case insensitive strstr for TCHARs

[Syslock]

Apparently, the runtime library doesn't have a case insensitive strstr function, so I had to write one myself.

I took the old one and just replaced everything with TCHARs.
Is there anything inheritly wrong with it?

I don't think it matters if I use _totlower or _totupper?

The SDK docs says:
"In order for tolower to give the expected results, __isascii and isupper must both return nonzero."

I tested it with some non-ASCII characters, and tolower returned the same character, so I hope that's ok.

TCHAR *_tcsistr( const TCHAR * str1,
                 const TCHAR * str2 ) {

 TCHAR *cp = (TCHAR *)str1;
 TCHAR *s1, *s2;

 if (!*str2) {
    return (TCHAR *)str1;
 }

 while (*cp) {
    s1 = cp;
    s2 = (TCHAR *)str2;

    // Matching characters will return zero if they are subtracted

    while ( *s1 && *s2 && !(_totlower(*s1) - _totlower(*s2)) ) {
       s1++, s2++;
    }

    if (!*s2) {
       return cp;
    }

    cp++;
 }

 return NULL;
}

Original runtime function:

char * __cdecl strstr (
        const char * str1,
        const char * str2
        )
{
        char *cp = (char *) str1;
        char *s1, *s2;

        if ( !*str2 )
            return((char *)str1);

        while (*cp)
        {
                s1 = cp;
                s2 = (char *) str2;

                while ( *s1 && *s2 && !(*s1-*s2) )
                        s1++, s2++;

                if (!*s2)
                        return(cp);

                cp++;
        }

        return(NULL);

}