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关于信号量和共享内存的相关知识可参考下面链接：

进程间通信方式介绍_夜雨听萧瑟的博客-CSDN博客

C++ 创建共享内存_c共享内存_夜雨听萧瑟的博客-CSDN博客

 信号量SytemV与Posix信号量的介绍与用法_夜雨听萧瑟的博客-CSDN博客

直接上代码，代码如下：

#include <iostream>
#include <string>
#include <unistd.h>
#include <errno.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <sys/shm.h>
#include <string.h>
#define SHARE_MEM_SIZE 2048
class Semaphore
{
private:union semVar{int val;struct semid_ds *buf;unsigned short *array;};int sem_id;
public:bool init(key_t key);bool wait();bool post();bool destroy();
};bool Semaphore::init(key_t key)
{sem_id = semget(key,0,0640);if(-1 == sem_id){if(2 == errno){sem_id = semget(key,1,0640|IPC_CREAT);if(-1 == sem_id){std::cout << "init 1 semget() error" << std::endl;return false;}else{union semVar semTmp;semTmp.val = 1;if(semctl(sem_id,0,SETVAL,semTmp) < 0){std::cout << "init 1 semctl() error" << std::endl;return false;}else{return true;}}}else{std::cout << "init 2 semget() error" << std::endl;return false;}}else{return true;}}bool Semaphore::wait(){struct sembuf sem_b;sem_b.sem_num = 0;sem_b.sem_op = -1;sem_b.sem_flg = SEM_UNDO;if(-1 == semop(sem_id,&sem_b,1)){std::cout << "wait semop failed." << std::endl;return false;}return true;
}bool Semaphore::post()
{struct sembuf sem_b;sem_b.sem_num = 0;sem_b.sem_op = 1;sem_b.sem_flg = SEM_UNDO;if(-1 == semop(sem_id,&sem_b,1)){std::cout << "post semop failed." << std::endl;return false;}return true;
}bool Semaphore::destroy()
{if(semctl(sem_id,0,IPC_RMID) == -1){std::cout << "destroy semctl failed." << std::endl;return false;}return true;
}int main()
{Semaphore sem;//初始化信号灯if(false == sem.init(0x5000)){std::cout << "sem init failed." << std::endl;return -1;}std::cout << "sem init ok." << std::endl;int shmid = 0;  //内存标识符//创建共享内存shmid = shmget((key_t)0x5005,SHARE_MEM_SIZE,0640|IPC_CREAT);if(-1 == shmid){std::cout << "create shareMem failed." << std::endl;return -1;}char* pMemSharedMem = 0;/////等待信号灯挂出，等待成功后，将持有锁if(false == sem.wait()){std::cout << "sem wait failed." << std::endl;return -1;}std::cout << "sem wait ok." << std::endl;sleep(10);//将当前进程与共享内存shmid建立链接,shmat返回指定共享内存的映射地址pMemSharedMem = (char*)shmat(shmid,0,0);std::cout << "read context: " << pMemSharedMem << std::endl;std::string strContext = "hello world, "+ std::to_string(getpid());strncpy(pMemSharedMem,strContext.c_str(),strContext.length());std::cout << "write after: " << pMemSharedMem << std::endl;//shmat的反操作,将共享内存与当前进程分离shmdt(pMemSharedMem);//挂出信号灯if(false == sem.post()){std::cout << "sem post failed." << std::endl;return -1;}std::cout << "sem post ok." << std::endl;//销毁信号灯//if(false == sem.destroy())//{//    std::cout << "sem destroy failed." << std::endl;//    return -1;//}//std::cout << "sem destroy ok." << std::endl;return 0;
}

同时运行3个进程，运行结果如下：