When you need to update a file that will continue to be "live", you have to create a new file and then rename it over the old one so that the data appears to be atomically updated.

However, if you naively use flock, you run the risk of getting exclusive access to the now-dead file, rather than the live file, so you need to verify that you really have the live file exclusively flocked. Here's a template to do that.

The checks for the device/inode number verify that the filehandle FILE is really the same as the file $file.

{
  open FILE, $file or die;
  my ($dev,$ino) = stat(FILE);
  flock FILE, 2 or die;
  my ($dev1, $ino1) = stat($file);
  redo unless "$dev $ino" eq "$dev1 $ino1"; 
}
open FILE_OUT, ">$file.tmp" or die;
### ... process FILE into FILE_OUT
close FILE_OUT;
rename "$file.tmp", "$file" or die;
close FILE;

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In reply to Exclusively updating a file that continues to be repeatedly read by merlyn

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