foo.c

#include <stdio.h>

struct CrystalString {
  char *ptr;
  int size;
};

void dump(struct CrystalString str) {
  printf("%iB @ %p\n", str.size, str.ptr);
}

void bla(void *eins, struct CrystalString zwei, struct CrystalString drei, struct CrystalString vier) {
  printf("eins: %p\n", eins);  // This is fine
  printf("zwei: "); dump(zwei); // Same
  printf("drei: "); dump(drei); // Great too
  printf("vier: "); dump(vier); // What's going on?!
}

foo.cr

@[Link(ldflags: "#{__DIR__}/foo.c")] # <- Don't try this at home
lib Bind
  struct CrystalString
    ptr : LibC::Char*
    size : LibC::Int
  end

  fun bla(eins : Void*, zwei : CrystalString, drei : CrystalString, vier : CrystalString) : Void
end

def wrap(str)
  Bind::CrystalString.new(ptr: str.to_unsafe, size: str.size)
end

Bind.bla(nil, wrap("Zwei"), wrap("Drei"), wrap("Vier"))