# Calling JL\_GC\_PUSH1 multiple times

**URL:** <https://discourse.julialang.org/t/calling-jl-gc-push1-multiple-times/18666>\
**Category:** General Usage\
**Created:** [December 14, 2018, 4:49pm UTC](https://discourse.julialang.org/t/calling-jl-gc-push1-multiple-times/18666 "2018-12-14T16:49:59Z")\
**Posts on this page:** 7\
**Page:** 1

<div class="post-metadata">

**Author:** ![Ronis\_BR](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ronis_br/32/50999_2.png) [@Ronis\_BR](https://discourse.julialang.org/u/Ronis_BR)\
**Post date:** [December 14, 2018, 4:49pm UTC](https://discourse.julialang.org/t/calling-jl-gc-push1-multiple-times/18666/1 "2018-12-14T16:49:59Z")

</div>

Hi guys,

I am creating a C++ software that calls multiple functions of my Julia package. Something like:

```nohighlight
    jl_init();

    jl_eval_string("using SatelliteToolbox");
    jl_module_t* SatelliteToolbox =
        (jl_module_t*)jl_eval_string("SatelliteToolbox");

    jl_function_t *read_tle = jl_get_function(SatelliteToolbox, "read_tle");

    jl_value_t *filename = jl_eval_string("\"amz1.tle\"");
    jl_array_t *array_tle = (jl_array_t*)jl_call1(read_tle, filename);

    jl_function_t* getindex = jl_get_function(jl_main_module, "getindex");
    jl_value_t* tle =
        (jl_value_t*)jl_call2(getindex, (jl_value_t*)array_tle, jl_box_int64(1));

    jl_function_t* init_orbit_propagator =
        jl_get_function(SatelliteToolbox, "init_orbit_propagator");

    jl_value_t* tipo_prop = jl_eval_string("Val{:J2}");
    jl_value_t* orb_prop = jl_call2(init_orbit_propagator, tipo_prop, tle);

    jl_function_t* propagate = jl_get_function(SatelliteToolbox, "propagate!");

    jl_value_t* t = jl_box_float64(0.0);
    jl_value_t* ret = jl_call2(propagate, orb_prop, t);
    jl_value_t* orb = jl_fieldref(ret,0);

```

By reading the documentation, I saw that I need to use `JL_GC_PUSH1` to avoid, for example, `array_tle` to be freed by the GC. However, when I call `JL_GC_PUSH1` twice in this function, I get the following error:

```julia
main.cpp:43:5: error: redefinition of '__gc_stkf'
    JL_GC_PUSH1(&tle);
    ^
/Applications/Julia-1.0.app/Contents/Resources/julia/include/julia/julia.h:667:9: note: expanded from macro 'JL_GC_PUSH1'
  void *__gc_stkf[] = {(void*)3, jl_pgcstack, arg1}; \
        ^
main.cpp:35:5: note: previous definition is here
    JL_GC_PUSH1(&array_tle);
    ^
/Applications/Julia-1.0.app/Contents/Resources/julia/include/julia/julia.h:667:9: note: expanded from macro 'JL_GC_PUSH1'
  void *__gc_stkf[] = {(void*)3, jl_pgcstack, arg1}; \
        ^
1 error generated.

```

So, can anyone tell me what is the best practice here?

---

<div class="post-metadata">

**Author:** ![Ronis\_BR](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ronis_br/32/50999_2.png) [@Ronis\_BR](https://discourse.julialang.org/u/Ronis_BR)\
**Post date:** [December 14, 2018, 5:12pm UTC](https://discourse.julialang.org/t/calling-jl-gc-push1-multiple-times/18666/2 "2018-12-14T17:12:22Z")

</div>

Another problem is that I would like that the variable `orb_prop`, which will reside in a class, to never be GCed, since it will be used many times. How can I do that?

