Calling local Julia package from C - c

Question

The Julia documentation shows examples of how to call Base Julia functions from C (e.g. sqrt), which I've been successful in replicating. What I'm really interested in doing is calling locally developed Julia modules and it's not at all clear from the documentation how one would call non-Base functions. There are some discussion threads on the issue from a few years ago, but the APIs appear to have changed in the meantime. Any pointers would be appreciated.

Answer 1

The reason why jl_eval_string("using SomeModule") returns NULL is simply because using SomeModule returns nothing.
You can use functions from other modules by first importing the module, and then retrieving function objects from that Julia module into C. For example, let's use the package GR and its plot function. We can get the plot function with
jl_eval_string("using GR") // this returns nothing
jl_module_t* GR = (jl_module_t *)jl_eval_string("GR") // this returns the module
/* get `plot` function */
jl_function_t *plot = jl_get_function(GR, "plot");
Here we passed GR module as the first argument to jl_get_function. We can, knowing the fact that things will be loaded into the module Main and plot is exported from GR, use the following snippet instead to do the same. Note that jl_main_module holds a pointer to the module Main.
jl_eval_string("using GR")
/* get `plot` function */
jl_function_t *plot = jl_get_function(jl_main_module, "plot");
We can also use plots qualified name.
/* get `plot` function */
jl_function_t *plot = jl_get_function(jl_main_module, "GR.plot");
That said, here is the complete example plotting an array of values using GR. The example uses the first style to get the function GR.plot.
#include <julia.h>
JULIA_DEFINE_FAST_TLS() // only define this once, in an executable (not in a shared library) if you want fast code.
#include <stdio.h>
int main(int argc, char *argv[])
{
/* required: setup the Julia context */
jl_init();
/* create a 1D array of length 100 */
double length = 100;
double *existingArray = (double*)malloc(sizeof(double)*length);
/* create a *thin wrapper* around our C array */
jl_value_t* array_type = jl_apply_array_type((jl_value_t*)jl_float64_type, 1);
jl_array_t *x = jl_ptr_to_array_1d(array_type, existingArray, length, 0);
/* fill in values */
double *xData = (double*)jl_array_data(x);
for (int i = 0; i < length; i++)
xData[i] = i * i;
/* import `Plots` into `Main` module with `using`*/
jl_eval_string("using GR");
jl_module_t* GR = (jl_module_t *)jl_eval_string("GR");;
/* get `plot` function */
jl_function_t *plot = jl_get_function(GR, "plot");
/* create the plot */
jl_value_t* p = jl_call1(plot, (jl_value_t*)x);
/* display the plot */
jl_function_t *disp = jl_get_function(jl_base_module, "display");
jl_call1(disp, p);
getchar();
/* exit */
jl_atexit_hook(0);
return 0;
}
Including a Julia module from a local file and use it in C
I do not know what is exactly meant by a local Julia package, but, you can include your files and then import the modules in those files to do the same. Here is an example module.
# Hello.jl
module Hello
export foo!
foo!(x) = (x .*= 2) # multiply entries of x by 2 inplace
end
To include this file you need to use jl_eval_string("Base.include(Main, \"Hello.jl\")");. For some reason, embedded Julia cannot access include directly. You need to use Base.include(Main, "/path/to/file") instead.
jl_eval_string("Base.include(Main, \"Hello.jl\")");
jl_eval_string("using Main.Hello"); // or just '.Hello'
jl_module_t* Hello = (jl_module_t *)jl_eval_string("Main.Hello"); // or just .Hello
Here is the complete example in C.
#include <julia.h>
JULIA_DEFINE_FAST_TLS() // only define this once, in an executable (not in a shared library) if you want fast code.
#include <stdio.h>
int main(int argc, char *argv[])
{
/* required: setup the Julia context */
jl_init();
/* create a 1D array of length 100 */
double length = 100;
double *existingArray = (double*)malloc(sizeof(double)*length);
/* create a *thin wrapper* around our C array */
jl_value_t* array_type = jl_apply_array_type((jl_value_t*)jl_float64_type, 1);
jl_array_t *x = jl_ptr_to_array_1d(array_type, existingArray, length, 0);
JL_GC_PUSH1(&x);
/* fill in values */
double *xData = (double*)jl_array_data(x);
for (int i = 0; i < length; i++)
xData[i] = i * i;
/* import `Hello` module from file Hello.jl */
jl_eval_string("Base.include(Main, \"Hello.jl\")");
jl_eval_string("using Main.Hello");
jl_module_t* Hello = (jl_module_t *)jl_eval_string("Main.Hello");
/* get `foo!` function */
jl_function_t *foo = jl_get_function(Hello, "foo!");
/* call the function */
jl_call1(foo, (jl_value_t*)x);
/* print new values of x */
for (int i = 0; i < length; i++)
printf("%.1f ", xData[i]);
printf("\n");
JL_GC_POP();
getchar();
/* exit */
jl_atexit_hook(0);
return 0;
}