C++ pthread_cond_wait/signal和mutex未按预期工作

我正在尝试学习pthread/mutex，但尽管在web上进行了大量的研究/阅读，我还是无法理解这段代码到底出了什么问题：

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <unistd.h>

struct data
{
    int Counter = 0;
    int calls = -1;
    int iteration = -1;
    pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
    pthread_cond_t condition = PTHREAD_COND_INITIALIZER;
};

void* threadAlarm (void* arg);
void* threadCounter (void* arg);

int main (void)
{
    pthread_t monThreadCounter;
    pthread_t monThreadAlarm;

    struct data mydata;

    if (pthread_create (&monThreadAlarm, NULL, threadAlarm,(void*)&mydata)>0)
        printf("Pthread Alarme error\n");
    if (pthread_create (&monThreadCounter, NULL, threadCounter, (void*)&mydata)>0)
        printf("Pthread Counter error\n");

    pthread_join (monThreadCounter, NULL);
    pthread_join (monThreadAlarm, NULL);

    return 0;
}

void* threadCounter (void *arg)
{
    struct data *myarg = (struct data *)arg;
    srand(time(NULL));

    pthread_mutex_lock (&myarg->mutex);

    while(1)
    {
        myarg->Counter += rand()%10; /* We add a random number to the counter */

        if(myarg->Counter > 20) /* If Counter is greater than 20, we should trigger the alarm*/
        {
            myarg->iteration += 1; /* Iteration counter, to check any shift between expected triggers and reality */

            printf("Counter = %i(%i)-->",myarg->Counter,myarg->iteration);

            pthread_mutex_unlock (&myarg->mutex); /* Unlock mutex before sending signal */

            if (pthread_cond_signal (&myarg->condition) >0)
            {
                printf("COND SIGNAL ERROR\n");
                pthread_exit(NULL);
            }

            usleep(10000); /* The shorter the sleep is, the weirder the output is */

            pthread_mutex_lock (&myarg->mutex); /* We should get the lock again before testing/modifying any shared variable */
        }
    }
}

void* threadAlarm (void* arg)
{
    struct data *myarg = (struct data *)arg;

    while(1)
    {
        pthread_mutex_lock(&myarg->mutex);

        //while(myarg->Counter<21) // Uneeded? Since we'll never get the lock before the Counter thread detects condition and release it
        {
            printf("\nWAITING for trigger...\n",myarg->Counter);
            if (pthread_cond_wait (&myarg->condition, &myarg->mutex)>0)
            {
                printf("ERROR COND WAIT\n");
                pthread_exit(NULL);
            }
        }

        myarg->calls+=1; // Calls counter, should be equal to iteration counter, overwise calls have been missed

        printf("ALARM TRIGGERED! Call #%i/Iteration #%i -> COUNTER RESET\n",myarg->calls, myarg->iteration);

        // Counter reset
        myarg->Counter = 0;

        pthread_mutex_unlock(&myarg->mutex);
    }
}

调用/迭代计数器是同步的，保证每次达到条件时，都能正确触发“报警”线程

但是，如果我减少睡眠，结果会变得很奇怪。在完全没有睡眠的情况下（被注释掉），我得到例如：

WAITING for trigger...
Counter = 21(57916)-->Counter = 23(57917)-->Counter = 29(57918)-->Counter = 38(57919)-->Counter = 45(57920)-->Counter = 45(57921)-->Counter = 45(57922)-->Counter = 49(57923)-->Counter = 52(57924)-->Counter = 55(57925)-->Counter = 61(57926)-->Counter = 65(57927)-->Counter = 70(57928)-->Counter = 77(57929)-->Counter = 83(57930)-->Counter = 86(57931)-->Counter = 92(57932)-->Counter = 95(57933)-->Counter = 99(57934)-->Counter = 107(57935)-->ALARM TRIGGERED! Call #4665/Iteration #57935 -> COUNTER RESET

WAITING for trigger...
Counter = 24(57936)-->Counter = 28(57937)-->Counter = 31(57938)-->Counter = 31(57939)-->Counter = 36(57940)-->Counter = 41(57941)-->Counter = 45(57942)-->Counter = 47(57943)-->Counter = 54(57944)-->Counter = 54(57945)-->Counter = 56(57946)-->Counter = 62(57947)-->Counter = 64(57948)-->Counter = 66(57949)-->Counter = 66

