Day 4: The Sizeof Trap: Understanding Array Decay in C

If you pass an array to a function in C, you might be in for a rude awakening when you check its size.

This is called Array Decay.

When an array is passed to a function, it "decays" into a simple pointer to its first element. It forgets how long it is.

• Inside main, sizeof(arr) tells you the total bytes of the array.
• Inside a function, sizeof(arr) only tells you the size of the pointer address (usually 8 bytes on 64-bit systems).

The Code

Run this code to see the "Lie" in action.

// Day 4: Arrays vs. Pointers (The Nuance)

#include <stdio.h>

void print_size(int arr[]) {
    // WARNING: 'arr' here has decayed to a pointer!
    // It is no longer an array.
    printf("Size inside function: %zu bytes (Pointer size)\n", sizeof(arr));
}

int main() {
    int arr[] = {1, 2, 3, 4, 5};

    // 1. Size of the Array in local scope
    // Returns 20 (5 ints * 4 bytes each)
    printf("Size in main: %zu bytes\n", sizeof(arr));

    // 2. Pass it to a function
    print_size(arr);

    return 0;
}

📂 View the source code on GitHub:https://github.com/Ujjawal0711/30-Days/