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| Perl: the Markov chain saw | |
| PerlMonks |
Current Perl documentation can be found at perldoc.perl.org.
Here is our local, out-dated (pre-5.6) version:
A common bit of code NOT TO USE is this:
sleep(3) while -e "file.lock"; # PLEASE DO NOT USE
open(LCK, "> 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(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 involving
link() have been suggested, but these tend to involve busy-wait, which is also subdesirable.