# Embedding Julia: GC race conditions

**URL:** <https://discourse.julialang.org/t/embedding-julia-gc-race-conditions/108346>\
**Category:** General Usage\
**Tags:** embedding\
**Created:** [January 4, 2024, 2:29pm UTC](https://discourse.julialang.org/t/embedding-julia-gc-race-conditions/108346 "2024-01-04T14:29:27Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![sijo](https://avatars.discourse-cdn.com/v4/letter/s/da6949/32.png) [@sijo](https://discourse.julialang.org/u/sijo)\
**Post date:** [January 4, 2024, 2:29pm UTC](https://discourse.julialang.org/t/embedding-julia-gc-race-conditions/108346/1 "2024-01-04T14:29:27Z")

</div>

I’m trying to understand how GC interaction from C works in case of multiple threads. How are race conditions avoided?

The manual [says](https://docs.julialang.org/en/v1/manual/embedding/):

> When writing code that embeds Julia, it is generally safe to use `jl_value_t*` values in between `jl_...` calls (as GC will only get triggered by those calls). But in order to make sure that values can survive `jl_...` calls, we have to tell Julia that we still hold a reference to Julia [root](https://www.cs.purdue.edu/homes/hosking/690M/p611-fenichel.pdf) values, a process called “GC rooting”. Rooting a value will ensure that the garbage collector does not accidentally identify this value as unused and free the memory backing that value. This can be done using the `JL_GC_PUSH` macros:
> 
> ```julia
> jl_value_t *ret = jl_eval_string("sqrt(2.0)");
> JL_GC_PUSH1(&ret);
> // Do something with ret
> JL_GC_POP();
> 
> ```

Is this not a race condition? If a `jl_...` call is made in another thread, just after the `jl_eval_string` call here, can’t we have the GC running before the `JL_GC_PUSH1` is in effect?

(Note that a `jl_...` call occuring in another thread is indeed possible: further down this page of the manual it shows how this can happen from threads started by Julia itself.)