Semaphores

• A synchronization tool called semaphore.Semaphores are variables that are used to signal the status of shared resources to processes.
• Dijkstra (1965) suggested using an integer variable to count the number of wakeups saved for future use. In his proposal, a new variable type, called a semaphore, was introduced.
• A semaphore $S$ is an integer variable that, apart from initialization, is accessed only through two standard atomic operations: $wait()$ (sleep) and $signal()$ (wakeup).
• A semaphore could have the value 0, indicating that no wakeups were saved, or some positive value if one or more wakeups were pending.
• Two operations, down and up (generalizations of sleep and wakeup, respectively);
• The definition of $wait()$ is as follows:

  wait (S) {
    while S <= 0
      ;// no-op
    S--;
  }
  

• The definition of $signal()$ is as follows:

  signal (S) {
    S++;
  }
  

• All the modifications to the integer value of the semaphore in the $wait()$ and $signal()$ operations must be executed indivisibly.
• That is, when one process modifies the semaphore value, no other process can simultaneously modify that same semaphore value.
• In addition, in the case of $wait(S)$, the testing of the integer value of $S$ ($S\leq0$), and its possible modification ($S--$), must also be executed without interruption.