尽管计数器已达到触发状态，但它似乎不会触发报警线程，并继续增加，并且调用/迭代计数器完全不同步，从而证明错过了许多调用

我如何确保每次发出pthread_cond_信号时，等待的线程都被真正触发，而调用线程将等待直到被触发的线程释放互斥锁

为了以防万一，我目前正在Linux Ubuntu上编写代码

---

谢谢你的帮助。

这是意料之中的行为。一旦您向条件变量发送信号，等待的线程最终将唤醒并争夺互斥体，但不能保证在这之前发送信号的线程将无法重新获取互斥体

如果您想让计数器线程等待报警被消耗，您需要对其进行实际编程。您可以通过另一种方式使用相同的条件变量-在计数器线程中：

if (pthread_cond_signal (&myarg->condition) >0)
{
    printf("COND SIGNAL ERROR\n");
    pthread_exit(NULL);
}

pthread_mutex_lock (&myarg->mutex); /* We should get the lock again before testing/modifying any shared variable */

/* Wait for alarm to happen */
while (myarg->calls < myarg->iteration)
{
    pthread_cond_wait(&myarg->condition, &myarg->mutex);
}

if（pthread\u cond\u信号（&myarg->condition）>0）
{
printf（“COND信号错误\n”）；
pthread_exit（NULL）；
}
pthread_mutex_lock（&myarg->mutex）；/*在测试/修改任何共享变量之前，我们应该再次获得锁*/
/*等待警报发生*/
while（myarg->调用迭代）
{
pthread_cond_wait（&myarg->condition，&myarg->mutex）；
}

在报警线程中，在递增

myarg->calls

后的某个时间点调用

pthread\u cond\u signal（&myarg->condition）

---

顺便说一下，您确实需要将

while（myarg->CounterCounter

的值设置为大于20。它在报警线程有机会运行之前解锁互斥锁并向条件变量发送信号。然后报警线程运行，获取互斥锁并在

pthread\u cond\u wait（）中阻塞

-它将永远在这里等待，因为我们现在已经确保计数器线程将在继续之前等待警报被消耗

报警线程刚刚将计数器减为零，解锁互斥锁，立即在循环顶部重新锁定它，并调用

pthread\u cond\u wait（）

pthread\u cond\u wait（）

立即返回（由于允许“虚假唤醒”）在计数器线程有机会获取互斥锁之前，报警线程现在将继续，即使计数器仍然为零

---

这是工作版本，以防对其他人有用：

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <unistd.h>


struct data
{
    int Counter = 0;
    int calls = -1;
    int iteration = -1;
    pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
    pthread_cond_t condition = PTHREAD_COND_INITIALIZER;
};

void* threadAlarm (void* arg);
void* threadCounter (void* arg);

int main (void)
{
    pthread_t monThreadCounter;
    pthread_t monThreadAlarm;

    struct data mydata;

    if (pthread_create (&monThreadAlarm, NULL, threadAlarm, (void*)&mydata)>0)
        printf("Pthread Alarme error\n");
    if (pthread_create (&monThreadCounter, NULL, threadCounter, (void*)&mydata)>0)
        printf("Pthread Counter error\n");

    pthread_join (monThreadCounter, NULL);
    pthread_join (monThreadAlarm, NULL);

    return 0;
}

void* threadCounter (void *arg)
{
    struct data *myarg = (struct data *)arg;
    srand(time(NULL));

    if (pthread_mutex_lock(&myarg->mutex) > 0)
        {
            printf("ERROR Mutex lock1 Counter\n");
            pthread_exit(NULL);
        }

    while(1)
    {
        myarg->Counter += rand()%10; /* We add a random number to the counter */

        if(myarg->Counter > 20) /* If Counter is greater than 20, we should trigger the alarm*/
        {
            myarg->iteration += 1; /* Iteration counter, to check any shift between expected triggers and reality */

            printf("Counter = %i(%i)-->",myarg->Counter,myarg->iteration);

            if (pthread_mutex_unlock(&myarg->mutex) > 0) /* Unlock mutex before sending signal */
                {
                    printf("ERROR Mutex Unlock Counter\n");
                    pthread_exit(NULL);
                }

            if (pthread_cond_signal (&myarg->condition) >0)
            {
                printf("COND SIGNAL ERROR\n");
                pthread_exit(NULL);
            }

            if (pthread_mutex_lock(&myarg->mutex) > 0) /* We should get the lock again before testing/modifying any shared variable */
                {
                    printf("ERROR Mutex lock2 Counter\n");
                    pthread_exit(NULL);
                }

