FAQ 5.22 Why can't I just open(FH, ">file.lock")?
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From: PerlFAQ Server on 7 May 2010 12:00 This is an excerpt from the latest version perlfaq5.pod, which comes with the standard Perl distribution. These postings aim to reduce the number of repeated questions as well as allow the community to review and update the answers. The latest version of the complete perlfaq is at http://faq.perl.org . -------------------------------------------------------------------- 5.22: Why can't I just open(FH, ">file.lock")? A common bit of code NOT TO USE is this: sleep(3) while -e 'file.lock'; # PLEASE DO NOT USE open my $lock, '>', 'file.lock'; # THIS BROKEN CODE This is a classic race condition: you take two steps to do something which must be done in one. That's why computer hardware provides an atomic test-and-set instruction. In theory, this "ought" to work: sysopen my $fh, "file.lock", O_WRONLY|O_EXCL|O_CREAT or die "can't open file.lock: $!"; except that lamentably, file creation (and deletion) is not atomic over NFS, so this won't work (at least, not every time) over the net. Various schemes involving link() have been suggested, but these tend to involve busy-wait, which is also less than desirable. -------------------------------------------------------------------- The perlfaq-workers, a group of volunteers, maintain the perlfaq. They are not necessarily experts in every domain where Perl might show up, so please include as much information as possible and relevant in any corrections. The perlfaq-workers also don't have access to every operating system or platform, so please include relevant details for corrections to examples that do not work on particular platforms. Working code is greatly appreciated. If you'd like to help maintain the perlfaq, see the details in perlfaq.pod. |