---

<div class="post-metadata">

**Author:** ![Ronis\_BR](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ronis_br/32/50999_2.png) [@Ronis\_BR](https://discourse.julialang.org/u/Ronis_BR)\
**Post date:** [December 14, 2018, 5:48pm UTC](https://discourse.julialang.org/t/calling-jl-gc-push1-multiple-times/18666/3 "2018-12-14T17:48:44Z")

</div>

Well, I am going to describe what I did here, because I am finding very difficult to obtain clear information about the process of embedding Julia into C++ (when I get better, I will submit a PR to improve the documentation).

(Please, correct me if I am wrong!)

In fact, Julia will not GCed a variable if it is referenced somewhere. Since I want a variable inside a C++ class to not be freed, I need to make it global or add it reference to a global object. Since the second option seems better, I create a global `IdDict` at the initialization:

```nohighlight
    jl_value_t* refs = jl_eval_string("refs = IdDict()");

```

Then, when my variable inside the class is created, I add the reference to this `IdDict` by:

```nohighlight
    jl_value_t* refs = jl_eval_string("refs");
    jl_function_t* setindex = jl_get_function(jl_main_module, "setindex!");

    ...

    tle = (jl_value_t*)jl_call2(getindex, (jl_value_t*)array_tle, jl_box_int64(1));

    jl_call3(setindex, refs, tle, tle);

```

In this case, since `tle` is referenced in the global variable `refs`, it seems that the GC will not delete it.

Seems right?

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<div class="post-metadata">

**Author:** ![danielmatz](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/danielmatz/32/2285_2.png) [@danielmatz](https://discourse.julialang.org/u/danielmatz)\
**Post date:** [December 14, 2018, 6:16pm UTC](https://discourse.julialang.org/t/calling-jl-gc-push1-multiple-times/18666/4 "2018-12-14T18:16:57Z")

</div>

The `JL_GC_PUSH1` macro (and the other variations) use the stack. In the past, I’ve just introduced a new scoping block for every push/pop pair.

```nohighlight
jl_value_t *a = jl_eval_string("foo()");
JL_GC_PUSH1(&a);
{
    jl_value_t *b = jl_eval_string("bar()");
    JL_GC_PUSH1(&b);
    // do something with a and b
    JL_GC_POP();
}
JL_GC_POP();

```

As for keeping a value alive beyond a given scope, I think you are on the right track. I’ve only ever had to do it for instances of a `mutable struct`, so it sufficed to just generate a new global symbol and bind it to my value. If you are dealing with immutables, I think you need to do something more involved, like wrap it in a `Ref`.

---

<div class="post-metadata">

**Author:** ![Ronis\_BR](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ronis_br/32/50999_2.png) [@Ronis\_BR](https://discourse.julialang.org/u/Ronis_BR)\
**Post date:** [December 15, 2018, 11:37am UTC](https://discourse.julialang.org/t/calling-jl-gc-push1-multiple-times/18666/5 "2018-12-15T11:37:33Z")

</div>

If this approach (to add the variable ref to a global container), can I submit a PR with a doc update? I am not sure if this is described in the section [High-Level Embedding](https://docs.julialang.org/en/v1/manual/embedding/#High-Level-Embedding-1) of the manual.

---

<div class="post-metadata">

**Author:** ![vitreo12](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/vitreo12/32/4028_2.png) [@vitreo12](https://discourse.julialang.org/u/vitreo12)\
**Post date:** [December 15, 2018, 1:04pm UTC](https://discourse.julialang.org/t/calling-jl-gc-push1-multiple-times/18666/6 "2018-12-15T13:04:05Z")

</div>

I had a similar problem and I went for the same approach, which appears to be working just fine. I think you are on the right track with this.

---

<div class="post-metadata">

**Author:** ![Ronis\_BR](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ronis_br/32/50999_2.png) [@Ronis\_BR](https://discourse.julialang.org/u/Ronis_BR)\
**Post date:** [December 16, 2018, 11:35am UTC](https://discourse.julialang.org/t/calling-jl-gc-push1-multiple-times/18666/7 "2018-12-16T11:35:59Z")

</div>

I created a PR to make this clear:

[https://github.com/JuliaLang/julia/pull/30399](https://github.com/JuliaLang/julia/pull/30399)

Any suggestions are welcome!