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course/03-Semaphores.md

+---
+author: Florent Gluck - Florent.Gluck@hesge.ch
+
+title: Semaphores
+
+date: \vspace{.5cm} \footnotesize \today
+
+pandoc-latex-fontsize:
+  - classes: [tiny]
+    size: tiny
+  - classes: [verysmall]
+    size: scriptsize
+  - classes: [small]
+    size: footnotesize
+  - classes: [huge, important]
+    size: huge
+---
+
+[//]: # ----------------------------------------------------------------
+## What is a semaphore?
+
+- A semaphore is a **synchronization mechanism** between threads (or processes)
+- Used to synchronize the execution of threads (or processes)
+- Waiting on a semaphore is a **passive** operation
+- Invented by E.W. Dijkstra in 1965
+
+[//]: # ----------------------------------------------------------------
+## Anatomy of a semaphore
+
+A semaphore can be seen as an integer variable associated with two operations:
+
+  - incrementation
+  - decrement
+
+These two operations are **atomic**
+
+[//]: # ----------------------------------------------------------------
+## Using a semaphore
+
+- A semaphore is **shared** by several threads/processes
+- A semaphore is **always** initialized to an integer value $\geq$ 0
+- The only possible operations are **incrementation** and **decrementation**
+- In general, the value of a semaphore **cannot** be determined
+
+[//]: # ----------------------------------------------------------------
+## Behavior of a semaphore
+
+- When a thread **decrements** a semaphore, if the result is **< 0**, the thread is **suspended**
+- When a thread **increments** a semaphore, if one or more threads are suspended on the semaphore, then **one thread is awakened** and continues its execution
+  - it is **impossible to determine** in advance which thread will be woken up
+
+[//]: # ----------------------------------------------------------------
+## The meaning of a semaphore's value
+
+- Value > 0:
+  - represents the number of threads that can decrement the semaphore without being suspended
+- Value < 0:
+  - represents the number of threads suspended on this semaphore
+- Value == 0:
+  - means that no thread is suspended, but that if a thread decrements the semaphore, then the thread will be suspended
+
+[//]: # ----------------------------------------------------------------
+## Properties of semaphore
+
+- Before decrementing a semaphore, there is **no way of knowing** whether the thread will be suspended or not
+- After incrementing a semaphore, another thread is awakened and the two threads **run concurrently**
+  - **no way of knowing** which thread will immediately continue
+- When a semaphore is incremented, there's **no way of knowing** whether a thread is already suspended
+
+[//]: # ----------------------------------------------------------------
+## Semaphore terminology
+
+Other terminologies exist besides increment/decrement:
+
+- P/V
+- signal/wait
+- post/wait
+
+[//]: # ----------------------------------------------------------------
+## Semaphores with POSIX.1-2001
+
+- Header file: `semaphore.h`
+- Type: `sem_t`
+- Fonctions:
+  - `sem_init`
+  - `sem_destroy`
+  - `sem_wait`
+  - `sem_post`
+
+[//]: # ----------------------------------------------------------------
+## Usage example
+
+```{.tiny .c}
+// Errors are purposedly ignored
+#include <pthread.h>
+#include <semaphore.h>
+
+#define SEM_THREAD  0  // use 1 for inter-process sempahores
+
+static sem_t sem;
+
+static void *f1(void *data) {
+    sem_wait(&sem);
+    puts("f1: I'm always last :-(");
+}                                                                                                                                                            
+
+static void *f2(void *data) {
+    puts("f2: I'm always first!");
+    sem_post(&sem);
+}
+
+int main() {
+    sem_init(&sem, SEM_THREAD, 0);
+    pthread_t t1, t2;
+    pthread_create(&t1, NULL, f1, NULL);
+    pthread_create(&t2, NULL, f2, NULL);
+    pthread_join(t1, NULL);
+    pthread_join(t2, NULL);
+    sem_destroy(&sem);
+    return 0;
+}
+```
+
+[//]: # ----------------------------------------------------------------
+## Resources
+
+\small
+- Man pages\
+\footnotesize `man sem_overview`
+
+\small
+- Operating Systems: Three Easy Pieces, Remzi H. and Andrea C. Arpaci-Dusseau. Arpaci-Dusseau Books\
+\footnotesize [\textcolor{myblue}{http://pages.cs.wisc.edu/~remzi/OSTEP/}](http://pages.cs.wisc.edu/~remzi/OSTEP/)