overflow-underflow-casting

overunder.c

#include <stdio.h>

int main() {
    // Down Casting an unsigned long to an unsigned char
    unsigned long a = 257;
    printf("%lu == %d \n", a, (unsigned char)a);
    // Output: 257 == 1
    // This is obviously not true, but there are not enough bits in a
    // char (8 bits) to hold 257, so it will carry over the extra into the value

    // Upcasting a signed negative char into an unsigned long value
    char b = -1; 
    printf("%d == %lu \n", b, (unsigned long)b);
    // Output: -1 == 18446744073709551615
    // This is obviously not true, but since computers represent signed negative
    // numbers as ones compliment, the -1 signed char will be represented like
    // in binary like this: 11111110
    // During this operation, there is an underflow.

    return 0;
}