sem_release(): failed to release key 0xc: Invalid argument


I'm using semaphore to synchronize some parts in my application.

On releasing of the semaphore (sem_release) I'm getting this warning:

sem_release(): failed to release key 0xc: Invalid argument

First I don't know whether the semaphore released, but since I'm not getting "true" as result, I guess it's not releasing.

PHP Version: 5.6.30

ipcs -V => ipcs from util-linux 2.25.2

Here is the semaphore:

key        semid      owner      perms      nsems 
0x0000000c 4124122    myUser      666        3

Here is part of the code (class Synchronization):

if ( !( $this->semaphoreId = sem_get( $this->id, 1 ) ) )
    throw new RuntimeException( 'Error getting Semaphore.');

if ( !sem_acquire( $this->semaphoreId ) )
   throw new RuntimeException( 'Error acquiring Semaphore.');
if ( !sem_release( $this->semaphoreId ) )
    throw new RuntimeException( 'Error releasing Semaphore.');

P.S. I'm getting this error only in my productive environment and I'm not able to reproduce/debug in in my test environment.

I searched on Internet for this error message, but I found nothing.

Does anyone knows what this message means?


  1. The error message does not appear every time the script is running.
  2. I indeed get sometimes the error 'Error acquiring Semaphore' with the similar warining 'sem_acquire(): failed to acquire key 0xc: Identifier removed', BUT not at the same time(day) I get the 'Error releasing Semaphore'
  3. The class above is used all over the place with different keys to synchronize part of application code. I'm not having ANY issues with other keys. And yes, this key "12"/"0xc" is beeing used ONLY in one place and from the same user.
  4. Issues with permission should not occure, because if you check the permisson of the semaphore "0xc" is "666"
asked on Stack Overflow May 17, 2017 by dritan • edited May 23, 2017 by dritan

1 Answer


Could you follow the steps from the beginning:

  • Getting the semaphore resource sem_get ( int $key [, int $max_acquire = 1 [, int $perm = 0666 [, int $auto_release = 1 ]]] )
  • Acquiring the semaphore bool sem_acquire ( resource $sem_identifier [, bool $nowait = false ] )

and add sanity checks to make sure that the above function are returning the expected value.

Could you also check if the other part of your application runs under the same user to avoid permission issues.

answered on Stack Overflow May 21, 2017 by Rares Harnagea

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