/* assume C, therefore character literals are ints. */
unsigned asDigit(int ch)
{
if (ch >= '0' && ch <= '0' + 10) return ch - '0';
return -1u;
}
int parseAsUInt(char const *input, unsigned upperBound, unsigned *presult)
{
unsigned result = 0;
unsigned const threshold10 = upperBound / 10, threshold1 = upperBound % 10;
unsigned current;
if (!input) return 0;
for (; (int)*input; ++input)
{
if ((current = asDigit((int)*input)) == -1u) continue;
if (result > threshold10) return 0;
if (result == threshold10 && current > threshold1) return 0;
result = result * 10u + current;
}
if (presult)
*presult = result;
return 1;
}

脑补的