[julia-users] Passing structs to C using StrPack

[Karanveer Mohan]

Hi, 

I'm a new Julia user, so bear with me. I believe I can convert a type into 
a structure input for ccall using StrPack. What I'm stuck on is when a 
struct points to another struct like in this case:


struct structB
{
  int val;
};

struct structA
{
  int val;
  struct structB * struct_b;
};

int test(struct structA *struct_a)
{
  fprintf(stderr, "%d\n", struct_a->val);
  fprintf(stderr, "Got val from struct_a\n");
  fprintf(stderr, "%d\n", (struct_a->struct_b)->val);
  return 1;
}



When I try to access val of structA, everything works fine, but it faults 
when trying to access val of structB through structA 
(struct_a->struct_b->val).

My Julia code looks like this:
using StrPack
@struct type StructB
  val::Int32

  function StructB(val)
    new(convert(Int32, val))
  end
end

@struct type StructA
  val_one::Int32
  struct_b::StructB

  function StructA(val_one, struct_b::StructB)
    new(convert(Int32, val_one), struct_b)
  end
end

function test()
  struct_b = StructB(1)
  struct_a = StructA(1, struct_b)

  struct_a_str = IOBuffer()
  pack(struct_a_str, struct_a)

  return_val = ccall((:test, "test.so"), Int32, (Ptr{Void}, ), 
struct_a_str.data)
end

When this didn't work, I tried to be a bit more 'clever' and use 
pointer_from_objref to point to structB like this:
using StrPack
@struct type StructB
  val::Int32

  function StructB(val)
    new(convert(Int32, val))
  end
end

@struct type StructA
  val_one::Int32
  ptr::Ptr{None}

  function StructA(val_one, ptr::Ptr{None})
    new(convert(Int32, val_one), ptr)
  end
end

  struct_b = StructB(10)
  struct_b_str = IOBuffer()
  pack(struct_b_str, struct_b)
  struct_a = StructA(1, pointer_from_objref(struct_b_str.data))
  struct_a_str = IOBuffer()
  pack(struct_a_str, struct_a)

  return_val = ccall((:test, "test.so"), Int32, (Ptr{Void}, ), 
struct_a_str.data)



While this doesn't crash, it seems to be returning garbage values for 
struct_b->val. I also can't make these types immutable since I expect the C 
library I'm using to update them.

Any help would be appreciated. Apologies if this is a stupid question.