Embedding julia in C, Segmentation fault when call hcat to concatenate two arrays

General Usage
1

Hi,
I need to call some julia function in C, but meet some problem. For example, I want to use hcat to concatenate two arrays, but segmentation fault occurs. Below is my source code.

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

jl_value_t * checked_eval_string(const char* code)
{
    jl_value_t * result = jl_eval_string(code);
    if (jl_exception_occurred()) {
        // none of these allocate, so a gc-root (JL_GC_PUSH) is not necessary
        jl_call2(jl_get_function(jl_base_module, "showerror"),
                 jl_stderr_obj(),
                 jl_exception_occurred());
        jl_printf(jl_stderr_stream(), "\n");
        jl_atexit_hook(1);
        exit(1);
    }
    assert(result && "Missing return value but no exception occurred!");
    return result;
}

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!");

    double * existing_a = (double *)malloc(sizeof(double) * 4);
    existing_a[0] = 1;
    existing_a[1] = 2;
    existing_a[2] = 3;
    existing_a[3] = 4;
    jl_value_t * array_type = jl_apply_array_type((jl_value_t *)jl_float64_type, 1);
    jl_array_t * a = jl_ptr_to_array_1d(array_type, existing_a, 4, 0);
    jl_call3(setindex, refs, (jl_value_t *)a, (jl_value_t *)a);

    double * existing_b = (double *)malloc(sizeof(double) * 2);
    existing_b[0] = 5;
    existing_b[1] = 6;
    jl_array_t * b = jl_ptr_to_array_1d(array_type, existing_b, 2, 0);
    jl_call3(setindex, refs, (jl_value_t *)b, (jl_value_t *)b);

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

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

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

    jl_atexit_hook(0);
    free(existing_a);
    existing_a = NULL;
    free(existing_b);
    existing_b = NULL;
    return 0;
}

And the output is:

[1.0, 2.0, 3.0, 4.0]
[5.0, 6.0]

signal (11): Segmentation fault
in expression starting at none:0
sig_match_simple at /buildworker/worker/package_linux64/build/src/typemap.c:128 [inlined]
jl_typemap_entry_assoc_exact at /buildworker/worker/package_linux64/build/src/typemap.c:662
jl_typemap_assoc_exact at /buildworker/worker/package_linux64/build/src/julia_internal.h:1074 [inlined]
jl_typemap_level_assoc_exact at /buildworker/worker/package_linux64/build/src/typemap.c:715
jl_typemap_assoc_exact at /buildworker/worker/package_linux64/build/src/julia_internal.h:1078 [inlined]
jl_lookup_generic_ at /buildworker/worker/package_linux64/build/src/gf.c:2342 [inlined]
jl_apply_generic at /buildworker/worker/package_linux64/build/src/gf.c:2394
jl_apply at /buildworker/worker/package_linux64/build/src/julia.h:1690 [inlined]
jl_call3 at /buildworker/worker/package_linux64/build/src/jlapi.c:269
main at /home/fanxp/vscode/julia_test/main2.cpp:50
__libc_start_main at /lib/x86_64-linux-gnu/libc.so.6 (unknown line)
_start at ./build/julia_test (unknown line)
Allocations: 2546 (Pool: 2536; Big: 10); GC: 0
Segmentation fault

The expected output is:

[1.0, 2.0, 3.0, 4.0]
[5.0, 6.0]
[1.0, 2.0, 3.0, 4.0, 5.0, 6.0]

My environment is:

ubuntu 18.04
julia v1.5.3
gcc v7.5.0

The source code I provided uses hcat function in julia to concatenate 1d array a and b. I have already allocated the array and initialized them with some data, so I usejl_ptr_to_array_1d to wrap them. According to the output, array a and b are created successfully, but fail to call hcat. After exhaustive online searching/reading on google, I find nothing. Anyone can help me?
Thanks!

2

Since Julia’s arrays are column-major (1D arrays are column vectors), you probably want to use vcat.

julia> hcat([1, 2, 3, 4], [5, 6])
ERROR: DimensionMismatch("vectors must have same lengths")
Stacktrace:
 [1] hcat(::Vector{Int64}, ::Vector{Int64})
   @ Base .\array.jl:1710
 [2] top-level scope
   @ REPL[45]:1

julia> vcat([1, 2, 3, 4], [5, 6])
6-element Vector{Int64}:
 1
 2
 3
 4
 5
 6
3

Thank you very much. I should check my source code carefully.