[SOLVED] bug on localtime() function

Lobinho · 01-03-2011, 01:06 PM

Hi all,

Happy new year, happy new bugs.

I'm developing a log program with c++ on an embedded platform (ARM9). The program receive some status and write everything on a log (.txt) file.
Each line must contain the current date time and the status info, and there will be 4 or more lines per second to be 'logged'.

My problem starts when I try to get current date time. Sometimes the localtime() function doesn't work properly (1% or less of all tries):

Code:

 ...
1294044158 - | 03/01/2011 16:42:38:000 | 03/01/2011 16:42:38:000
1294044159 - | 03/01/2011 16:42:39:000 | 03/01/2011 16:42:39:000
1294044159 - | 03/01/2011 16:42:39:000 | 03/01/2011 16:42:39:000
1294044161 - | 03/01/2011 08:42:41:000 | 03/01/2011 08:42:41:000  <-- ERROR
1294044163 - | 03/01/2011 16:42:43:000 | 03/01/2011 16:42:43:000
1294044164 - | 03/01/2011 16:42:44:000 | 03/01/2011 16:42:44:000
 ...
1294044361 - | 03/01/2011 16:46:01:000 | 03/01/2011 16:46:01:000
1294044361 - | 03/01/2011 16:46:01:000 | 03/01/2011 16:46:01:000
1294044362 - | 03/01/2011 16:46:02:000 | 03/01/2011 16:46:02:000
1294044362 - | 03/01/2011 16:46:02:000 | 03/01/2011 16:46:02:000
1294044362 - | 03/01/2011 16:46:02:000 | 03/01/2011 16:46:02:000
1294044363 - | 03/01/2011 08:46:03:000 | 03/01/2011 08:46:03:000  <-- ERROR
1294044363 - | 03/01/2011 16:46:03:000 | 03/01/2011 16:46:03:000
1294044363 - | 03/01/2011 16:46:03:000 | 03/01/2011 16:46:03:000
1294044363 - | 03/01/2011 16:46:03:000 | 03/01/2011 16:46:03:000
 ...

I build a function to return the current date time (I put some debugs to find the problem):

Code:

int getCurrentDateTime(tm* dateTime) {
    time_t now;    
    tm* pLt = NULL;
    if ((now = time(NULL)) == (time_t) -1) {
        printf("error on time()\n"); //DEBUG
    }
    printf("%ld - ", now); //printing the time_t value
    pLt = localtime(&now);
    if (pLt != NULL) {
        //DEBUG - Checking the pLt structure
        printf("| %02d/%02d/%04d %02d:%02d:%02d:%03d | ", pLt->tm_mday,
            pLt->tm_mon + (1), pLt->tm_year + (1900), pLt->tm_hour,
            pLt->tm_min, pLt->tm_sec, 0);
        memcpy(dateTime, pLt, sizeof (tm));
        mktime(dateTime);
        //DEBUG - Checking the dateTime structure
        printf("%02d/%02d/%04d %02d:%02d:%02d:%03d\n", dateTime->tm_mday,
            dateTime->tm_mon + (1), dateTime->tm_year + (1900), dateTime->tm_hour,
            dateTime->tm_min, dateTime->tm_sec, 0);
    } else {
        printf("null pointer\n");
    }
    return OK;
}

The pattern is the hour, it's always 8 hours late. Anyone knows what is happening?
Am I missing something?

PS: The kernel version is 2.6.32.2.

Thanks in advance

Lobinho · 01-03-2011, 01:27 PM

With ctime() this error also occurs, but not at the same time as localtime().

Code:

printf("| %s | ", ctime(&now));

wje_lq · 01-03-2011, 01:47 PM

Three things. Not from the Orient, but three things nonetheless.

What time zone are you in? Any chance that sometimes localtime() and ctime() are actng as though you're in UTC, and sometimes not?
The purpose of mktime() is to take the date/time fields in the struct and make something of type time_t as the resultant value of the function. Why bother to call mktime() if you're not going to use the value of the function?
I have no experience with embedded system programming (none that I'm willing to admit to, anyway), but isn't it a good idea to make your code as reentrant as possible? Use localtime_r() or ctime_r(). Note that with localtime_r(), you'll need a local variable of type tm, not *tm. Similarly, for ctime_r, you'll need an actual array of characters as a local variable for the string to be written into.

Lobinho · 01-04-2011, 04:54 AM

Thanks for your response wje_lq,

1. My UTC is -3, but I'm not setting the timezone.
2. I was calling mktime() just hoping to normalize the structure and solve the problem, but this didn't work and doesn't make sense do that.
3. Excuse my ignorance, but what do you mean when you say that I should make the code more reentrant? I tried to understand the difference between a normal function and a reentrant function, but couldn't find a good explanation.

Anyway I'll follow your suggestion that make sense. I was reading this specification:

Quote:

The localtime_r() function shall convert the time in seconds since the Epoch pointed to by timer into a broken-down time stored in the structure to which result points. The localtime_r() function shall also return a pointer to that same structure.

Unlike localtime(), the reentrant version is not required to set tzname.

If the reentrant version does not set tzname, it shall not set daylight and shall not set timezone.

from http://pubs.opengroup.org/onlinepubs...localtime.html.

Lobinho · 01-04-2011, 05:09 AM

I finished my tests and guess the problem is solved.

I guess I understood the meaning of make the code reentrant. And this is critical :O

http://en.wikipedia.org/wiki/Reentrant_%28subroutine%29

my function now is:

Code:

int getCurrentDateTime(tm* dateTime) {
    time_t now = time(NULL);
    localtime_r(&now, dateTime);
    return OK;
}

I think I'll have to take a look over all my code.

Thanks wje_lq.