pthread_cond_wait and pthread_cond_signal

pthread_cond_signal函數的作用是發送一個訊號給另外一個正在處於阻塞等待狀態的線程,使其脫離阻塞狀態,繼續執行.如果沒有線程處在阻塞等待狀態,pthread_cond_signal也會成功返回。

但使用pthread_cond_signal不會有“驚群現象”產生，他最多隻給一個線程發訊號。假如有多個線程正在阻塞等待著這個條件變數的話，那麼
是根據各等待線程優先順序的高低確定哪個線程接收到訊號開始繼續執行。如果各線程優先順序相同，則根據等待時間的長短來確定哪個線程獲得訊號。但無論如何一個
pthread_cond_signal調用最多發信一次。

另外，互斥量的作用一般是用於對某個資源進行互斥性的存取,很多時候是用來保證操作是一個原子性的操作，是不可中斷的。

pthread_cond_signal函數與條件變數的典型應用就是用來實現producer/consumer模型。

[code]
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <pthread.h>

#define BUFFER_SIZE 8

struct Products
{
int buffer[BUFFER_SIZE];
pthread_mutex_t locker;           //保證存取操作的原子性 互斥性
pthread_cond_t notEmpty; //是否可讀      
pthread_cond_t notFull;          //是否可寫
int posReadFrom;
int posWriteTo;
};

int BufferIsFull(struct Products* products)
{
if ((products->posWriteTo + 1) % BUFFER_SIZE == products->posReadFrom)
{
return (1);
}
return (0);
}

int BufferIsEmpty(struct Products* products)
{
if (products->posWriteTo == products->posReadFrom)
{
return (1);
}

return (0);
}

//製造產品。

void Produce(struct Products* products, int item)
{
pthread_mutex_lock(&products->locker); //原子操作

while (BufferIsFull(products))
{
pthread_cond_wait(&products->notFull, &products->locker);
} //無空間可寫入

//寫入資料
products->buffer[products->posWriteTo] = item;
products->posWriteTo++;

if (products->posWriteTo >= BUFFER_SIZE)
products->posWriteTo = 0;

pthread_cond_signal(&products->notEmpty);     //發信
pthread_mutex_unlock(&products->locker);    //解鎖
}

int Consume(struct Products* products)
{
int item;

pthread_mutex_lock(&products->locker);

while (BufferIsEmpty(products))
{
pthread_cond_wait(&products->notEmpty, &products->locker);
} //為空白時持續等待,無資料可讀

//提取資料
item = products->buffer[products->posReadFrom];
products->posReadFrom++;

if (products->posReadFrom >= BUFFER_SIZE) //如果到末尾,從頭讀取
products->posReadFrom = 0;

pthread_cond_signal(&products->notFull);
pthread_mutex_unlock(&products->locker);

return item;
}

#define END_FLAG (-1)

struct Products products;

void* ProducerThread(void* data)
{
int i;
for (i = 0; i < 16; ++i)
{
printf("producer: %d/n", i);
Produce(&products, i);
}
Produce(&products, END_FLAG);
return NULL;
}

void* ConsumerThread(void* data)
{
int item;

while (1)
{
item = Consume(&products);
if (END_FLAG == item)
break;
printf("consumer: %d/n", item);
}
return (NULL);
}

int main(int argc, char* argv[])
{
pthread_t producer;
pthread_t consumer;
int result;

pthread_create(&producer, NULL, &ProducerThread, NULL);
pthread_create(&consumer, NULL, &ConsumerThread, NULL);

pthread_join(producer, (void *)&result);
pthread_join(consumer, (void *)&result);

exit(EXIT_SUCCESS);
}

[/code]