Unix timestamp to FAT timestamp

问题

Im trying to convert a time structure to a FAT timestamp. My code looks like:

unsigned long Fat(tm_struct pTime)
{
    unsigned long FatTime = 0;

    FatTime |= (pTime.seconds / 2) >> 1;
    FatTime |= (pTime.minutes) << 5;
    FatTime |= (pTime.hours) << 11;
    FatTime |= (pTime.days) << 16;
    FatTime |= (pTime.months) << 21;
    FatTime |= (pTime.years + 20) << 25;

    return FatTime;
}

Does someone have the correct code?

回答1:

The DOS date/time format is a bitmask:

               24                16                 8                 0
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
|Y|Y|Y|Y|Y|Y|Y|M| |M|M|M|D|D|D|D|D| |h|h|h|h|h|m|m|m| |m|m|m|s|s|s|s|s|
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
 \___________/\________/\_________/ \________/\____________/\_________/
    year        month       day      hour       minute        second

The year is stored as an offset from 1980. 
Seconds are stored in two-second increments. 
(So if the "second" value is 15, it actually represents 30 seconds.)

I dont know the tm_struct you are using but if it's http://www.cplusplus.com/reference/ctime/tm/ then

unsigned long FatTime = ((pTime.tm_year - 80) << 25) | 
                        (pTime.tm_mon << 21) |
                        (pTime.tm_mday << 16) |
                        (pTime.tm_hour << 11) |
                        (pTime.tm_min << 5) |
                        (pTime.tm_sec >> 1);

回答2:

Lefteris E gave almost correct answer but here's a little mistake

you should add 1 to tm_mon, because tm struct keeps month as number from 0 to 11 (struct tm), but DOS date/time from 1 to 12 (FileTimeToDosDateTime). so correctly is

unsigned long FatTime = ((pTime.tm_year - 80) << 25) | 
    ((pTime.tm_mon+1) << 21) |
    (pTime.tm_mday << 16) |
    (pTime.tm_hour << 11) |
    (pTime.tm_min << 5) |
    (pTime.tm_sec >> 1);

回答3:

Also you can use bit fields and library time.h. In my project I convert UNIX timestamp to FatFs time.

        #include <time.h>

        #pragma pack (push,1)
        typedef struct{
            unsigned Sec :5;
            unsigned Min :6;
            unsigned Hour :5;
            unsigned Day :5;
            unsigned Month :4;
            unsigned Year :7;
        }FatDate_t;
        #pragma pack (pop)

        DWORD get_fattime(void){ 
            time_t ts=GetDateTimeNow();
            struct tm * tmInfo = gmtime( &ts );
            uint32_t Result=0;
            FatDate_t *fatDate;
            fatDate=(FatDate_t *)&Result;
            fatDate->Year=tmInfo->tm_year-80;
            fatDate->Month=tmInfo->tm_mon+1;
            fatDate->Day=tmInfo->tm_mday;

            fatDate->Hour=tmInfo->tm_hour;
            fatDate->Min=tmInfo->tm_min;
            fatDate->Sec=tmInfo->tm_sec>>1;

            return Result;
        }

来源：https://stackoverflow.com/questions/15763259/unix-timestamp-to-fat-timestamp