            /* Wait for alarm to happen */
            while (myarg->calls < myarg->iteration)
            {
                pthread_cond_wait(&myarg->condition, &myarg->mutex);
            }
        }
    }
}

void* threadAlarm (void* arg)
{
    struct data *myarg = (struct data *)arg;

    while(1)
    {
        if (pthread_mutex_lock(&myarg->mutex) > 0)
            {
                printf("ERROR Mutex lock Alarm\n");
                pthread_exit(NULL);
            }

        while(myarg->Counter<21)
        {
            printf("\nWAITING for trigger...\n");
            if (pthread_cond_wait (&myarg->condition, &myarg->mutex)>0)
            {
                printf("ERROR COND WAIT\n");
                pthread_exit(NULL);
            }
        }

        myarg->calls+=1; // Calls counter, should be equal to iteration counter, overwise calls have been missed

        printf("ALARM TRIGGERED! Call #%i/Iteration #%i -> COUNTER RESET\n",myarg->calls, myarg->iteration);

        // Counter reset
        myarg->Counter = 0;

        if (pthread_mutex_unlock(&myarg->mutex) > 0)
            {
                printf("ERROR Mutex Unlock Alarm\n");
                pthread_exit(NULL);
            }


        if (pthread_cond_signal (&myarg->condition) >0) //Signal back to Counter thread
        {
            printf("COND SIGNAL ERROR\n");
            pthread_exit(NULL);
        }

    }
}

#包括
#包括
#包括
#包括
结构数据
{
int计数器=0；
int调用=-1；
int迭代=-1；
pthread\u mutex\u t mutex=pthread\u mutex\u初始值设定项；
pthread_cond_t condition=pthread_cond_初始值设定项；
};
无效*螺纹报警（无效*参数）；
void*螺纹计数器（void*arg）；
内部主（空）
{
pthread\u t monthread计数器；
pthread\u t monthread报警；
结构数据mydata；
if（pthread_create（&monThreadAlarm，NULL，threadAlarm，（void*）&mydata）>0）
printf（“Pthread Alarme error\n”）；
if（pthread_create（&monThreadCounter，NULL，threadCounter，（void*）&mydata）>0）
printf（“Pthread Counter error\n”）；
pthread_join（monThreadCounter，NULL）；
pthread_join（monThreadAlarm，NULL）；
返回0；
}
void*threadCounter（void*arg）
{
结构数据*myarg=（结构数据*）arg；
srand（时间（空））；
如果（pthread\u mutex\u lock（&myarg->mutex）>0）
{
printf（“错误互斥锁1计数器”）；
pthread_exit（NULL）；
}
而(1)
{
myarg->Counter+=rand（）%10；/*我们向计数器添加一个随机数*/
如果（myarg->Counter>20）/*如果计数器大于20，我们应该触发警报*/
{
myarg->iteration+=1；/*迭代计数器，用于检查预期触发器和现实之间的任何转换*/
printf（“计数器=%i（%i）-->”，myarg->Counter，myarg->iteration）；
如果（pthread\u mutex\u unlock（&myarg->mutex）>0）/*在发送信号之前解锁mutex*/
{
printf（“错误互斥解锁计数器\n”）；
pthread_exit（NULL）；
}
if（pthread\u cond\u信号（&myarg->condition）>0）
{
printf（“COND信号错误\n”）；
pthread_exit（NULL）；
}
如果（pthread\u mutex\u lock（&myarg->mutex）>0）/*我们应该在测试/修改任何共享变量之前再次获得锁*/
{
printf（“错误互斥锁2计数器”）；
pthread_exit（NULL）；
}
/*等待警报发生*/
while（myarg->调用迭代）
{
pthread_cond_wait（&myarg->condition，&myarg->mutex）；
}
}
}
}
void*threadAlarm（void*arg）
{
结构数据*myarg=（结构数据*）arg；
而(1)
{
如果（pthread\u mutex\u lock（&myarg->mutex）>0）
{
printf（“错误互斥锁报警\n”）；
pthread_exit（NULL）；
}
while（myarg->Countercondition，&myarg->mutex）>0）
{
printf（“错误条件等待\n”）；
pthread_exit（NULL）；
}
}
myarg->calls+=1；//调用计数器，应等于迭代计数器，已错过过度调用
printf（“报警触发！调用#%i/迭代#%i->计数器重置\n”，myarg->调用，myarg->迭代）；
//计数器复位
myarg->计数器=0；
如果（pthread_mutex_unlock(&
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <unistd.h>


