Brothers,
I'm seeking wisdom and potentially flashes of inspiration... I have a process that receives files via NDM from an external partner in pairs - a data file and a status file. The files may arrive in any order, and could be anything from minutes apart to fractions of a second. I'm trying to create a process that populates the file content into an Oracle database via a Perl script triggered after each file arrives. Although the files can arrive in pseudo parallel, I need to process them in sequence, so I'm trying to semaphore lock them (IPC::Semaphore). If they arrive separated by, say, 1 second the lock works nicely, but if they arrive a tenth of a second apart, both Perl processes say that they're the first one and create and initialize the semaphore. 
$sem = IPC::Semaphore->new( 4321, 1, S_IRUSR | S_IWUSR );

if ( $sem ) {
# Semaphore already exists so just open it
    print "Semaphore already exists - just open it\n";
    $sem = IPC::Semaphore->new( 4321, 1, S_IRUSR | S_IWUSR );
} else {
# The semaphore didn't already exit so create it
    print "Create semaphore \n";
    $sem = IPC::Semaphore->new( 4321, 1, IPC_CREAT | S_IRUSR | S_IWUSR
+ );
    print "Semaphore created\n";
    $sem->setval(0,1);
    print "Semaphore initialised\n";
}

print "Locking other threads\n";

$sem->op(0, -1, SEM_UNDO);

[download]
Depending on exact timing, I see that it's possible for the first process to create the semaphore and attempt to lock the other process, but the other one running a fraction behind has not detected the creation so it does its own and sets the semaphore to 1, so revoking it's partner's lock! I can't see a fool proof way of getting around this, so any divine inspiration gratefully received.
Jerry

In reply to Semaphore puzzle by jerryhone

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