Interfacing C programs: Pass D string to C function

[Jonathan M Davis]

On Friday, December 13, 2013 09:00:20 Dfr wrote:
> Hello
> 
> I trying to write simple wrapper around pcre and have problem
> passing strings to it. As i understood, the best way is
> std.string.toStringZ.
> 
> So, my code look like:
> 
> string pattern = "....";
> pcre_compile2( toStringz(pattern), options, &errcode, &errmsg,
> &erroffset, cast(char*)null);
> 
> This gives me error:
> 
> Error: function pcre_compile2 (char*, int, int*, char**, int*,
> char*) is not callable using argument types (immutable(char)*,
> int, int*, char**, int*, char*)
> 
> Any ideas for better way to do this task ?

The problem is that toStringz returns immutable(char)* (because string is 
immutable(string)[]), and your function is taking char*. It will work to pass 
the result of toStringz to a C function which takes const char* (since 
immutable will implicitly convert to const), but immutable doesn't implicitly 
convert to mutable, and casting immutable mutable is almost always a bad idea.

The easiest solution would be to use std.utf.toUTFz. e.g.

auto cstr = str.toUTFz!(char*)();

or you could do something like

auto cstr = str.dup.ptr;

In either case, what you're doing is allocating a new char[] which holds the 
same elements as the original string and getting its ptr property so that you 
have a char*.

- Jonathan M Davis