Semaphore not working as expected in LAMP - php

Question

my code is the following
<?php
$semaphore_key = 2112;
$semaphore_max = 1;
$semaphore_permissions = 0666;
$semaphore_autorelease = 1;
$semaphore = sem_get($semaphore_key, $semaphore_max, $semaphore_permissions, $semaphore_autorelease);
if(!$semaphore)
{
echo "Failed to get semaphore - sem_get().\n";
exit();
}else{
echo "ok\n";
}
sem_acquire($semaphore);
sleep(10);
sem_release($semaphore);
?>
In the linux terminal, I run:
php semaphore.php &
and after 1 sec, I run
php semaphore.php
I expect the second call to the php to exit, but both print "ok" and arrive to the sleep(10), and I don't understand because the semaphore is locked by the other script running in background.
Since semaphore_max is set to 1, I expect the second call to the script to print "Failed to get semaphore - sem_get()." and terminate, but it is not happening.
echo $semaphore results in "Resource id #4", so I suppose sysvsem is working fine.
Someone could help?
Thanks

Answer 1

sem_get() just fetches/constructs the semaphore object, it will only return false if perhaps there are no more resources available to create a semaphore, or if your parameters are invalid.
sem_aquire() is where the expected locking behaviour happens, but you have to pass true into the 2nd argument to not block and return false immediately.
So your code should be:
$semaphore = sem_get($semaphore_key, $semaphore_max, $semaphore_permissions, $semaphore_autorelease);
if(sem_aquire($semaphore, true) === false) {
echo "Failed to get semaphore - sem_get().\n";
exit();
}else{
echo "ok\n";
}
sleep(10);
sem_release($semaphore);