IPC之信号量

信号量
* 2种类型
  - 二值信号量（取值为0或1的信号量，常用于进程间的"互斥锁"）
  - 计数信号量（如生产者消费者问题等）
* 2个操作
  - P：等待信号量>0，然后将信号量--；
  - V：信号量++，如果有则唤起一个等待信号量>0的进程

数据结构semid_ds（#include<sys/sem.h>）

struct semid_ds {truct ipc_perm sem_perm; /* operation permission struct */struct sem *sem_base;    /* ptr to first semaphore in set */ushort sem_nsems;        /* # of semaphores in set */time_t sem_otime;        /* last semop time */time_t sem_ctime;        /* last change time */};struct ipc_perm {    key_t key;            /* Key supplied to semget() */    uid_t uid;            /* Effective UID of owner */    gid_t gid;            /* Effective GID of owner */    uid_t cuid;           /* Effective UID of creator */    gid_t cgid;           /* Effective GID of creator */    unsigned short mode;  /* Permissions */    unsigned short seq;   /* Sequence number */};struct sem {ushort semval;   /* semaphore text map address */short sempid;    /* pid of last operation */ushort semncnt;  /* # awaiting semval > cval */ushort semzcnt;  /* # awaiting semval = 0 */};

相关函数

#include <sys/sem.h>// 创建或打开一个信号量int semget(key_t key, int num_sems, int sem_flags);// PV操作改变信号量的值int semop(int sem_id, struct sembuf *sem_ops, size_t num_sem_ops);// 控制操作，如删除等int semctl(int sem_id, int sem_num, int command, ...);

使用例子
[P操作：SemP.cpp，循环 { P操作 } ]

#include <iostream>#include <string.h>#include <sys/types.h>#include <sys/ipc.h>#include <sys/sem.h>using namespace std;int main(){    key_t iKey;    int iSemId;    // Get Sem Key    const char *pSemPath = "/data/home/andyawang/code/ipc/Semaphore";    if ((iKey = ftok(pSemPath, 1)) == -1) {        cout << "Ftok Err" << endl;        return -1;    }    // 1.semget    if ((iSemId = semget(iKey, 1, IPC_CREAT|0666)) == -1) {        cout << "Semget Err" << endl;        return -1;    }    // 2.semop(P,--)    for(int i = 0; i < 5; i ++)    {        struct sembuf sSem;        sSem.sem_num = 0;        sSem.sem_op = -1;        sSem.sem_flg = 0;        if (semop(iSemId, &sSem, 1) == -1) {            cout << "Semop Err" << endl;            return -1;        }        cout << "Semop P Suc" << endl;    }    // 3.semctl    if (semctl(iSemId, 0, IPC_RMID) == -1) {        cout << "Semctl Err" << endl;        return -1;    }    return 0;}

[V操作：SemV.cpp，进行一次V操作 ]

#include <iostream>#include <string.h>#include <sys/types.h>#include <sys/ipc.h>#include <sys/sem.h>using namespace std;int main(){    key_t iKey;    int iSemId;    // Get Sem Key    const char *pSemPath = "/data/home/andyawang/code/ipc/Semaphore";    if ((iKey = ftok(pSemPath, 1)) == -1) {        cout << "Ftok Err" << endl;        return -1;    }    // 1.semget    if ((iSemId = semget(iKey, 1, IPC_CREAT|0666)) == -1) {        cout << "Semget Err" << endl;        return -1;    }    // 2.semop(V,++)    struct sembuf sSem;    sSem.sem_num = 0;    sSem.sem_op = 1;    sSem.sem_flg = 0;    if (semop(iSemId, &sSem, 1) == -1) {        cout << "Semop Err" << endl;        return -1;    }    cout << "Semop V Suc" << endl;    return 0;}