我正在尝试使用 POSIX 信号量在 C 中编写线程安全的读写锁。您可以在此处查看源代码的当前状态。我按照这个创建了一个读者首选的锁。
问题是我想为调用 rwl_destroy() 时可能出现的任何可能状态处理锁的破坏。
如果调用了destroy并且没有其他线程在锁上，那么它将锁定wrt（由写入器使用）以防止任何其他线程访问由锁保护的数据。接下来，destroy 函数应该销毁信号量并释放为 ReadWriteLock 结构分配的内存。但是如果另一个线程现在正在等待锁怎么办？根据文档，该线程将处于未定义状态。
这就是我试图避免的，以使锁更易于使用。
编辑：
当前代码是：
typedef struct ReadWriteLock
{
sem_t wrt;
sem_t mtx;
sem_t delFlag;
int readcount;
int active;
}ReadWriteLock;
//forward declaration
/* This function is used to take the state of the lock.
* Return values:
* [*] 1 is returned when the lock is alive.
* [*] 0 is returned when the lock is marked for delete.
* [*] -1 is returned if an error was encountered.
*/
int isActive(ReadWriteLock*);
int rwl_init(ReadWriteLock* lock)
{
lock = malloc(sizeof(ReadWriteLock));
if (lock == NULL)
{
perror("rwl_init - could not allocate memory for lock\n");
return -1;
}
if (sem_init(&(lock->wrt), 0, 1) == -1)
{
perror("rwl_init - could not allocate wrt semaphore\n");
free(lock);
lock = NULL;
return -1;
}
if (sem_init(&(lock->mtx), 0, 1) == -1)
{
perror("rwl_init - could not allocate mtx semaphore\n");
sem_destroy(&(lock->wrt));
free(lock);
lock = NULL;
return -1;
}
if (sem_init(&(lock->delFlag), 0 , 1) == -1)
{
perror("rwl_init - could not allocate delFlag semaphore\n");
sem_destroy(&(lock->wrt));
sem_destroy(&(lock->mtx));
free(lock);
lock = NULL;
return -1;
}
lock->readcount = 0;
lock->active = 1;
return 0;
}
int rwl_destroy(ReadWriteLock* lock)
{
errno = 0;
if (sem_trywait(&(lock->wrt)) == -1)
perror("rwl_destroy - trywait on wrt failed.");
if ( errno == EAGAIN)
perror("rwl_destroy - wrt is locked, undefined behaviour.");
errno = 0;
if (sem_trywait(&(lock->mtx)) == -1)
perror("rwl_destroy - trywait on mtx failed.");
if ( errno == EAGAIN)
perror("rwl_destroy - mtx is locked, undefined behaviour.");
if (sem_destroy(&(lock->wrt)) == -1)
perror("rwl_destroy - destroy wrt failed");
if (sem_destroy(&(lock->mtx)) == -1)
perror("rwl_destroy - destroy mtx failed");
if (sem_destroy(&(lock->delFlag)) == -1)
perror("rwl_destroy - destroy delFlag failed");
free(lock);
lock = NULL;
return 0;
}
int isActive(ReadWriteLock* lock)
{
errno = 0;
if (sem_trywait(&(lock->delFlag)) == -1)
{
perror("isActive - trywait on delFlag failed.");
return -1;
}
if ( errno == EAGAIN)
{//delFlag is down, lock is marked for delete
perror("isActive - tried to lock but ReadWriteLock was marked for delete");
return 0;
}
return 1;
}
我也有这些功能：
int rwl_writeLock(ReadWriteLock*);
int rwl_writeUnlock(ReadWriteLock*);
int rwl_readLock(ReadWriteLock*);
int rwl_readUnlock(ReadWriteLock*);
所以我的问题是如何更改这些函数以避免我上面描述的未定义状态。是否有可能，或者此代码的用户是否应该在尝试销毁 ReadWriteLock 之前负责释放所有锁？
isActive() 函数和 delFlag 信号量目前没有使用，它们是我试图解决问题的结果。