When you do two things, there is a gap between them.

You discover that the file does not exist with -e. Then there is a gap. Then you create the file by writing to it. That is a race condition - if another process created the file in that gap, you just overwrote it. Now both of you think you own the file.

(You also treat the disappearance of the file between finding it exists and trying to open it as a fatal error, but that's actually another race condition.)

It's quite a while since I did something like this, but as far as I remember a way that works looks something like:

  1. If the file exists, you don't own it, give up.
  2. Open the file for append, write your pid followed by a newline, close.
  3. Open the file for read; if the open failed, you didn't own the file, give up. (Whoever did own it probably just unlinked it, so optionally you could go back to step one to try again - but probably better not to.)
  4. Read the first line; if it is not your pid, you don't own the file, give up.
  5. You now own the file. When you're done, unlink the file.

In reply to Re^4: Mechanism for ensuring only one instance of a Perl script can only run? by hv
in thread Mechanism for ensuring only one instance of a Perl script can only run? by redapplesonly

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