A simple implementation of string (char *) to int conversion in ISO C.

strtoi.c

/* Naive implementation of string (char *) to int conversion in ISO C. */


int
strtoi(const char *str, unsigned int len)
{
    int i, neg, num;
    unsigned int val, pow;

    if (str == 0 || len == 0)
        return 0;

    if (*str == '-')
    {
        ++str;
        --len;
        neg = 1;
    } else
        neg = 0;

    for (i = len - 1, val = 0, pow = 1; i >= 0; --i)
    {
        num = str[i] - '0';

        if (num < 0 || num > 9)
            break;

        val += num * pow;
        pow *= 10;
    }

    return neg ? -val : val;
}