Why don’t pointers appear in Python Tutor’s memory visualization in C?

Question

I’m new to C and I’m trying to understand how pointers work. In this piece of code, when I run it on Python Tutor (pythontutor.com) to see how the stack and heap work, I never see the pointer being created in either the stack or the heap. Yet, I am able to printf the values that the pointer points to. I don’t understand why the pointer isn’t visible in the memory visualization.

#include <stdio.h>
#include <stdlib.h>

int main(void) {
    int length = 5;
  
    int *numbers[length];

    for (int i = 0; i < length; i++) {
        numbers[i] = malloc(sizeof(int));
        if (!numbers[i]) {
            perror("malloc");
            exit(1);
        }
        *numbers[i] = i * 10;
    }
     for (int i = 0; i < length; i++) {
        printf("numbers[%d] = %d (address=%p)\n", i, *numbers[i], (void*)numbers[i]); // It works
    }
    
    return 0;
}

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2025-09-26 19:39:33 +00:00
Commented Sep 26 at 19:39 — 0___________
– 0___________, Commented Sep 26 at 19:39

ruohola · Accepted Answer · 2025-09-26 20:15:35Z

8

This is currently a limitation of Python Tutor. From C and C++ known limitations:

doesn’t show some arrays like VLAs

If you change to a fixed-size array it will show the pointers.

#include <stdio.h>
#include <stdlib.h>

#define LENGTH 5

int main(void) {
    int *numbers[LENGTH];

    for (int i = 0; i < LENGTH; i++) {
        numbers[i] = malloc(sizeof(int));
        if (!numbers[i]) {
            perror("malloc");
            exit(1);
        }
        *numbers[i] = i * 10;
    }
     for (int i = 0; i < LENGTH; i++) {
        printf("numbers[%d] = %d (address=%p)\n", i, *numbers[i], (void*)numbers[i]); // It works
    }
    
    return 0;
}

edited Sep 26 at 20:15

ruohola

24.8k7 gold badges76 silver badges118 bronze badges

answered Sep 26 at 19:49

Barmar

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