isalnum

#include <stdio.h>
#include <ctype.h>
#include <locale.h>

int main(void)
{
    unsigned char c = '\xdf'; //     ISO-8859-1

    printf("isalnum('\\xdf') in default C locale returned %d\n", !!isalnum(c));

    if(setlocale(LC_CTYPE, "de_DE.iso88591"))
        printf("isalnum('\\xdf') in ISO-8859-1 locale returned %d\n", !!isalnum(c));
}