sem_destroy —
    destroy an unnamed semaphore
Standard C Library (libc, -lc)
The
    sem_destroy()
    function destroys the unnamed semaphore pointed to by
    sem. After a successful call to
    sem_destroy(), sem is unusable
    until re-initialized by another call to
    sem_init(3).
The sem_destroy() function returns the
    value 0 if successful; otherwise the value -1 is returned and
    the global variable errno is set to indicate the
    error.
The sem_destroy() function will fail
  if:
  - [
EINVAL] 
  - The sem argument points to an invalid
    semaphore.
 
  - [
EBUSY] 
  - There are currently threads blocked on the semaphore that
      sem points to.
 
The sem_destroy() function conforms to
    ISO/IEC 9945-1:1996 (“POSIX.1”).
POSIX does not define the behavior of
    sem_destroy() if called while there are threads
    blocked on sem, but this implementation is guaranteed
    to return -1 and set errno to
    EBUSY if there are threads blocked on
    sem.