What is the flaw in this string comparison logic?

Question

The following

#include <iostream>

unsigned short int stringCompare ( char * s1, char * s2 )
{
// returns 1 if the character arrays s1 and s2 are equal; 
// returns 0 otherwise
    while (*s1 && (*s1++ == *s2++));
    return (!(*s1) && !(*s2));
}

int main () 
{
    char str1 [] = "americano";
    char str2 [] = "americana";
    std::cout << stringCompare(str1,str2);
    return 0;
}

prints 1, meaning my function's logic is incorrect. I'd like to understand why. Let me explain my logic:

while (*s1 && (*s1++ == *s2++))

simultaneously increments pointers s1 and s2 so long as s1 is not equal to '\0' and the value s1 points to is the same as the value s2 points to. It is supposed to be a shorter way of writing

while (*s1 && *s2)
{
   if (*s1 != *s2) break;
   ++s1; ++s2;
}

and relies on fancy operator precedence to shorten it.

The statement

return (!(*s1) && !(*s2))

means

"If s1 and s2 are both the null character, return true; otherwise return false"

because if the strings are equal then s1 and s2 will both be the null character after the while loop.

Where am I wrong?

Answer 1

The flaw is that the ++ is done in the loop even on the last character, when they don't match. If the following character matches (which it does in this case, the null terminator) then they compare as true.