【linux高级程序设计】（第十一章）System V进程间通信 3

信号量通信机制

可以看到，跟消息队列类似，也是包括两个结构。

int semget (key_t __key, int __nsems, int __semflg) : 创建信号量集合

　　第一个参数：ftok产生的key值

　　第二个参数：创建的信号量个数

　　第三个参数：权限信息

创建信号量例子：

#include<stdio.h>
#include<unistd.h>
#include<stdlib.h>
#include<sys/ipc.h>
#include<sys/sem.h>
#include<sys/types.h>

int main(int argc, char * argv[])
{
    int sem;
    key_t key;
    if((key = ftok(".",'B'))==-1)
    {
        perror("ftok");
        exit(EXIT_FAILURE);
    }
    if((sem = semget(key, 3, IPC_CREAT|0770)) == -1)
    {
        perror("semget");
        exit(EXIT_FAILURE);
    }
    printf("sem1 id is: %d
", sem);
    semctl(sem, 0, IPC_RMID, (struct msquid_ds*)0);  //删除信号量集合
    return 0;
}

int semctl (int __semid, int __semnum, int __cmd, ...) ：控制信号量集合、信号量

　　第一个参数：要操作的信号量集合标识

　　第二个参数：信号量编号。如果操作整个信号量集合无意义；如果是某个信号量，为信号量下标。

　　第三个参数：执行的操作。 如果对整个集合操作，则包括：IPC_RMID, IPC_SET, IPC_STAT, IPC_INFO

　　　　　　　　　　　　　　如果是对某个或某些信号量操作，则包括GETPID、GETVAL、GETALL、GETNCNT、GETZCNT、SETVAL、SETALL.具体见图

设置读取信号量的例子

#include<stdio.h>
#include<sys/types.h>
#include<sys/sem.h>
#include<errno.h>
#define MAX_SEMAPHORES 5
int main(int argc, char *argv[])
{
    int i, ret, semid;
    unsigned short sem_array[MAX_SEMAPHORES];
    unsigned short sem_read_array[MAX_SEMAPHORES];
    union senmun
    {
        int val;
        struct semid_ds *buf;
        unsigned short *array;
    }arg;
    //创建信号量集合
    semid = semget(IPC_PRIVATE, MAX_SEMAPHORES, IPC_CREAT|0666);
    if(semid != -1)
    {
        //初始化
        for(i = 0; i < MAX_SEMAPHORES; i++)
            sem_array[i] = (unsigned short)(i + 1);
        arg.array = sem_array;
        //设置所有信号量的值
        ret = semctl(semid, 0, SETALL, arg);
        if(ret == -1)
            printf("SETALL failed (%d)
", errno);
        
        arg.array = sem_read_array;
        //获取所有信号量的值
        ret = semctl(semid, 0, GETALL, arg); //获取所有信号量的值
        if(ret == -1)
            printf("GETALL failed (%d)
", errno);
        
        for(i = 0; i < MAX_SEMAPHORES; i++)
            printf("Semaphore %d, value %d
", i, sem_read_array[i]);
        
        //逐次获取信号量值
        for(i = 0; i < MAX_SEMAPHORES; i++)
        {
            ret = semctl(semid, i, GETVAL);
            printf("Semaphore %d, value %d
", i, sem_read_array[i]);
        }
        //删除信号量集合
        ret = semctl(semid, 0, IPC_RMID);
    }
    else
    {
        printf("Could not allocate semaphore (%d)
", errno);
    }
    return 0;
}

int semop (int __semid, struct sembuf *__sops, size_t __nsops) ：操作信号量

第一个参数：要操作的信号量集合ID

第二个参数

struct sembug{
    unsigned short sem_num;
    short sem_op;
    short sem_flg;
};

• sem_num:操作的信号量编号
• sem_op:用于信号量的操作，正数表示增加信号量的值；负数表示减小信号量的值；0表示测试当前值是否为0
• sem_flg:操作标识，IPC_NOWAIT,如果等待立即返回；SEM_UNDO,当进程退出后，该进程对sem进行的操作将被撤销。

