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


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This is an excerpt from the latest version perlfaq5.pod, which
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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.



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