使用system V实现读者写者问题

#include <stdio.h>
#include <sys/sem.h>
#include <sys/ipc.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>
#include <pthread.h>
#include <sys/types.h>
#include <sys/wait.h>

typedef int semophore;
semophore mutex = 0;
semophore cnt = 0;
semophore db = 1;
//char mutex[] = "mutex";
//char db[] = "db";
int semId;
void read(int cnt)
{
    fprintf(stdout, "NO.%d reading...
",cnt);
    //for (int i = 0; i < 1000000; i++);
    sleep(1);
    fprintf(stdout, "NO.%d read finished.
",cnt);

}
void write()
{
    fprintf(stdout, "writing...
");
    //for (int i = 0; i < 1000000; i++);
    sleep(1);
    fprintf(stdout, "write finished.
");
}

union semun {
    int val;
    struct semid_ds *buf;
    unsigned short int *array;
    struct seminfo *__buf;
};
/*
struct  sembuf{                     //结构定义在 linux/sem.h，无需重复定义
    unsigned short   sem_num;     //信号在信号集中的索引，0代表第一个信号，1代表第二个信号  
    short          sem_op;     //操作类型
    short          sem_flg;   //操作标志
};
*/

/*
void p(const char *s)
{
    /*
    semun arg;
    int tmp;
    int r;
    if (strcmp(s, mutex) == 0)
    {
        //fprintf(stdout,"mutex:%d
",semctl(semId, 0, GETVAL, arg));
        while(1){
            if(semctl(semId, 0, GETVAL, arg)<=0)
                wait(&r);
            else break;
        }
        tmp = semctl(semId, 0, GETVAL, arg);
        //if(tmp<=0) waitpid();
        arg.val = tmp - 1;
        semctl(semId, 0, SETVAL, arg);
    }
    else if (strcmp(s, db) == 0)
    {
        //fprintf(stdout,"db:%d
",semctl(semId, 1, GETVAL, arg));
        while(1){
            if(semctl(semId, 1, GETVAL, arg)<=0)
                wait(&r);
            else break;
        }
        tmp = semctl(semId, 1, GETVAL, arg);
        arg.val = tmp - 1;
        semctl(semId, 1, SETVAL, arg);
    }
    
    sembuf arg;
    arg.sem_op = 1;
    arg.sem_flg = IPC_UNDO;
    if(strcmp(s, mutex)==0)
    {
        arg.sem_num = 0;
        semop(semId,&arg,1);
    }
    else if(strcmp(s, db)==0)
    {
        arg.sem_num = 1;
        semop(semId,&arg,1);
    }

}
*/
/*
void v(const char *s)
{
    /*
    semun arg;
    int tmp;
    if (strcmp(s, mutex) == 0)
    {
        tmp = semctl(semId, 0, GETVAL, arg);
        arg.val = tmp + 1;
        semctl(semId, 0, SETVAL, arg);
    }
    else if (strcmp(s, db) == 0)
    {
        tmp = semctl(semId, 1, GETVAL, arg);
        arg.val = tmp + 1;
        semctl(semId, 1, SETVAL, arg);
    }
    
    sembuf arg;
    arg.sem_op = -1;
    arg.sem_flg = IPC_UNDO;
    if(strcmp(s, mutex)==0)
    {    
        arg.sem_num = 0;
        semop(semId,&arg,1);
    }
    else if(strcmp(s, db)==0)
    {
        arg.sem_num = 1;
        semop(semId,&arg,1);
    }
}
*/

//P操作函数
int p(int index)
{
    struct sembuf buf = {0, -1};

    if (index < 0)
    {
        perror("index of array cannot equals a minus value!
");
        return -1;
    }
    buf.sem_num = index;
    if (semop(semId, &buf, 1) == -1)
    {
        perror(" a wrong operation to semaphore occurred!
");
        return -1;
    }
    return 0;
}

//V操作函数
int v(int index)
{
    struct sembuf buf = {0, 1};

    if (index < 0)
    {
        perror("index of array cannot equals a minus value!
");
        return -1;
    }
    buf.sem_num = index;
    if (semop(semId, &buf, 1) == -1)
    {
        perror(" a wrong operation to semaphore occurred!
");
        return -1;
    }
    return 0;
}
void* reader(void* args)
{
    p(mutex);
    cnt++;
    if (cnt == 1)
    {
        p(db);
    }
    v(mutex);

    read(cnt);

    p(mutex);
    cnt--;
    if (cnt == 0)
    {
        v(db);
    }
    v(mutex);
}
void* writer(void* args)
{
    p(db);
    write();
    v(db);
}

int main()
{
    semun arg;

    key_t key = 1234;
    if ((key = ftok(".", 11)) == -1)
    {
        perror("ftok error:");
        _exit(1);
    }
    semctl(semId, 0, IPC_RMID, arg);

    //semid = semget(key, 1, IPC_CREAT|0660);

    if ((semId = semget(key, 2, IPC_CREAT | IPC_EXCL | 0666)) >= 0)
    {
        arg.val = 1;
        if (semctl(semId, 0, SETVAL, arg) < 0)
        {
            fprintf(stdout, "semctl error %s
", strerror(errno));
            return -1;
        }
        if (semctl(semId, 1, SETVAL, arg) < 0)
        {
            fprintf(stdout, "semctl error %s
", strerror(errno));
            return -1;
        }

    }
    else if (errno == EEXIST)
    {
        semId = semget(key, 2, 0666);
        //fprintf(stdout,"Taskr :: errno==EEXIST
");
        arg.val = 1;
        if (semctl(semId, 0, SETVAL, arg) < 0)
        {
            fprintf(stdout, "semctl error %s
", strerror(errno));
            return -1;
        }
        if (semctl(semId, 1, SETVAL, arg) < 0)
        {
            fprintf(stdout, "semctl error %s
", strerror(errno));
            return -1;
        }
    }
    else
    {
        fprintf(stdout, "semget error %s
", strerror(errno));
        return -1;
    }
    pthread_t tids[5];
    for(int i = 0; i < 5; ++i)
    {
        
        //参数依次是：创建的线程id，线程参数，调用的函数，传入的函数参数
        if(i==0 || i==2 || i==4)
        {
            fprintf(stdout,"turn NO.%d want to read.
",i);
            int ret = pthread_create(&tids[i], NULL, reader, NULL);
            if (ret != 0)
            {
                fprintf(stdout, "reading error
");
            }
        }
        else{
            fprintf(stdout,"turn NO.%d want to write.
",i);
            int ret = pthread_create(&tids[i], NULL, writer, NULL);
            if (ret != 0)
            {
                fprintf(stdout, "writing error
");
            }
        }
    }

    pthread_exit(NULL);
    return 0;
}

结果如图（读者优先）：

非常感谢 @神一城 老师的指点，之前自己没有学清楚 system V 中关于信号量的内容，使用了 semctl 这种直接赋值的操作，其实这样和直接使用 int 变量再加一些条件判断实现一样，而这样就等同于没有原子性，完全没有体现出信号量的作用，只是把信号量当作了一个普通变量。

这里向之前受到错误博客内容误导的朋友道歉。