convert.c

#include <stdio.h>
#include <limits.h>

int main(void) {
    int a;
    unsigned int b;

    a = -10;
    printf("Before: %d\n", a);

    b = a; /* If a is negative, b will be converted adding UINT_MAX + 1. */
    if (b > INT_MAX) {
        /* b > INT_MAX only if the above conversion happened. We need to
         * subtract UNIT_MAX + 1.
         *
         * UINT_MAX + 1 = (INT_MAX+1)*2 so we need to subtract
         * INT_MAX + 1 two times.
         *
         * First: subtract INT_MAX+1 from the unsigned integer. */
        b = b - INT_MAX - 1;

        /* Convert to signed, we should now always be within range. */
        a = b;

        /* Finally subtract it again to reconstruct the original value. */
        a = a - INT_MAX - 1;
    } else {
        a = b;
    }
    printf("After: %d\n", a);
}