strtol with a lenght limit.

nstrtol

long nstrtol(const char *string, size_t len)
{
	long sig = 1;
	long value = 0;
	size_t i = 0;

	if(string[0] == '-')
		sign *= -1, i++;
   
	for(; len > 0 && isdigit(string[i]); i++, len--)
 	{
  		value *= 10;
		value += *string - '0';
		if(string[i] == 0) break;
  	}
  	return sign * value;
}

Refined from a rough outline on stack overflow. This function reads a string until len bytes or null is reached. This is helpful if dealing with untrusted content such as environment variables. Which could be literally anything like one fallowed by a million zeros. Or something more sinister.