Q190315: PRB: Some CRT File Functions Adjust For Daylight Savings Time
Article: Q190315
Product(s): Microsoft C Compiler
Version(s): 5.0,6.0
Operating System(s):
Keyword(s): kbcode kbCRT kbVC500
Last Modified: 07-JAN-2002
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The information in this article applies to:
- The C Run-Time (CRT), included with:
- Microsoft Visual C++, 32-bit Enterprise Edition, versions 5.0, 6.0
- Microsoft Visual C++, 32-bit Professional Edition, versions 5.0, 6.0
- Microsoft Visual C++, 32-bit Learning Edition, version 6.0
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SYMPTOMS
========
You can adjust the time of last access and time of last modification reported by
the CRT functions _stat, _fstat, _findfirst, and _findnext for daylight savings
time under the NTFS file system. This manifests itself as a change in the time
last modified when making the switch to daylight savings time from standard time
or vice-versa.
CAUSE
=====
All of the previously referenced CRT functions call the Win32 API
FileTimeToLocalFileTime. FileTimeToLocalFileTime adjusts the file times for
daylight savings time if "Automatically adjust clock for daylight saving
changes" is selected in the Date/Time Properties dialog box for the system
clock. This behavior is by design under the NTFS file system.
RESOLUTION
==========
One way to avoid the behavior is to clear "Automatically adjust clock for
daylight saving changes" in the Date/Time Properties dialog box for the system
clock. Other workarounds depend upon the calculations or assumptions that are
made based on the date/time last modified that is reported for a file.
NOTE: If the time last modified is between 12:00 am and 1:00 am, the date last
modified is also changed when the adjustment subtracts an hour from the time
last modified in the Fall. Similarly, files with a time last modified between
11:00 pm and midnight will have their date last modified when the adjustment
advances the time one hour in the Spring.
STATUS
======
This behavior is by design.
MORE INFORMATION
================
This behavior is specific to the NTFS file system, and does not reproduce on a
system using the FAT file system. Note that the CRT and the operating system
report the same time last modified under all circumstances.
In the steps below, references to stat.exe mean the program generated by
compiling stat.c, which appears below:
Sample Code:
------------
/* STAT.C: This program uses the _stat function to
* report information about the file named STAT.C.
* Compile options: none.
*/
#include <time.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <stdio.h>
int main(int ac, char **av)
{
struct _stat buf;
if(ac != 2)
{
printf("Usage: %s <FileName>\n",av[0]);
return -1;
}
/* Get File Statistics for stat.c. */
if( _stat( av[1], &buf ) != 0 )
perror( "Problem getting information" );
else /* print the date/time last modified */
printf( "Date/Time modified : %s", ctime( &buf.st_mtime ) );
}
/* End stat.c. */
Steps To Reproduce Behavior (must be an NTFS drive):
----------------------------------------------------
1. Open the Date/Time Properties dialog box and ensure that "Automatically
Adjust For Daylight Savings Time" is selected. Press OK.
2. Set your system date to a date that is during daylight savings time (for
example, 10/25/97 in the "Pacific Time (US & Canada); Tijuana" time
zone).
3. Create a new file called repro.txt.
4. Note the date/time last modified for repro.txt, as reported by either the
Windows Explorer or using an MS-DOS dir command.
5. At the MS-DOS prompt, run "c:\>stat.exe repro.txt", and note that the
date/time last modified matches the date/time last modified that the system
reports.
6. Reset your system date to a date that is in the standard time period.
7. Look again at an MS-DOS dir command (or in the Windows Explorer) and you can
see that the time last modified for repro.txt is now one hour earlier than
before.
8. Repeat step 5, and note that the date/time reported by stat.exe is also one
hour earlier, matching what the operating system reports.
9. Clear "Automatically Adjust for Daylight Savings Time."
10. Repeat steps 7 and 8, noting that the adjustment is not being made, and the
time last modified has returned to its original value.
A similar series of steps can be followed to observe the time being adjusted
forward one hour by creating the file during standard time and referencing it
after the switch to daylight savings time.
Additional query words: _stat, _fstat, _findfirst, _findnext, NTFS
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Keywords : kbcode kbCRT kbVC500
Technology : kbVCsearch kbAudDeveloper kbCRT
Version : :5.0,6.0
Issue type : kbprb
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