Passing static array to C variadic function

[John Colvin via Digitalmars-d]

On Sunday, 20 July 2014 at 16:00:41 UTC, Daniel Gibson wrote:
> Hi,
> I have a C variadic function (passed from C code into my D code 
> via function pointer) that I need to call with a static array.
>
> So according to the D documentation, static arrays are passed 
> by value in D2 and by reference in C and D1.
> (Even though http://dlang.org/abi.html claims "Static arrays 
> are passed as pointers to their first element." - I guess this 
> just wasn't updated for D2)
>
> For "normal" functions http://dlang.org/interfaceToC.html tells 
> me to add a "ref" in the function signature, to tell D to pass 
> it by reference (couldn't this be implicit for extern (C) 
> functions?) - But I obviously can't to this for vararg function 
> arguments.
>
> So let's imagine the following code:
>
>   extern (C) alias funPtr_t = ptrdiff_t function( ptrdiff_t 
> arg, ... );
>
>   funPtr_t fun = ...; // is assigned somewhere..
>
>   void bla( float[3] v ) {
>       fun( 42, v );
>   }
>
> This produces the following compiler error (DMD 2.065 linux 
> amd64):
> "Error: cannot pass static arrays to extern(C) vararg functions"
>
> /However/, if I typedef a float[3] type, the compiler does not 
> complain (not sure if the code behaves like expected, though, 
> or if it's still passed by value instead of by reference as 
> expected by the C code):
>
>   typedef float[3] vec3_t;
>
>   void bla( vec3_t v ) {
>       fun( 42, v );
>   }
>
> Then again, if I use alias instead of the deprecated typedef:
>   alias vec3_t = float[3];
> I again get "Error: cannot pass static arrays to extern(C) 
> vararg functions".
>
>
> Is there a "proper" way to make this work?
> If not, any ideas for a viable workaround?
>
> Cheers,
> Daniel

C functions takes arrays by pointer to first element. fun(42, 
v.ptr) should work.