Problems with file buffers. Flushing synchronously

Posted on 2004-04-15
Last Modified: 2010-04-21
I am trying to write a file to and then I want to ensure that all the data is written to it. I was going to the the flush command, but snooping on a serial port I can see that the file buffer is not fully flushing the file.

I'm converting from Windows to Linux and in Win I see that it uses flushfilebuffers on a drive letter so I was going to do the same on the drive relating to the device : /dev/sdf

In the fstab file I have configured my drive to be synchronous :

/dev/sdf     /media/mydisk        auto       noauto,users,exec,sync      0 0

Now this seems to help but the final portions of the file are not copied and because of this my program cannot complete the updating command.

I am using QFile, this is my code:

QFile myFile("/media/myDisk/myfile");

I have also tried flushing the actual device /media/myDisk, but this does not make any difference either.

Atm I am getting round the problem by going to sleep for a while until the delayed write finishes but this is not really much of a solution.

Any ideas?


Question by:jeffreyg
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LVL 45

Expert Comment

ID: 10831781
Hi jeffreyg,

If you want to eliminate buffering alltogther, use setvbuf() towards the beginning of the program ... I suspect Qt uses stdio functions so this should do the trick

Another option will be to use read and write for file operations as these calls are not buffered

LVL 12

Expert Comment

ID: 10832786
Hi jeffreyg,

If you use buffered I/O, you need a two-stage sync:

fflush(your_FILE);         // Flush your buffers to system
fsync(fileno(your_FILE));  // Flush system's write cache to disk

LVL 12

Accepted Solution

stefan73 earned 250 total points
ID: 10834622
For a QFile, the flush to disk is slightly different:

QFile myFile;


Author Comment

ID: 11013108

Thanks for the input. Got it working now. Used your fsync code and implemented it in the following way:

QFile myFile;

Thanks, points r yours!
Thanks also to sunnycoder for your time.



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