ID Number: Q51123
5.10 6.00 6.00a 6.00ax 7.00
MS-DOS
Summary:
SYMPTOMS
In Microsoft C versions 5.1, 6.0, 6.0a, 6.0ax, and C/C++ version
7.0, changing the time and date of a file with _dos_setftime() may
appear to fail if the file is opened for write and all data has not
been written to disk.
CAUSE
The MS-DOS and OS/2 update file dates whenever a file gets
modified. Therefore, attempts to change a file date before the
operating system has written all data to the physical disk may
appear to be ignored. This is because the date is changed again
when the file is closed or flushed if there is data still to be
written.
RESOLUTION
One way to work around this feature of the operating system is to
put a flush() statement before the call to _dos_setftime(). This
ensures that all remaining data has been written to disk before the
date is changed and the file is closed. The following program
changes the date to 6-1-89 10:00 successfully.
void main(void)
{
FILE *fp;
fp = fopen( "outfile.txt", "wt" );
fprintf( fp, "Start of file\n" );
fprintf( fp, "End of file\n" );
fflush(fp); // <---- flush the data to disk
_dos_setftime( fileno( fp ), 0x12c1, 0x5000) // 10:00 6-1-89
fclose(fp);
}
More Information:
The following program will attempt to modify the date and time of the
output file to 6-1-89 10:00. Because the file has not been flushed or
closed, it is likely that there is still data in the file buffer
waiting to be written to disk. When the file is closed, the program
will flush all remaining data to the disk, changing the date and time
to the current date and time in the process.
void main(void)
{
FILE *fp;
fp = fopen( "outfile.txt", "wt" );
fprintf( fp, "Start of file\n" );
fprintf( fp, "End of file\n" );
_dos_setftime( fileno( fp ), 0x12c1, 0x5000) // 10:00 6-1-89
fclose(fp);
}
Additional reference words: 5.10 6.00 6.00a 6.00ax 7.00 s_quickc