AM, Andrea Ferretti <ferrettiand...@gmail.com>
wrote:
> I am trying to interface with C code. Everything seems to work more or
> less fine, until I have to deal with output parameters, that is,
> parameters in C functions which are meant to be passed buffers which
> are filled by the function.
>
> I have made a very basic example which only copies a string and I have
> made a shared library called "libexample.so" which exports
>
> void example_cp(char *in, char *out);
>
> I have verified that I can call the shared library from C.
>
> Now in factor I have defined an interface like this
>
> USING: alien alien.c-types alien.syntax alien.libraries ;
> IN: example-alien
>
> <<
> "libexample" "/home/papillon/esperimenti/ssl/libexample.so" cdecl
> add-library
> >>
>
> LIBRARY: libexample
> FUNCTION: void example_cp ( c-string in, c-string out ) ;
>
> In the listener I try to use example_cp, but I am not sure how to pass
> a preallocated char buffer. If I try
>
> { { c-string } } [ "hello world" swap example_cp ] with-out-parameters
>
> I get "index out of bounds: 0" which seems reasonable, since I pass an
> empty c-string. But if I try something like
>
> { { c-string initial: "hello earth" } } [ "hello world" swap
> example_cp ] with-out-parameters
>
> I get "local-allocation-error".
>
> What is the correct way to pass a preallocated buffer?
>
>
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