Re: singleton lock not reliable

You are trying to obtain an exclusive lock on a filehandle that you've opened for reading. I am no expert on file locking but I don't think that's possible. The FAQ is explicit is stating that the filehandle must be opened for writing (or appending or read+write) when using lockf at least. Perhaps that is the cause of your problem. The canonical example in the Monastery is Highlander - allow only one invocation at a time of an expensive CGI script and that also uses a writing filehandle.

I suggest you try switching to a writing filehandle (obviously not on $0) and see if that enables the lock for you reliably across your various OSes.

🦛

Comment on Re: singleton lock not reliable

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Re^2: singleton lock not reliable by LanX (Saint) on Jun 24, 2021 at 09:51 UTC
> The FAQ is explicit is stating that the filehandle must be opened for writing (or appending or read+write) it depends on the implementation, the perldocs refer to multiple different OS functions potentially used for `flock` and say `lockf(3) does not provide shared locking, and requires that the filehandle be open for writing (or appending, or read/writing).` So yes, it's possible that some Perl ports will fail when opening with `<` , if Perl was compiled to use `lockf` Personally I'm doing it deliberately on WIN (where it works) to make sure my colleagues understand that the lockfile is an empty semaphore only. (Opening for writing with `>` has it's own hazards, because the content will be deleted each time.) It really depends on the OS and FS and should be tested for each combination. Cheers Rolf _{(addicted to the Perl Programming Language :) Wikisyntax for the Monastery}	[reply] [d/l] [select]
Re^2: singleton lock not reliable by ikegami (Patriarch) on Jun 24, 2021 at 04:24 UTC
I am no expert on file locking but I don't think that's possible. It is. `$ touch lock $ perl -M5.010 -MFcntl=LOCK_EX -e' open(my $fh, "<", "lock") or die $!; flock($fh, LOCK_EX) or die $!; say "ok"; ' ok` [download] `flock` locks have nothing to do with reading and writing, and they prevent neither. They merely prevent locks from being obtained. Seeking work! You can reach me at ikegami@adaelis.com	[reply] [d/l] [select]


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