Why does casting a char array to an int pointer and writing to it using the pointer make the data reversed?

Question

I created some simple code to test casting a char array to int pointer. This works fine as I expected, but when I wrote to the array using the pointer, the data got swapped MSB<-->LSB when I print the c array back out. Why does this happen? Is this an OS dependent thing?

#include "stdio.h"

const int SIZE = 12;

int _tmain(int argc, _TCHAR * argv[]) {
    unsigned char c[SIZE] = {
        1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12
    };
    unsigned int * ptr = (unsigned int * ) c;
    int i;

    printf("Int size=%d\n", sizeof(unsigned long));

    for (i = 0; i < sizeof(c); i++) {
        printf("%X, ", c[i]);
    }
    printf("\n\n");

    for (i = 0; i < sizeof(c) / sizeof(unsigned long); i++) { * ptr++ = i;
    }

    for (i = 0; i < sizeof(c); i++) {
        printf("%X, ", c[i]);
    }

    printf("\n");
    return 0;
}

Here is the output:

Int size=4
1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C,
0, 0, 0, 0, 1, 0, 0, 0, 2, 0, 0, 0,

Answer 1

Your architecture is Little Endian, which means the least significant byte is stored first in memory:

In your case 0x00000001 is written in the order [0x01, 0x00, 0x00, 0x00].

Answer 2

This is caused by endianness of your CPU architecture. Your architecture seems to be little endian to cause this kind of inversions.