C loader: Cannot pass array (int[]) from Python to C functions #570
Description
🐛 Bug Report
When calling a C function that takes an array (e.g., int pixels[]) from Python via MetaCall, the arguments fail with a type mismatch error.
Minimal example:
Python code
from PIL import Image
import numpy as np
from metacall import metacall, metacall_load_from_file
# Load C file
metacall_load_from_file('c', ['main.c'])
def process_image(image_path, output_path):
img = Image.open(image_path).convert("RGB")
pixels = np.array(img, dtype=np.uint8)
flat_pixels = pixels.flatten().tolist()
# Call C functions
brightness = metacall("calculate_brightness", flat_pixels, len(flat_pixels))
metacall("apply_blur_filter", flat_pixels, img.width, img.height)
print("Brightness:", brightness)
process_image("input.jpeg", "output.jpeg")
C code
#include <stdio.h>
void apply_blur_filter(int pixels[], int width, int height) {
int size = width * height * 3;
for (int i = 0; i < size; i++) {
pixels[i] = pixels[i] / 2;
}
printf("C: Blur filter applied on %d pixels\n", size);
}
double calculate_brightness(int pixels[], int size) {
long sum = 0;
for (int i = 0; i < size; i++) {
sum += pixels[i];
}
double avg = (double)sum / size;
printf("C: Average brightness = %f\n", avg);
return avg;
}
Error Output:
[Sat Aug 16 15:10:06] #452654 @Error : Type mismatch in when calling calculate_brightness in argument number 0 (expected (null) of type Invalid and received Array). Canceling call in order to avoid a segfault.
[Sat Aug 16 15:10:06] #452654 @Error : Type mismatch in when calling apply_blur_filter in argument number 0 (expected (null) of type Invalid and received Array). Canceling call in order to avoid a segfault.
Brightness: None
Expected Behavior
MetaCall should pass the Python list/NumPy array into the C function as an int[] (inout pointer), allowing modification and correct return values.
This seems related to missing inout pointer support in the C loader. I found that PR #568
addresses exactly this issue.