bicycle1885 - 10 months ago 49

C Question

My question is simple but I don't know the best way to do that (or Julia doesn't offer such a way at the moment): how can I set a field value of a C struct from Julia?

Imagine you have a struct type to represent a node of a tree in a C library:

`typedef struct node_s`

{

int type;

node_t* next;

node_t* children;

node_t* parent;

} node_t;

and copy it in Julia:

`immutable node_t`

typ::Cint

next::Ptr{node_t}

children::Ptr{node_t}

parent::Ptr{node_t}

end

Now assume that you have a pointer to

`node_t`

`parent`

`unsafe_store!`

`parent`

`sizeof(Int) + sizeof(Ptr{node_t}) * 2`

Answer Source

The `fieldoffset`

function is provided for this use case:

```
julia> immutable node_t
typ::Cint
next::Ptr{node_t}
children::Ptr{node_t}
parent::Ptr{node_t}
end
julia> fieldoffset(node_t, 1)
0x0000000000000000
julia> fieldoffset(node_t, 2)
0x0000000000000008
julia> fieldoffset(node_t, 3)
0x0000000000000010
julia> fieldoffset(node_t, 4)
0x0000000000000018
```

But also don't be worried about simply storing the whole `immutable`

, with one field changed; it will be optimized away.

```
julia> k = Ptr{node_t}(Libc.malloc(sizeof(node_t)))
Ptr{node_t} @0x00000000036194c0
julia> unsafe_load(k)
node_t(29544064,Ptr{node_t} @0x3038662d34363a34,Ptr{node_t} @0x3a386e2d3832313a,Ptr{node_t} @0x34363a32333a3631)
julia> update_parent(x::node_t, n::Ptr{node_t}) =
node_t(x.typ, x.next, x.children, n)
update_parent (generic function with 1 method)
julia> mutate!(k) = unsafe_store!(k, update_parent(unsafe_load(k), Ptr{node_t}(0)))
mutate! (generic function with 1 method)
julia> @code_llvm mutate!(k)
define %node_t* @"julia_mutate!_70963"(%node_t*) #0 {
top:
%1 = load %node_t, %node_t* %0, align 1
%.fca.3.insert = insertvalue %node_t %1, %node_t* null, 3
store %node_t %.fca.3.insert, %node_t* %0, align 1
ret %node_t* %0
}
```