Hex to Int (Kernighan & Ritchie 2-3)

htoi.c

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

int htio(char s[])
{
    int i,n;
    n=0;
    for (i=0; isxdigit(s[i]); ++i)
    {
        n *= 16;
        if(isdigit(s[i]))
            n += s[i] - '0';
        else
            n += toupper(s[i]) - 'A' + 0xA;
    }
    return n;
};

int main(void) {
    char *s;
    s = "12ab";
    printf("%d", htio(s));
};