minilibc 中 time 相关函数的疑问

发布于 2011-01-09 20:48:05 浏览：9394 订阅该版

前几天发现在 finsh 上使用 `set_date()` 和 `list_date()` 总是得不到正确的输出。
今天使用 mini2440 模拟器测试了一下。

过程是，为了方便测试，俺先修改了 `_libcpu/arm/s3c24x0/rtc.c`_ 
```c
static rt_uint32_t rtc_time = 0;
static rt_err_t rtc_control(rt_device_t dev, rt_uint8_t cmd, void *args)
{
	struct tm tm, *tm_ptr;
    time_t *time;
	RT_ASSERT(dev != RT_NULL);

time = (time_t *)args;
	switch (cmd)
	{
	case RT_DEVICE_CTRL_RTC_GET_TIME:
		/* read device */
		//rt_hw_rtc_get(&tm);
		//*((rt_time_t *)args) = mktime(&tm);
		*(rt_uint32_t *)args = rtc_time;
		rt_kprintf("get_time: %d
", rtc_time);
		break;

case RT_DEVICE_CTRL_RTC_SET_TIME:
		//tm_ptr = localtime(time);
		/* write device */
		//rt_hw_rtc_set(tm_ptr);
		rtc_time = *(rt_uint32_t *)args;
		rt_kprintf("set_time: %d
", rtc_time);
		break;
	}

return RT_EOK;
}
```
rtc_control() 成了伪函数，get_time 会得到 0 。

```c
void list_date()
{
	time_t time, time1;
	rt_device_t device;

device = rt_device_find("rtc");
	if (device != RT_NULL)
	{
		rt_device_control(device, RT_DEVICE_CTRL_RTC_GET_TIME, &time);

rt_kprintf("%d, %s
", time, ctime(&time));
	}

time1 = 0;
    rt_kprintf("time1 = %d -> %s
", time1, ctime(&time1));

time1 = 1;
    rt_kprintf("time1 = %d -> %s
", time1, ctime(&time1));

time1 = 60;
    rt_kprintf("time1 = %d -> %s
", time1, ctime(&time1));

time1 = 60*60*24;
    rt_kprintf("time1 = %d -> %s
", time1, ctime(&time1));	
}
```
在 list_date() 中加了些 test code 。

在 finsh 中敲 list_date() 便会输出测试结果。
[attach]0[/attach]

我研究了一下 time.c ，觉得 ctime() 的结果似乎是随机的。以下是 `_components/libc/minilibc/time.c_` 中的相关代码。
```c
char *asctime(const struct tm *timeptr)
{
	static char buf[25];
	return asctime_r(timeptr, buf);
}

char *ctime(const time_t *timep)
{
	return asctime(localtime(timep));
}
```
```c
struct tm* localtime_r(const time_t* t, struct tm* r)
{
	time_t tmp;
	struct timezone tz;
	gettimeofday(0, &tz);
	timezone = tz.tz_minuteswest * 60L;
	tmp = *t + timezone;
	return gmtime_r(&tmp, r);
}

struct tm* localtime(const time_t* t)
{
	static struct tm tmp;
	return localtime_r(t, &tmp);
}
```
```c
int gettimeofday(struct timeval *tp, void *ignore)
{
	time_t time;
	rt_device_t device;

device = rt_device_find("rtc");
	if (device != RT_NULL)
	{
		rt_device_control(device, RT_DEVICE_CTRL_RTC_GET_TIME, &time);
		if (tp != RT_NULL)
		{
			tp->tv_sec = time;
			tp->tv_usec = 0;
		}

return time;
	}

return 0;
}
```
```c
int __isleap(int year)
{
	/* every fourth year is a leap year except for century years that are
	 * not divisible by 400. */
	/*  return (year % 4 == 0 && (year % 100 != 0 || year % 400 == 0)); */
	return (!(year % 4) && ((year % 100) || !(year % 400)));
}

struct tm *gmtime_r(const time_t *timep, struct tm *r)
{
	time_t i;
	register time_t work = *timep % (SPD);
	r->tm_sec = work % 60;
	work /= 60;
	r->tm_min = work % 60;
	r->tm_hour = work / 60;
	work = *timep / (SPD);
	r->tm_wday = (4 + work) % 7;
	for (i = 1970;; ++i)
	{
		register time_t k = __isleap(i) ? 366 : 365;
		if (work >= k)
			work -= k;
		else
			break;
	}
	r->tm_year = i - 1900;
	r->tm_yday = work;

r->tm_mday = 1;
	if (__isleap(i) && (work > 58))
	{
		if (work == 59)
			r->tm_mday = 2; /* 29.2. */
		work -= 1;
	}

for (i = 11; i && (__spm* > work); --i)
		;
	r->tm_mon = i;
	r->tm_mday += work - __spm*;
	return r;
}
```
ctime() 中间有掉用到 
`gettimeofday(0, &tz) ：`
`**ctime() -> localtime() -> localtime_r() -> gettimeofday() + gmtime_r()**`
调用时，第一个参数是空指针，第二个参数被 gettimeofday() 内部忽略。

那么，疑问一：在调用 localtime_r()时，tz，timezone 和 tmp 岂不是成了未定义的值？若如此，localtime_r() 最后调用 gmtime_r() 并返回的结果依然是未定义的。

疑问二：
`localtime()` 和 asctime() 最终返回的是自己局部变量的指针。那么，这些指针所指的数据岂不是有被覆盖的危险？ 
<br> 
![time_test.png](https://oss-club.rt-thread.org/uploads/2183_0b6fd23ed37e0fca70395e930f72d7ad.png)
 
<br> 
![time.png](https://oss-club.rt-thread.org/uploads/2183_0411d978d72d495e84838a2b2e8be4c4.png)

10 个回答

onelife 2011-06-17

这家伙很懒，什么也没写！

俺在 minilibc 的 timegm() 函数中发现一个 bug ：
```c
	/* After 2100 we have to substract 3 leap years for every 400 years
	 This is not intuitive. Most mktime implementations do not support
	 dates after 2059, anyway, so we might leave this out for it's
	 bloat. */
	if ((years -= 131) >= 0)
	{
		years /= 100;
		day -= (years >> 2) * 3 + 1;
		if ((years &= 3) == 3)
			years--;
		day -= years;
	}
```
可改为：
```c
	/* After 2100 we have to substract 3 leap years for every 400 years
	 This is not intuitive. Most mktime implementations do not support
	 dates after 2059, anyway, so we might leave this out for it's
	 bloat. */
	if (years >= 131)
	{
		years -= 131;
		years /= 100;
		day -= (years >> 2) * 3 + 1;
		if ((years &= 3) == 3)
			years--;
		day -= years;
	}
```

此 bug 导致时间计算错误。另外，在32位的系统中，一个32位的计数器只能从1970年数到2038年。请各位留意。

若干掉这个 bug ，再对 “struct timezone tz;” 进行一下初始化（共两处，如下），则在 FINSH 中敲入
- “set_time()”
- “set_date()” 
- “list_date()”
可以得到正确的结果。
此方法使用 EFM32 分支在硬件上测试通过。

```c
struct tm* localtime_r(const time_t* t, struct tm* r)
{
	time_t tmp;
	struct timezone tz = {0};
	gettimeofday(0, &tz);
	timezone = tz.tz_minuteswest * 60L;
	tmp = *t + timezone;
	return gmtime_r(&tmp, r);
}

time_t mktime(register struct tm* const t)
{
	time_t x = timegm(t);
	struct timezone tz = {0};
	gettimeofday(0, &tz);
	timezone = tz.tz_minuteswest * 60L;
	x += timezone;
	return x;
}
```