struct data
{
    int Counter = 0;
    int calls = -1;
    int iteration = -1;
    pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
    pthread_cond_t condition = PTHREAD_COND_INITIALIZER;
};

void* threadAlarm (void* arg);
void* threadCounter (void* arg);

int main (void)
{
    pthread_t monThreadCounter;
    pthread_t monThreadAlarm;

    struct data mydata;

    if (pthread_create (&monThreadAlarm, NULL, threadAlarm, (void*)&mydata)>0)
        printf("Pthread Alarme error\n");
    if (pthread_create (&monThreadCounter, NULL, threadCounter, (void*)&mydata)>0)
        printf("Pthread Counter error\n");

    pthread_join (monThreadCounter, NULL);
    pthread_join (monThreadAlarm, NULL);

    return 0;
}

void* threadCounter (void *arg)
{
    struct data *myarg = (struct data *)arg;
    srand(time(NULL));

    if (pthread_mutex_lock(&myarg->mutex) > 0)
        {
            printf("ERROR Mutex lock1 Counter\n");
            pthread_exit(NULL);
        }

    while(1)
    {
        myarg->Counter += rand()%10; /* We add a random number to the counter */

        if(myarg->Counter > 20) /* If Counter is greater than 20, we should trigger the alarm*/
        {
            myarg->iteration += 1; /* Iteration counter, to check any shift between expected triggers and reality */

            printf("Counter = %i(%i)-->",myarg->Counter,myarg->iteration);

            if (pthread_mutex_unlock(&myarg->mutex) > 0) /* Unlock mutex before sending signal */
                {
                    printf("ERROR Mutex Unlock Counter\n");
                    pthread_exit(NULL);
                }

            if (pthread_cond_signal (&myarg->condition) >0)
            {
                printf("COND SIGNAL ERROR\n");
                pthread_exit(NULL);
            }

            if (pthread_mutex_lock(&myarg->mutex) > 0) /* We should get the lock again before testing/modifying any shared variable */
                {
                    printf("ERROR Mutex lock2 Counter\n");
                    pthread_exit(NULL);
                }

            /* Wait for alarm to happen */
            while (myarg->calls < myarg->iteration)
            {
                pthread_cond_wait(&myarg->condition, &myarg->mutex);
            }
        }
    }
}

void* threadAlarm (void* arg)
{
    struct data *myarg = (struct data *)arg;

    while(1)
    {
        if (pthread_mutex_lock(&myarg->mutex) > 0)
            {
                printf("ERROR Mutex lock Alarm\n");
                pthread_exit(NULL);
            }

        while(myarg->Counter<21)
        {
            printf("\nWAITING for trigger...\n");
            if (pthread_cond_wait (&myarg->condition, &myarg->mutex)>0)
            {
                printf("ERROR COND WAIT\n");
                pthread_exit(NULL);
            }
        }

        myarg->calls+=1; // Calls counter, should be equal to iteration counter, overwise calls have been missed

        printf("ALARM TRIGGERED! Call #%i/Iteration #%i -> COUNTER RESET\n",myarg->calls, myarg->iteration);

        // Counter reset
        myarg->Counter = 0;

        if (pthread_mutex_unlock(&myarg->mutex) > 0)
            {
                printf("ERROR Mutex Unlock Alarm\n");
                pthread_exit(NULL);
            }


        if (pthread_cond_signal (&myarg->condition) >0) //Signal back to Counter thread
        {
            printf("COND SIGNAL ERROR\n");
            pthread_exit(NULL);
        }

    }
}