Semaphore (信号量)

Abstract

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• Used to limit the number of Process (进程)/Thread running on a particular resource at any time with a non-negative integer
• Used in environments with Inter-Process Communication (IPC) and can also be useful in managing synchronisation across different systems, such as in Distributed Systems

Code Snippets

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• sem_t semaphore; & sem_init(&semaphore, 0, 1);
• Decrement the value when a Process (进程)/Thread running - sem_wait(&semaphore);
• Increase the value when a Process (进程)/Thread finishes running - sem_post(&semaphore);

#include <semaphore.h>
 
int main() {
  sem_t semaphore;
  // 0 means it is not shared with other processes, 1 means only 1 thread can run at a time
  sem_init(&semaphore, 0, 1);
  
  sem_wait(&semaphore); // semmaphore--
  sem_post(&semaphore); // semmaphore++
  
  sem_destroy(&semaphore);
  return 0;
}

Differences compared to Mutex (互斥体)

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Simultaneous Multi-Locking

• Semaphore can be used to lock a resource at more than one Process (进程)/Thread at the same time
• For example, maximum 5 database connections & online game queueing system

Decoupled Locking & Releasing

• The locking & releasing can be decoupled into 2 different Process (进程)/Thread
• We can use this property to control how many times a Process (进程)/Thread runs (locking) based on the number of Interrupts (中断) (releasing )