the proper way to declare C void pointers in Julia

Question

Ok, I originally badly screwed up my formulation of this question (it's more than a year now since I seriously wrote C++ code and I have pretty limited experience with pure C), so let's try again.

Some C code is written to expect you to do something like the following

void* p;
create_new_thing(&p);  //p is now a new thing
do_stuff_to_thing(p);  //something happened to p

My question is how to create the object p in Julia. Right now I believe the answer to be

p = Ref{Ptr{Void}}()
ccall((:create_new_thing, :lib), Void, (Ptr{Ptr{Void}},), p)
ccall((:do_stuff_to_thing, :lib), Void, (Ptr{Void},), p)

Furthermore, I believe the same code but with p declared instead as p = Array(Ptr{Void}, 1) also works.

I do however find the whole distinction between Ref and Ptr in Julia very confusing, mostly because they seem to get converted between each other in ways I cannot keep track of.

Answer 1

Your code looks almost fine. But be careful! Any small error, like the one you have here, can cause a segmentation fault:

p = Ref{Ptr{Void}}()
ccall((:create_new_thing, :lib), Void, (Ptr{Ptr{Void}},), p)
ccall((:do_stuff_to_thing, :lib), Void, (Ptr{Void},), p)
                                        # error here  ^

The correct way to do it is

p = Ref{Ptr{Void}}()
ccall((:create_new_thing, :lib), Void, (Ptr{Ptr{Void}},), p)
ccall((:do_stuff_to_thing, :lib), Void, (Ptr{Void},), p[])
                                             # fixed  ^

The easiest way to understand where to use p and p[] is to think of the corresponding C code. In C, we write

void *p;
create_new_thing(&p)
do_stuff_to_thing(p)

Julia objects do not have first-class memory addresses like C objects do, so we must use p = Ref{Ptr{Void}}() in Julia to get a memory address. This object, as a ref, behaves like &p in C. That means to get the object itself, p in C, we need to use p[] in Julia.

So the equivalent in Julia is

p = Ref{Ptr{Void}}()                 # this p corresponds to &p in C
ccall(:create_new_thing, ..., p)     # like &p
ccall(:do_stuff_to_thing, ..., p[])  # like *(&p); that is, like p