下面的代码，对信号量1做P操作(get)，对信号量2做V操作(release)

struct sembuf sops[4];
sops[0].sem_num = 1;
sops[0].sem_op = -1;
sops[0].sem_flg = 0;

sops[1].sem_num = 2;
sops[1].sem_op = 1;
sops[1].sem_flg = 0;

semop(mysemid, sops, 2)

信号量解决生产者消费者问题，设有100个空位的缓冲池。

下面代码，经验证可用。

生产者代码：

#include<unistd.h>
#include<stdlib.h>
#include<stdio.h>
#include<sys/types.h>
#include<sys/ipc.h>
#include<sys/sem.h>
#include<errno.h>
int sem_id;
void init()
{
    key_t key;
    int ret;
    unsigned short sem_array[2];
    union semun
    {
        int val;
        struct semid_ds *buf;
        unsigned short *array;
    }arg;
    key = ftok(".",'s');
    sem_id = semget(key, 2, IPC_CREAT|0644);
    sem_array[0] = 0;   //0号下标 生产者
    sem_array[1] = 100;
    arg.array = sem_array;
    ret = semctl(sem_id, 0, SETALL, arg);  //初始化
    if(ret == -1)
        printf("SETALL failed (%d)
", errno);
    printf("productor init is %d
", semctl(sem_id, 0, GETVAL));
    printf("space init is %d

", semctl(sem_id, 1, GETVAL));
}
void del()
{
    semctl(sem_id, 0, IPC_RMID);
}
int main(int argc, char *argv[])
{
    struct sembuf sops[2];

    sops[0].sem_num = 0;
    sops[0].sem_op = 1;          //生产者数量加一
    sops[0].sem_flg = 0;
    
    sops[1].sem_num = 1;
    sops[1].sem_op = -1;         //消费者数量减一
    sops[1].sem_flg = 0;
    
    init();
    printf("this is productor
");
    while(1)
    {
        printf("

before produce:
");
        printf("productor number is %d
", semctl(sem_id, 0, GETVAL));
        printf("space init is %d

", semctl(sem_id, 1, GETVAL));
        semop(sem_id, (struct sembuf *)&sops[1], 1);  //先预定一个空位
        printf("now producing.....
");
        semop(sem_id,(struct sembuf *)&sops[0], 1); //生产出一个产品 释放一个产品
        printf("
after produce:
");
        printf("productor number is %d
", semctl(sem_id, 0, GETVAL));
        printf("space number is %d
", semctl(sem_id, 1, GETVAL));
        sleep(4);
    }
    del();
}

消费者代码：

#include<unistd.h>
#include<stdlib.h>
#include<stdio.h>
#include<sys/types.h>
#include<sys/ipc.h>
#include<sys/sem.h>
#include<errno.h>
int sem_id;
void init()
{
    key_t key;
    key = ftok(".", 's');
    sem_id = semget(key, 2, IPC_CREAT|0644);
}
int main(int argc, char *argv[])
{
    init();
    struct sembuf sops[2];

    sops[0].sem_num = 0;
    sops[0].sem_op = -1;          //产品数量减一
    sops[0].sem_flg = 0;
    
    sops[1].sem_num = 1;
    sops[1].sem_op = 1;         //空位数量加一
    sops[1].sem_flg = 0;
    printf("this is customer
");
    
    while(1)
    {
        printf("

before consume:
");
        printf("productor is %d
", semctl(sem_id, 0, GETVAL));
        printf("space is %d
", semctl(sem_id, 1, GETVAL));
        semop(sem_id, (struct sembuf *)&sops[0], 1); //预定一个要消费的产品
        printf("now consuming.....
");
        semop(sem_id,(struct sembuf *)&sops[1], 1); //增加一个空位
        printf("
after consume:
");
        printf("productor is %d
", semctl(sem_id, 0, GETVAL));
        printf("space is %d
", semctl(sem_id, 1, GETVAL));
        sleep(3);
    }
}