linux 读写锁

特性：
    读写锁也叫共享——排他锁，因为有3种状态, 所以可以有更高的并行性。使用mutex，它的状态要么处于锁住和未锁状态，只有一个线程可以上锁。而读写锁有更多的状态：在读状态锁住，在写状态锁住，未锁住。只有一个线程可以获得写锁，多个线程可以同时获得读锁。
• 当读写锁是写加锁状态时, 在这个锁被解锁之前, 所有试图对这个锁加锁的线程都会被阻塞。
• 当读写锁在读加锁状态时, 所有试图以读模式对它进行加锁的线程都可以得到访问权, 但是如果线程希望以写模式对此锁进行加锁, 它必须阻塞知道所有的线程释放锁。
• 通常, 当读写锁处于读模式锁住状态时, 如果有另外线程试图以写模式加锁, 读写锁通常会阻塞随后的读模式锁请求, 这样可以避免读模式锁长期占用, 而等待的写模式锁请求长期阻塞。

适用性：
    读写锁适合读比写频繁情形。读写锁和互斥量一样也需要在使用前初始化，在释放他们内存的时候销毁。

初始化和销毁：

    int pthread_rwlock_init(pthread_rwlock_t *restrict rwlock, const pthread_rwlockattr_t *restrict attr); 
    int pthread_rwlock_destroy(pthread_rwlock_t *restrict rwlock);

    一个读写锁可以调用pthread_rwlock_init来初始化，我们可以传递NULL作为attr的参数，这样会使用读写锁的默认属性。我们可以调用pthread_rwlock_destroy来清理，销毁它所占的内存空间。

读和写：

    int pthread_rwlock_rdlock(pthread_rwlock_t *rwlock);  
    int pthread_rwlock_wrlock(pthread_rwlock_t *rwlock);  

    实现上可能会对读写锁中读模式的锁锁住次数有一定的限制，所以我们需要检查返回值，以确定是否成功。而其他的两个函数会返回错误，但是只要我们的锁设计的恰当，我们可以不必做检查。

非阻塞的函数为：

    int pthread_rwlock_tryrdlock(pthread_rwlock_t *rwlock);  
    int pthread_rwlock_trywrlock(pthread_rwlock_t *rwlock);

    当锁成功获取时，返回0，否则返回EBUSY。这两个函数可以避免死锁。
    如果针对未初始化的读写锁调用进行读写操作，则结果是不确定的。

释放：

    int pthread_rwlock_unlock(pthread_rwlock_t *rwlock);

    用来释放在 rwlock 引用的读写锁对象中持有的锁。
    如果调用线程未持有读写锁 rwlock，或者针对未初始化的读写锁调用该函数，则结果是不确定的。

例子：

#define   _XOPEN_SOURCE   500  
#include   <pthread.h>  
#define   PTHREAD_RWLOCK_INITIALIZER_READ_PREF { {0, 0}, 0, NULL, NULL, NULL, PTHREAD_RWLOCK_PREFER_READER_NP,   PTHREAD_PROCESS_PRIVATE   }  
   
  static   pthread_rwlock_t   a   =   PTHREAD_RWLOCK_INITIALIZER;  
   
  void   *route_3   (void   *p)  
  {   
        sleep(2);   
        printf("locking   3   =   %d\n",   pthread_rwlock_rdlock(&a));   
        pause();   
        return   NULL;  
  }  
   
  void   *route_2   (void   *p)  
  {   
        sleep(1);   
        printf("locking   2   =   %d\n",   pthread_rwlock_wrlock(&a));   
        pause();   
        return   NULL;  
  }  
   
  void   *route_1   (void   *p)  
  {   
        printf("locking   1   =   %d\n",   pthread_rwlock_rdlock(&a));   
        pause();   
        return   NULL;  
  }  
   
  main()  
  {   
        pthread_t   t1,   t2,   t3;   
        pthread_create(&t1,   NULL,   route_1,   NULL);   
        pthread_create(&t2,   NULL,   route_2,   NULL);   
        pthread_create(&t3,   NULL,   route_3,   NULL);   
        pthread_join(t1,   NULL);   
        pthread_join(t2,   NULL);   
        pthread_join(t3,   NULL);  
  } 

#include <errno.h>
#include <pthread.h>

static pthread_rwlock_t listlock;
static int lockiniterror = 0;
static pthread_once_t lockisinitialized = PTHREAD_ONCE_INIT;

static void ilock(void) {
   lockiniterror = pthread_rwlock_init(&listlock, NULL);
}

int initialize_r(void) {    /* must be called at least once before using list */
   if (pthread_once(&lockisinitialized, ilock))
      lockiniterror = EINVAL;
   return lockiniterror;
}

int accessdata_r(void) {               /* get a nonnegative key if successful */
   int error;
   int errorkey = 0;
   int key;
   if (error = pthread_rwlock_wrlock(&listlock)) {  /* no write lock, give up */
      errno = error;
      return -1;
   }
   key = accessdata();
   if (key == -1) {
      errorkey = errno;
      pthread_rwlock_unlock(&listlock);
      errno = errorkey;
      return -1;
   }
   if (error = pthread_rwlock_unlock(&listlock)) {
      errno = error;
      return -1;
   }
   return key;
}

