There are several ways to implement the semaphore abstract data type.

Looking at one possible way can help elaborate the missing details from the abstract description.

• The integer can be initialzed to any value >= 0.
• sem_wait:
  • first decrement the integer
  • if the value is now < 0, the calling thread is blocked and placed on the wait list for this semaphore
  • return
• sem_post:
  • first increment the integer
  • if the value is now <= 0, the wait list is not empty. Unblock one of the threads (presumably first in, first out)
  • return