I have encountered some weird behavior of programs or programming languages over the past couple of years and want to collect them here.

C

#include <stdio.h>

void pass_by_value(int a)
{ }

void pass_by_magic(void)
{
        int magic;
        printf("%d\n", magic);
}

int main(void)
{
        int number = 69;
        pass_by_value(number);
        pass_by_magic();
        return 0;
}

This code produces 69 on my machine when compiled with gcc 11.1.0 without any optimizations. Why? The first argument is passed in register rdi, but int a in line 3 is allocated on the stack so the top of the stack will be set to 69. int magic in line 9 is not initialized, but refers to the top of the stack which still contains 69 as we didn’t overwrite it with anything.

#include <stdio.h>

int main(void)
{
        int test[100];
        for (size_t i = 0; i < 100; i++) {
                test[i] = i;
        }

        for (size_t i = 0; i < 100; i++) {
                printf("%d ", i[test]);
        }

        printf("\n");

        return 0;
}

This program prints all numbers from 0 to 99. Doesn’t seem unusual, does it? Well, take a closer look at line 11. We would expect test[i], but not i[test]! Why does it work? Array indexing in C is syntactic sugar for adding an offset to the pointer and deferencing the resulting address. So test[i] is equivalent to *(test + i) which in turn is equivalent to *(i + test) because of commutativity of the addition, finally giving us i[test].