【Linux】Semaphore信号量线程同步的例子-优快云博客

0、信号量

Linux下的信号量和windows下的信号量稍有不同。

Windows

Windows下的信号量有一个最大值和一个初始值，初始值和最大值可以不同。而且Windows下的信号量是一个【内核对象】，在整个OS都可以访问到。

Linux

Linux下的信号量在创建的时候可以指定一个初始值，这个初始值也是最大值。而且Linux下的信号量可以根据需要设置为是否是【进程间共享】的，如果不是进程间共享的则就是一个本进程局部信号量。

1、相关API

int semt_init( semt_t* sem,     //a semaphore pointer
 int     pshared, //0 as a local semaphore of cuurent process, or the semaphore can be shared between mulit processes
 unsigned value   //the init value of this memaphore
 ) //minus ONE value of semaphore
int sem_wait(sem_t* sem); //add ONE value of semaphore
int sem_post(sem_t* sem); //destroy the semaphore
int sem_destroy(sem_t* sem); All the functions above Rerurn ZERO IF SUCCESS !

2、上代码

这个demo创建了5个线程，信号量的初始值为2，即同时最多有2个线程可以获得获得信号量从而得到执行。

#include <iostream> #include <pthread.h> #include <unistd.h> #include <semaphore.h>
using namespace std; sem_t g_semt; void* work_thread(void* p) { pthread_t tID = pthread_self(); cout << "-------" << tID << " is waiting for a semaphore -------" << endl; sem_wait(&g_semt); cout << "-------" << tID << " got a semaphore, is Runing -------" << endl << endl; usleep(1000 * 1000 * 2);  //2 seconds
    sem_post(&g_semt); static char* pRet = "thread finished! \n"; return pRet; } int main() { const size_t nThreadCount = 5; //amounts of thread array
    const unsigned int nSemaphoreCount = 2; //initial value of semaphore
    int nRet = -1; void* pRet = NULL; pthread_t threadIDs[nThreadCount] = {0}; nRet = sem_init(&g_semt, 0, nSemaphoreCount); if (0 != nRet) return -1; for (size_t i = 0; i < nThreadCount; ++ i) { nRet = pthread_create(&threadIDs[i], NULL, work_thread, NULL); if (0 != nRet) continue; } for (size_t i = 0; i < nThreadCount; ++ i) { int nRet2 = pthread_join(threadIDs[i], &pRet); cout << endl << threadIDs[i] << " return value is " << (char*)pRet << endl; } cout << endl << endl; sem_destroy(&g_semt); return 0; }