Hi Monks,

I need help with a file locking problem. Basically I have two processes (more than that, actually, but for the sake of explanation, two will do) that are messing with the same file. Some are attempting to read it, some are attempting to write to it. I was under the impression that if everyone uses flock(), things would be just dandy, but that is not the case. Here's what happens:

Process1 comes along and wants to read the file. It does: 
open HANDLE, "$file" or die "Can't open: $!";
flock(HANDLE, LOCK_SH);  #could use LOCK_EX, it doesn't prevent this p
+roblem

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It then starts reading the file. Before Process1 gets done, Process2 comes along and wants to write to the file. Process2 does:
open WRITER, ">$file" or die "Can't open: $!";
flock(WRITER, LOCK_EX);

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However, the damage is done before Process2 gets to the flock() call, because when Process2 calls open(), the contents of $file are immediately clobbered (even though Process1 has a flock on it). When Process2 does call flock(), of course, it blocks until Process1 releases his lock before it returns, so nothing gets written to the file until Process1 is done with it.

Is there a good way to ensure that this situation doesn't occur?

In reply to File Locking Problem by Anonymous Monk

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