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This is an excerpt from the latest version perlfaq5.pod, which
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5.21: How can I lock a file?

Perl's builtin flock() function (see perlfunc for details) will call
flock(2) if that exists, fcntl(2) if it doesn't (on perl version 5.004
and later), and lockf(3) if neither of the two previous system calls
exists. On some systems, it may even use a different form of native
locking. Here are some gotchas with Perl's flock():

1 Produces a fatal error if none of the three system calls (or their
close equivalent) exists.

2 lockf(3) does not provide shared locking, and requires that the
filehandle be open for writing (or appending, or read/writing).

3 Some versions of flock() can't lock files over a network (e.g. on
NFS file systems), so you'd need to force the use of fcntl(2) when
you build Perl. But even this is dubious at best. See the flock
entry of perlfunc and the INSTALL file in the source distribution
for information on building Perl to do this.

Two potentially non-obvious but traditional flock semantics are that
it waits indefinitely until the lock is granted, and that its locks
are *merely advisory*. Such discretionary locks are more flexible,
but offer fewer guarantees. This means that files locked with
flock() may be modified by programs that do not also use flock().
Cars that stop for red lights get on well with each other, but not
with cars that don't stop for red lights. See the perlport manpage,
your port's specific documentation, or your system-specific local
manpages for details. It's best to assume traditional behavior if
you're writing portable programs. (If you're not, you should as
always feel perfectly free to write for your own system's
idiosyncrasies (sometimes called "features"). Slavish adherence to
portability concerns shouldn't get in the way of your getting your
job done.)

For more information on file locking, see also "File Locking" in
perlopentut if you have it (new for 5.6).


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