sync( ) problem on SD card

Hi we have a customer designed arm9 board,
it has a software reset button.

when i press it, it'll trigger a configuration backup function,
which will save all my running configuration from ram disk to SD/EMMC card, using "cp" command.

In the end of the cp commands, i run "sync()" to make sure all stuff put back to SD or EMMC. Then board reboots.

Now after i reboot, i copy my original config from SD/EMMC to ram disk.
Every once a while it says inode problem, asking me to run e2fsck.
If i put several sync() in the code back to back, problem happens less but still happens.

my guess is it didn't write info to SD/EMMC completely during soft reset.
so how to make sure sync() can really flush info to the SD ?

Any help appreciated.

If you run sync from the command line, it waits for all writes to disk before it returns.

If you run the sync() C library procedure, then according to the documentation:

sync() schedules the writes, but may return before the actual writing is done.

You probably want to call fsync.

thanks for reply 1st,
check code again, we are doing system call actually,
system("sync");

my understanding is system() will guarantee "sync" completes and then return back.
system() executes a command specified in command by calling /bin/sh -c command, and returns after the command has been completed.

Originally Posted by smallpond If you run sync from the command line, it waits for all writes to disk before it returns. If you run the sync() C library procedure, then according to the documentation: You probably want to call fsync.

If the filesystem is still mounted read/write there is no assurance that the filesystem metadata has been written to the device. If the filesystem cannot be unmounted, you should at least remount it read-only before rebooting.