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一 原理

1. 和共享内存/消息队列一样都属于system v版本的进程间通信，接口大同小异。

2. 还是和之前的共享内存/消息队列一样，通过形成的key_t并创建一个信号量集，一个信号量集里面可以有多个信号量，并初始化信号量集合，后续对集合中某个特定的元素进行操作。

3. 信号量主要用来计数器，信号量++，--操作是原子的，自带同步机制，当某个信号量元素为0则阻塞/其他时间，阻塞有资源则就唤醒等。

二 接口

1. semget

#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>int semget(key_t key,   // ftok的返回值 int nsems,   // 信号量集合里的信号量的个数int semflg   // 怎么创建/权限是什么);

2. semctl

#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>int semctl(int semid,    // semget的返回值int semnum,   // 信号量集中某个元素的小标 , 0 ~ nint cmd,      // 怎么操作这个对应的下标的元素 ...           // 元素类型 -> 看下面);union semun
{int val;               /* Value for SETVAL */struct semid_ds *buf;  /* Buffer for IPC_STAT, IPC_SET */unsigned short *array; /* Array for GETALL, SETALL */struct seminfo *__buf; /* Buffer for IPC_INFO(Linux-specific) */
}

3. semop

#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>int semop(int semid,            // semget的返回值struct sembuf *sops,  // 信号集里面的某个元素size_t nsops          // 信号集里元素的个数);struct sembuf{unsigned short sem_num;  // 元素的下标short          sem_op;   // 怎么操作short          sem_flg;  // 不满足条件触发的功能}

三 二元信号量的demo代码 

#pragma once
#include <iostream>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <vector>const std::string key_path = "/home/CD/linux/Semaphore";
const int key_val = 123;int Create = IPC_CREAT | IPC_EXCL | 0666;
int User = IPC_CREAT;// 转16进制
void To_Hex(int val)
{char buff[1024] = {0};snprintf(buff, sizeof(buff), "0x%x", val);std::cout << buff << std::endl;return;
}// 获取 key_t
key_t Getkey(const std::string &mypath, int myproj)
{key_t key = ftok(mypath.c_str(), myproj);return key;
}class Semaphore
{
public:Semaphore(){create_key();}// 释放信号量~Semaphore(){if (_semid != -1){if (semop(_semid, 0, IPC_RMID) == -1){std::cout << " delete Semaphore failed" << std::endl;exit(3);}}}void P(){// 设置某个信号量元素的 -- 操作sembuf sem;sem.sem_num = 0;sem.sem_op = -1;int n = semop(_semid, &sem, _nums);if (n == -1){std::cout << "P semop set failed" << std::endl;exit(2);}}void V(){// 设置某个信号量元素的 ++ 操作sembuf sem;sem.sem_num = 0;sem.sem_op = 1;int n = semop(_semid, &sem, _nums);if (n == -1){std::cout << "V semop set failed" << std::endl;exit(2);}}// 创建/获取信号量     信号量集合的个数  初始值void identity(int flag, int nums, int val = -1){_nums = nums;if (flag == IPC_CREAT | IPC_EXCL | 0666){_semid = semget(_key, nums, flag);if (_semid == -1){std::cout << "msgget failed" << std::endl;exit(2);}std::cout << "create sem success" << std::endl;sem_init_nums(_nums, val);}else if (flag == IPC_CREAT){_semid = semget(_key, nums, flag);if (_semid == -1){std::cout << "msgid get failed" << std::endl;exit(2);}}}private:// 获取key_tvoid create_key(){_key = Getkey(key_path, key_val);if (_key == -1){std::cout << "key_t failed" << std::endl;exit(1);}std::cout << "key_t success" << std::endl;}// 初始化信号量集合的每个元素的初始值void sem_init_nums(int nums, int val){if (_nums <= 0){std::cout << "_nums<=0" << std::endl;exit(3);}union semun{int val;               struct semid_ds *buf;  unsigned short *array; struct seminfo *__buf; } sem;sem.val = val;for (int i = 0; i < _nums; i++){semctl(_semid, i, SETVAL, sem);}}private:key_t _key;int _semid;int _nums;
};