Re: C strings in O'Caml

[Xavier Leroy <Xavier.Leroy@inria.fr>]

> I'm   interfacing  a C  library in  O'Caml  and I  faced the following
> problem. I have a C string declared in the  library, and I want to see
> it as a Caml string. Is it  possible to do  it without copying it ?

If you have control over the allocation of the string, then you can
put a Caml string header in the word preceding the beginning of the
string, and twiddle the last byte of the last word of the string as
described in byterun/alloc.c. Then, you can just pass the char * as a
Caml value and any Caml code should work on it quite happily. (The
char * pointer must be word-aligned, by the way.)

If the C string is allocated outside of your code (e.g. in a library),
then you can't fully disguise it as a Caml string. You can still return
the char * as a Caml value (provided it's word-aligned), but you can't
use String.length neither any of the "safe" string functions (those
that checks the bounds) on this string. You can still use the
undocumented (and highly unsafe) String.unsafe_get, String.unsafe_set,
String.unsafe_blit and String.unsafe_fill on that pseudo Caml string.
Just be very, very careful with out-of-bounds accesses.

- Xavier Leroy