在进程之间共享条件变量和互斥：是否必须先锁定互斥锁？

我需要一些帮助来了解如何在C中使用条件变量来解决练习。 这是一个小例子：

#include  #include  #include  #include  #include  #include  #include  #include  #include  #include  #define OKTOWRITE "/oktowrite" #define MESSAGE "/message" #define MUTEX "/lock" int main(int argc, char** argv) { pthread_cond_t* condition; pthread_mutex_t *mutex; char* message; int des_cond, des_msg, des_mutex; int mode = S_IRWXU | S_IRWXG; des_mutex = shm_open(MUTEX, O_CREAT | O_RDWR | O_TRUNC, mode); if (des_mutex < 0) { perror("failure on shm_open on des_mutex"); exit(1); } if (ftruncate(des_mutex, sizeof(pthread_mutex_t)) == -1) { perror("Error on ftruncate to sizeof pthread_cond_t\n"); exit(-1); } mutex = (pthread_mutex_t*) mmap(NULL, sizeof(pthread_mutex_t), PROT_READ | PROT_WRITE, MAP_SHARED, des_mutex, 0); if (mutex == MAP_FAILED ) { perror("Error on mmap on mutex\n"); exit(1); } pthread_mutex_init(mutex, NULL ); des_cond = shm_open(OKTOWRITE, O_CREAT | O_RDWR | O_TRUNC, mode); if (des_cond < 0) { perror("failure on shm_open on des_cond"); exit(1); } if (ftruncate(des_cond, sizeof(pthread_cond_t)) == -1) { perror("Error on ftruncate to sizeof pthread_cond_t\n"); exit(-1); } condition = (pthread_cond_t*) mmap(NULL, sizeof(pthread_cond_t), PROT_READ | PROT_WRITE, MAP_SHARED, des_cond, 0); if (condition == MAP_FAILED ) { perror("Error on mmap on condition\n"); exit(1); } pthread_cond_init(condition, NULL ); if (!fork()) { sleep(3); pthread_mutex_lock(mutex); pthread_cond_signal(condition); pthread_mutex_unlock(mutex); printf("son signaled\n"); exit(0); } else { printf("wait on condition\n"); pthread_mutex_lock(mutex); pthread_cond_wait(condition, mutex); pthread_mutex_unlock(mutex); printf("Signaled by son process, wake up\n"); pthread_mutex_destroy(mutex); pthread_cond_destroy(condition); shm_unlink(OKTOWRITE); shm_unlink(MESSAGE); shm_unlink(MUTEX); return 0; } } 

问题是，即使在儿子发出信号后，该过程的父亲仍然被锁定。 一切都在共享内存中（使用shm_open和mmap ），因此两个进程的条件应该相同。 在调用wait或signal之前锁定互斥锁可能会犯错吗？

