Re: 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.