Semaphores: A Definition

This lesson covers the basics of semaphores and its two main routines.

We'll cover the following

A semaphore is an object with an integer value that we can manipulate with two routines; in the POSIX standard, these routines are sem_wait() and sem_post()Historically, sem_wait() was called P() by Dijkstra and sem_post() called V(). These shortened forms come from Dutch words; interestingly, which Dutch words they supposedly derive from has changed over time. Originally, P() came from “passering” (to pass) and V() from “vrijgave” (release); later, Dijkstra wrote P() was from “prolaag”, a contraction of “probeer” (Dutch for “try”) and “verlaag” (“decrease”), and V() from “verhoog” which means “increase”. Sometimes, people call them down and up. Use the Dutch versions to impress your friends, or confuse them, or both. See item?id=8761539) for details.. Because the initial value of the semaphore determines its behavior, before calling any other routine to interact with the semaphore, we must first initialize it to some value, as the code below does.

Get hands-on with 1200+ tech skills courses.