Re: [Caml-list] Wrapping OCaml function returning a variant

[Richard Jones <rich@annexia.org>]

On Wed, Mar 28, 2007 at 12:33:46PM +0100, Joel Reymont wrote:
> The other issue is that I don't know how to deal with strings  
> allocated in OCaml. Will they be  collected automatically?
> 
> The strings _must_ be allocated in OCaml but I can guarantee that  
> they will not be modified in the C code. Any suggestions on how to  
> deal with this?

String_val (ocaml_str) will give you a pointer to the bytes in the
string.  It's even ASCIIZ (ie. ASCII NUL-terminated) so you can
directly use C str* functions on it.

There are two catches: The small one is that OCaml strings may contain
NUL characters internally, which is obviously not allowed in C
strings, so you have to either guarantee on the OCaml side that this
won't be the case, or else go through hoops on the C side.

The bigger catch is that the garbage collector can and will move the
string around whenever it likes.  So if you save the pointer from
String_val, go back into OCaml code and back into C, and try to reuse
the saved pointer, it won't necessarily point to the string any more.
The string might even have been freed by the GC.  If you want to do
that, either rewrite your code so it doesn't save the pointer like
this, strdup the string to make a private copy in C, or register a
global root.

Rich.

-- 
Richard Jones
Red Hat