int adddata_r(data_t data) {          /* allocate a node on list to hold data */
   int error;
   if (error = pthread_rwlock_wrlock(&listlock)) { /* no writer lock, give up */
      errno = error;
      return -1;
   }
   if (adddata(data) == -1) {
      error = errno;
      pthread_rwlock_unlock(&listlock);
      errno = error;
      return -1;
   }
   if (error = pthread_rwlock_unlock(&listlock)) {
      errno = error;
      return -1;
   }
   return 0;
}

int getdata_r(int key, data_t *datap) {               /* retrieve node by key */
   int error;
   if (error = pthread_rwlock_rdlock(&listlock)) { /* no reader lock, give up */
      errno = error;
      return -1;
   }
   if (getdata(key, datap) == -1) {
      error = errno;
      pthread_rwlock_unlock(&listlock);
      errno = error;
      return -1;
   }
   if (error = pthread_rwlock_unlock(&listlock)) {
      errno = error;
      return -1;
   }
   return 0;
}

int freekey_r(int key) {                                      /* free the key */
   int error;
   if (error = pthread_rwlock_wrlock(&listlock)) {
      errno = error;
      return -1;
   }
   if (freekey(key) == -1) {
      error = errno;
      pthread_rwlock_unlock(&listlock);
      errno = error;
      return -1;
   }
   if (error = pthread_rwlock_unlock(&listlock)) {
      errno = error;
      return -1;
   }
   return 0;
}

#include <pthread.h> 
#include <sys/types.h>
#include <sys/stat.h> //文件状态结构
#include <unistd.h>
#include <sys/mman.h> //mmap头文件


#define BSIZE 10
typedef struct {
char buf[BSIZE];
int occupied;
int nextin;
int nextout;
pthread_mutex_t mutex;
pthread_cond_t more;
pthread_cond_t less;
} buffer_t;

char consumer(buffer_t *b)
{
	char item;
	pthread_mutex_lock(&b->mutex);
	while(b->occupied <= 0)
	pthread_cond_wait(&b->more, &b->mutex);
	assert(b->occupied > 0);
	item = b->buf[b->nextout++];
	b->nextout %= BSIZE;
	b->occupied--;
	/* now: either b->occupied > 0 and b->nextout is the index
	of the next occupied slot in the buffer, or
	b->occupied == 0 and b->nextout is the index of the next
	(empty) slot that will be filled by a producer (such as
	使用条件变量
	120 多线程编程指南• 2006年10月
	示例4–13 生成方和使用者问题：使用者（续）
	b->nextout == b->nextin) */
	pthread_cond_signal(&b->less);
	pthread_mutex_unlock(&b->mutex);
	return(item);
}

void producer(buffer_t *b, char item)
{
	pthread_mutex_lock(&b->mutex);
	while (b->occupied >= BSIZE)
	pthread_cond_wait(&b->less, &b->mutex);
	 assert(b->occupied < BSIZE);
	b->buf[b->nextin++] = item;
	b->nextin %= BSIZE;
	b->occupied++;
	/* now: either b->occupied < BSIZE and b->nextin is the index
	of the next empty slot in the buffer, or
	b->occupied == BSIZE and b->nextin is the index of the
	next (occupied) slot that will be emptied by a consumer
	(such as b->nextin == b->nextout) */
	pthread_cond_signal(&b->more);
	pthread_mutex_unlock(&b->mutex);
}

void producer_driver(buffer_t *b) {
	int item;
	while (1) {
		item = getchar();
		if (item == EOF) {
			producer(b, ‘\0’);
		break;
	} else
		producer(b, (char)item);
	}

	return 0
}

void consumer_driver(buffer_t *b) {
	char item;
	 
	while (1) {
		if ((item = consumer(b)) == ’\0’)
		break;
		putchar(item);
	}
}

int main() {
	int zfd;
	buffer_t *buffer;
	
	pthread_mutexattr_t mattr;
	pthread_condattr_t cvattr_less, cvattr_more;
	
	zfd = open("/dev/zero", O_RDWR);
	buffer = (buffer_t *)mmap(NULL, sizeof(buffer_t),PROT_READ|PROT_WRITE, MAP_SHARED, zfd, 0);

	buffer->occupied = buffer->nextin = buffer->nextout = 0;

	pthread_mutex_attr_init(&mattr);
	pthread_mutexattr_setpshared(&mattr,PTHREAD_PROCESS_SHARED);
	pthread_mutex_init(&buffer->lock, &mattr);

	pthread_condattr_init(&cvattr_less);
	pthread_condattr_setpshared(&cvattr_less, PTHREAD_PROCESS_SHARED);
	pthread_cond_init(&buffer->less, &cvattr_less);

	pthread_condattr_init(&cvattr_more);
	pthread_condattr_setpshared(&cvattr_more,PTHREAD_PROCESS_SHARED);
	pthread_cond_init(&buffer->more, &cvattr_more);
 
	if (fork() == 0)
		consumer_driver(buffer);
	else
		producer_driver(buffer);


}