atol实现

ong __cdecl atol(
  const char *nptr
  )
{
  int c; /* current char */
  long total; /* current total */
  int sign; /* if '-', then negative, otherwise positive */

  /* skip whitespace */
  while ( isspace((int)(unsigned char)*nptr) )
  ++nptr;

  c = (int)(unsigned char)*nptr++; sign = c; /* save sign indication */
  if (c == '-' || c == '+')
  c = (int)(unsigned char)*nptr++; /* skip sign */

  total = 0;

  while (isdigit(c)) {
  total = 10 * total + (c - '0'); /* accumulate digit */
  c = (int)(unsigned char)*nptr++; /* get next char */
  }

  if (sign == '-')
  return -total;
  else
  return total; /* return result, negated if necessary */
}