编辑：感谢所有帮助过我的人。 这是正确的代码，标有CRITICAL部分：

 #include  #include  #include  #include  #include  #include  #include  #include  #include  #include  #define OKTOWRITE "/condwrite" #define MESSAGE "/msg" #define MUTEX "/mutex_lock" int main(int argc, char** argv) { pthread_cond_t* condition; pthread_mutex_t* mutex; char* message; int des_cond, des_msg, des_mutex; int mode = S_IRWXU | S_IRWXG; des_mutex = shm_open(MUTEX, O_CREAT | O_RDWR | O_TRUNC, mode); if (des_mutex < 0) { perror("failure on shm_open on des_mutex"); exit(1); } if (ftruncate(des_mutex, sizeof(pthread_mutex_t)) == -1) { perror("Error on ftruncate to sizeof pthread_cond_t\n"); exit(-1); } mutex = (pthread_mutex_t*) mmap(NULL, sizeof(pthread_mutex_t), PROT_READ | PROT_WRITE, MAP_SHARED, des_mutex, 0); if (mutex == MAP_FAILED ) { perror("Error on mmap on mutex\n"); exit(1); } des_cond = shm_open(OKTOWRITE, O_CREAT | O_RDWR | O_TRUNC, mode); if (des_cond < 0) { perror("failure on shm_open on des_cond"); exit(1); } if (ftruncate(des_cond, sizeof(pthread_cond_t)) == -1) { perror("Error on ftruncate to sizeof pthread_cond_t\n"); exit(-1); } condition = (pthread_cond_t*) mmap(NULL, sizeof(pthread_cond_t), PROT_READ | PROT_WRITE, MAP_SHARED, des_cond, 0); if (condition == MAP_FAILED ) { perror("Error on mmap on condition\n"); exit(1); } /* HERE WE GO */ /**************************************/ /* set mutex shared between processes */ pthread_mutexattr_t mutexAttr; pthread_mutexattr_setpshared(&mutexAttr, PTHREAD_PROCESS_SHARED); pthread_mutex_init(mutex, &mutexAttr); /* set condition shared between processes */ pthread_condattr_t condAttr; pthread_condattr_setpshared(&condAttr, PTHREAD_PROCESS_SHARED); pthread_cond_init(condition, &condAttr); /*************************************/ if (!fork()) { sleep(10); pthread_mutex_lock(mutex); pthread_cond_signal(condition); printf("son signaled\n"); pthread_mutex_unlock(mutex); exit(0); } else { printf("father waits on condition\n"); pthread_mutex_lock(mutex); pthread_cond_wait(condition, mutex); pthread_mutex_unlock(mutex); printf("Signaled by son process, wake up!!!!!!!!\n"); pthread_condattr_destroy(&condAttr); pthread_mutexattr_destroy(&mutexAttr); pthread_mutex_destroy(mutex); pthread_cond_destroy(condition); shm_unlink(OKTOWRITE); shm_unlink(MESSAGE); shm_unlink(MUTEX); } return 0; } 

Answer 1

要在进程之间进行共享，需要通过正确初始化的属性相应地初始化互斥锁： http ： //pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_mutexattr_setpshared.html

 #include  ... pthread_mutex_t * pmutex = NULL; pthread_mutexattr_t attrmutex; /* Initialise attribute to mutex. */ pthread_mutexattr_init(&attrmutex); pthread_mutexattr_setpshared(&attrmutex, PTHREAD_PROCESS_SHARED); /* Allocate memory to pmutex here. */ /* Initialise mutex. */ pthread_mutex_init(pmutex, &attrmutex); /* Use the mutex. */ /* Clean up. */ pthread_mutex_destroy(pmutex); pthread_mutexattr_destroy(&attrmutex); 

^{（为了这个例子的可读性，遗漏了错误检查）}

这同样适用于应在进程之间共享的条件变量： http ： //pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_condattr_setpshared.html

 #include  ... pthread_cond_t * pcond = NULL; pthread_condattr_t attrcond; /* Initialise attribute to condition. */ pthread_condattr_init(&attrcond); pthread_condattr_setpshared(&attrcond, PTHREAD_PROCESS_SHARED); /* Allocate memory to pcond here. */ /* Initialise condition. */ pthread_cond_init(pcond, &attrcond); /* Use the condition. */ /* Clean up. */ pthread_cond_destroy(pcond); pthread_condattr_destroy(&attrcond); 

^{（为了这个例子的可读性，遗漏了错误检查）}

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另请参阅此答案： https ： //stackoverflow.com/a/2390670/694576

Answer 2

等待条件应该在while语句之前，如下所示：

 pthread_mutex_lock(mutex); while(!conditionSatisfied) pthread_cond_wait(condition, mutex); pthread_mutex_unlock(mutex); 

信令应按以下方式完成：

 pthread_mutex_lock(mutex); conditionSatisfied = true; pthread_cond_signal(condition); pthread_mutex_unlock(mutex); 

Answer 3

是的，必须在pthread_cond_wait之前锁定互斥锁，但不必为pthread_cond_signal锁定。 如果你回头看看你的代码，你会看到互斥锁将被解锁两次，这是一个错误的迹象……孩子也可以在已经被父母摧毁的互斥锁上调用解锁…

顺便睡觉并不能保证父母会先执行。 为了确保这一点，你需要…条件变量……