I want to embed a Julia module inside a C program. The documentation is not very c

I want to embed a Julia module inside a C program. The documentation is not very clear. Does someone know how to import a module? The example uses jl_function_t *func = jl_get_function(jl_base_module, "sqrt"); and I would like to do something like
jl_function_t *myfunc = jl_get_function(jl_mymodule_module, "myfunc");

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I got answers from @mbauman and Xirui Zhao. Got these good references:

small breadcrumb: How to use Julia special functions inside c++ - Stack Overflow

Perhaps you need to call jl_load julia/toplevel.c at 985158ff5b138954c87db273ebfed2ca4321a67c · JuliaLang/julia · GitHub, also see Eval of Julia code · The Julia Language

Although my recommendation is to write your C program as a library and call it from Julia. You can pass Julia function pointers to C functions. Calling C and Fortran Code · The Julia Language.

I modified the example from the post from SGJ that @mbauman referred me to. I want to modify it in order to send back and forth arrays with zero copy. However, I don’t know what is missing. I’m sure that it is something silly.

#include <julia.h>
JULIA_DEFINE_FAST_TLS() // only define this once, in an executable (not in a shared library) if you want fast code.

typedef jl_array_t* (*erf_ptr)(jl_array_t * z);
// erf_ptr erf = NULL;

#include <iostream>
int main(int argc, char *argv[])
{
    jl_init();
    jl_value_t* array_type = jl_apply_array_type((jl_value_t*)jl_float64_type, 1);
    double v[] = { .1, .2, .3, .4 };

    // Expected output
    // julia> [.1, .2, .3, .4] .|> SpecialFunctions.erf
    // 4-element Vector{Float64}:
    // 0.1124629160182849
    // 0.22270258921047847
    // 0.3286267594591274
    // 0.42839235504666845

    jl_array_t *x = jl_ptr_to_array_1d(array_type, v, 4, 0);
    // get a C function pointer to the Julia erf function compiled for
    jl_eval_string("import SpecialFunctions");
    jl_value_t *ret = jl_eval_string("@cfunction(x->SpecialFunctions.erf.(x), Vector{Float64},(Vector{Float64},))");
    erf_ptr erf = (erf_ptr) jl_unbox_voidpointer(ret);

    jl_array_t*  out = erf(x); // Probably doing something wrong here
    double *outData = (double*)jl_array_data(out);
        // Print out the vector
    for(size_t i=0; i<4; i++)
        std::cout << outData[i] << ' ';
    std::cout << "\n";
    // Returns garbage
    // 4.94066e-324 0 0 6.91799e-310 
    jl_atexit_hook(0);
    return 0;
}

I would suggest not wrapping your array in a Julia array from the C side but let the Julia code handle the raw data. Cf Add array example by GunnarFarneback · Pull Request #3 · GunnarFarneback/DynamicallyLoadedEmbedding.jl · GitHub