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FAQ 5.22 Why can't I just open(FH, ">file.lock")?

 
 
PerlFAQ Server
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      02-04-2011
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.
 
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