用SysV计数信号量

Question

我试图通过一个例子来理解计数信号量的概念。但是我想在Linux中使用SysV来实现这一点。

我熟悉二进制信号量和计数信号量的理论部分。

我已经提到了这个链接。

从概念上讲，信号量被用作从一个进程到另一个进程的信号机制，因此我试图编写一个简单的程序。

在下面的程序中，我希望thread_1等待，直到它没有从thread_2得到一个信号，类似地，thread_2应该等到它没有从thread_3得到一个信号。

所以输出应该如下所示：Hello From thread 3 Hello from thread 2 Hello from thread 1

我知道可以正确地使用pthread_join()来实现它，但是我想使用信号量来实现它。

代码：

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

int sem_id;
struct sembuf sops[3];
void thread_1(void)
{
    sops[0].sem_num = 0;
    sops[0].sem_op = 0;
    sops[0].sem_flg = 0;

    if(semop(sem_id, sops, 1) < 0)
        perror("Semop In thread 3");
    else
        printf("Hello From thread 1\n");
}

void thread_2(void)
{
    sops[0].sem_num = 0;
    sops[0].sem_op = -1;
    sops[0].sem_flg = 0;

    if(semop(sem_id, sops, 1) < 0)
        perror("Semop In thread 2");
    else
        printf("Hello from thread 2\n");
}

void thread_3(void)
{

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

    if(semop(sem_id, sops, 1) < 0)
        perror("Semop In thread 3");
    else
        printf("Hello from thread 3\n");
}

int main(void)
{
    void (*funct[]) = {thread_1, thread_2, thread_3};

    key_t semkey;
    char i;
    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) */
           }arg;
    pthread_t thread_id[3];

    semkey = ftok("/tmp", 'a');

    if(semkey < 0)
        perror("Cannot Create Semaphore Key");
    else
    {
        sem_id = semget(semkey, 1, (IPC_CREAT|IPC_EXCL|0666));
        if(sem_id < 0)
            perror("Cannot create semaphore\n");
        else
        {
            arg.val = 3;
            if (semctl(sem_id, 0, SETVAL, arg) == -1) {
                perror("semctl");
                exit(1);
             }   
        }
    }

    for(i = 0; i < 3; i++)
    {
        if(pthread_create(&thread_id[i], NULL, funct[i], NULL) < 0)
            perror("Cannot Create thread\n");
    }

   for(i = 0; i < 3; i++)
        pthread_join(thread_id[i], NULL);

    if(semctl(sem_id, 0, IPC_RMID, NULL) == -1)
        perror("semctl");

    return 0;
}

我是否必须使用多个信号量集来实现我所要做的事情？

Answer 1

您需要两个带有计数1的互斥/信号量。假设您的线程名为t0,t1,t2，您的信号量为sem0和sem1，那么t0可以自由运行并递增sem0，t1等待sem0和增量sem1，t2等待sem1。

这里有一个完整的草案，没有错误检查：

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

static int sem0, sem1;
#define POST1(Sem) semop(Sem, &(struct sembuf){0,1,0}, 1)
#define WAIT1(Sem) semop(Sem, &(struct sembuf){0,-1,0}, 1)

static void* t0(void *unused) { puts("hello from t0"); POST1(sem0); return 0; }
static void* t1(void *unused) { WAIT1(sem0); puts("hello from t1"); POST1(sem1); return 0; }
static void* t2(void *unused) { WAIT1(sem1); puts("hello from t2"); return 0; }

int main(void)
{
    key_t sem0_k, sem1_k;
    sem0_k = ftok("/tmp", '0');
    sem1_k = ftok("/tmp", '1');

    sem0 = semget(sem0_k, 1, (IPC_CREAT|IPC_EXCL|0666));
    sem1 = semget(sem1_k, 1, (IPC_CREAT|IPC_EXCL|0666));

    pthread_t tids[3];
    pthread_create(tids+2, NULL, t2, NULL);
    sleep(1);
    pthread_create(tids+1, NULL, t1, NULL);
    sleep(1);
    pthread_create(tids+0, NULL, t0, NULL);

    for(int i = 0; i < 3; i++)
        pthread_join(tids[i], NULL);

    semctl(sem0, 0, IPC_RMID, NULL);
    semctl(sem1, 0, IPC_RMID, NULL);

    return 0;
}

我以反向顺序运行线程，在t0和t1之间等待1秒，t1和t2显示信号量，完成将线程从t0排序到t2的工作。

Answer 2

@PSkocik，基于您的回答，我修改了我的代码，使用了一组两个信号量。以下是代码：

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

int sem_id;

struct sembuf sops;
void thread_1(void)
{
    /*Wait on Set1 of Semaphore*/
    sops.sem_num = 1;
    sops.sem_op = -1;
    sops.sem_flg = 0;

    if(semop(sem_id, &sops, 1) < 0)
        perror("Semop Wait In thread 3");
    else
        printf("Hello From thread 1\n");
}


void thread_2(void)
{
    /*Wait on Set0 of Semaphore*/
    sops.sem_num = 0;
    sops.sem_op = -1;
    sops.sem_flg = 0;

    if(semop(sem_id, &sops, 1) < 0)
        perror("Semop Wait In thread 2");
    else
        printf("Hello from thread 2\n");

    /*Post on Set1 of Semaphore*/        
    sops.sem_num = 1;
    sops.sem_op = 1;
    sops.sem_flg = 0;

    if(semop(sem_id, &sops, 1) < 0)
        perror("Semop Post In thread 2");
}

void thread_3(void)
{
    printf("Hello from thread 3\n");

    /*Post operation on Set0 of semaphore*/
    sops.sem_num = 0;
    sops.sem_op = 1;
    sops.sem_flg = 0;

    if(semop(sem_id, &sops, 1) < 0)
        perror("Semop In thread 3");
    else
    { ; }
}

int main(void)
{
    void (*funct[]) = {thread_1, thread_2, thread_3};

    key_t semkey;
    char i;
    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) */
           }arg;
    pthread_t thread_id[3];

    semkey = ftok("/tmp", 'a');

    if(semkey < 0)
        perror("Cannot Create Semaphore Key");
    else
    {
        sem_id = semget(semkey, 2, (IPC_CREAT|IPC_EXCL|0666));
        if(sem_id < 0)
            perror("Cannot create semaphore\n");
        else
        {
            /*arg.val = 3;
            if (semctl(sem_id, 0, SETVAL, arg) == -1) {
                perror("semctl");
                exit(1);
             }*/   
        }
    }

    for(i = 0; i < 3; i++)
    {
        if(pthread_create(&thread_id[i], NULL, funct[i], NULL) < 0)
            perror("Cannot Create thread\n");
    }

   for(i = 0; i < 3; i++)
        pthread_join(thread_id[i], NULL);

    if(semctl(sem_id, 0, IPC_RMID, NULL) == -1)
        perror("semctl");

    return 0;
}

顺便谢谢你的解释。