How to access the value of a RefValue{Any} Julia variable in C?

General Usage
1

Hi,

I use RefValue{Any} to protect a scalar (immutable type) inside the IdDict, according to Embedding Julia · The Julia Language. But I don’t know how to use the RefValue{Any} variable ('b" in the following example) in C (access its value , add something to its value, etc.). Below is a simple example:

#include <stdlib.h>
#include <julia.h>

int main(void) {
    jl_init();
    // gc protect
    jl_value_t * refs = jl_eval_string("refs = IdDict()");
    jl_function_t * setindex = jl_get_function(jl_base_module, "setindex!");
    jl_datatype_t* ref_type = (jl_datatype_t*)jl_eval_string("Base.RefValue{Any}");

    jl_value_t * a = jl_eval_string("[1.0,2.0,3.0,4.0]");
    jl_call3(setindex, refs, a, a);

    jl_value_t * b = 0;
    jl_value_t * ref_b = 0;
    {
        b = jl_box_float64(0.0001);
        JL_GC_PUSH1(&b);
        ref_b = jl_new_struct(ref_type, b);
        JL_GC_POP();
        jl_call3(setindex, refs, ref_b, ref_b);
    }

    jl_function_t * print = jl_get_function(jl_base_module, "show");
    jl_function_t * vcat = jl_get_function(jl_base_module, "vcat");

    printf("print a:\n");
    jl_call1(print, (jl_value_t *)a);
    printf("\n");
    printf("print ref_b:\n");
    jl_call1(print, ref_b);
    printf("\n");

    jl_value_t * c = jl_call2(vcat, (jl_value_t *)a, (jl_value_t *)ref_b);
    jl_call3(setindex, refs, c, c);
    printf("print vcat(a, ref_b):\n");
    jl_call1(print, c);
    printf("\n");

    jl_atexit_hook(0);
    return 0;
}

And the output is:

print a:
[1.0, 2.0, 3.0, 4.0]
print ref_b:
Base.RefValue{Any}(0.0001)
print vcat(a, ref_b):
Any[1.0, 2.0, 3.0, 4.0, Base.RefValue{Any}(0.0001)]

Basically, I want to protect immutable variable b. So I wrap it in RefValue{Any} as ref_b, and then push it to refs, which is an IdDict. But when I want to call vcat to concatenate a and ref_b, ref_b is Base.RefValue{Any}(0.0001) instead of 0.0001, I have googled extensivly but found nothing. How can I get the value of b (=0.0001) instead of Base.RefValue{Any}(0.0001) through ref_b?
Thanks!

2

You call getindex on it (ref_b[]).

3

Thank you very much and sorry to reply so late.
I complete my code according to your suggestion and everything works well. I attach my code below as an example to help others who have the same problem.

#include <stdlib.h>
#include <julia.h>

int main(void) {
    jl_init();
    // gc protect
    jl_value_t * refs = jl_eval_string("refs = IdDict()");
    jl_function_t * setindex = jl_get_function(jl_base_module, "setindex!");
    jl_function_t * getindex = jl_get_function(jl_base_module, "getindex");
    jl_datatype_t* ref_type = (jl_datatype_t*)jl_eval_string("Base.RefValue{Any}");

    jl_value_t * a = jl_eval_string("[1.0,2.0,3.0,4.0]");
    jl_call3(setindex, refs, a, a);

    jl_value_t * b = 0;
    jl_value_t * ref_b = 0;
    {
        b = jl_box_float64(0.0001);
        JL_GC_PUSH1(&b);
        ref_b = jl_new_struct(ref_type, b);
        JL_GC_POP();
        jl_call3(setindex, refs, ref_b, ref_b);
    }

    jl_function_t * print = jl_get_function(jl_base_module, "show");
    jl_function_t * vcat = jl_get_function(jl_base_module, "vcat");

    printf("print a:\n");
    jl_call1(print, (jl_value_t *)a);
    printf("\n");
    printf("print ref_b:\n");
    jl_call1(print, ref_b);
    printf("\n");
    b = jl_call1(getindex, ref_b);
    JL_GC_PUSH1(&b);
    printf("print getindex(ref_b)\n");
    jl_call1(print, b);
    printf("\n");

    jl_value_t * c = jl_call2(vcat, (jl_value_t *)a, (jl_value_t *)b);
    jl_call3(setindex, refs, c, c);
    printf("print vcat(a, b):\n");
    jl_call1(print, c);
    printf("\n");

    jl_atexit_hook(0);
    return 0;
}

But I have another question. I call getindex on variable ref_b and get the data b(=0.0001). I think the memory storing the data 0.0001 is protected by IdDict, so I don’t need to call JL_GC_PUSH1(&b) again and this line is redundant in my example. Is my idea correct?