1、需求
有消費者A和生產者B線程,共享變量n,生存者對n++,消費者對n--,消費者線程A必須要等到n>0纔可以執行下去,當然生產者B線程就執行n++,讓消費者能執行下去。
2、生產者和消費者模型中互斥鎖和條件變量的使用流程圖如下,其中藍色代表消費者的執行流,紅色是生產者的執行流。
3、幾個函數
/* 初始化一個條件變量 */
int pthread_cond_init (pthread_cond_t* cond, pthread_condattr_t *cond_attr);
/* 銷燬一個條件變量 */
int pthread_cond_destroy(pthread_cond_t* cond);
/* 令一個消費者等待在條件變量上 */
int pthread_cond_wait(pthread_cond_t *cv, pthread_mutex_t *mutex)
/* 生產者通知等待在條件變量上的消費者 */
int pthread_cond_signal(pthread_cond_t* cond);
/* 生產者向消費者廣播消息 */
int pthread_cond_broadcast(pthread_cond_t* cond);
消費者等待條件的僞代碼:
pthread_mutex_lock(&mutex); // 拿到互斥鎖,進入臨界區
while( 條件爲假)
pthread_cond_wait(cond, mutex); // 令進程等待在條件變量上
修改條件
pthread_mutex_unlock(&mutex); // 釋放互斥鎖
生產者通知消費者的僞代碼:
pthread_mutex_lock(&mutex); // 拿到互斥鎖,進入臨界區
while( 條件爲假)
pthread_cond_wait(cond, mutex); // 令進程等待在條件變量上
修改條件
pthread_mutex_unlock(&mutex); // 釋放互斥鎖
4、實例代碼
/***************************************************************
* Copyright (C) 2016 chengonghao
* All rights reserved.
*
* [email protected]
***************************************************************/
#include <unistd.h>
#include <pthread.h>
#define CONSUMERS_COUNT 2
#define PRODUCERS_COUNT 1
pthread_mutex_t g_mutex ;
pthread_cond_t g_cond ;
pthread_t g_thread[CONSUMERS_COUNT + PRODUCERS_COUNT] ;
int share_variable = 0 ;// this is the share variable, shared by consumer and producer
void* consumer( void* arg )
{
int num = (int)arg ;
while ( 1 )
{
/******* critical section begin *******/
pthread_mutex_lock( &g_mutex ) ;
// if share_variable == 0, means consumer shell stop here
while ( share_variable == 0 )
{
printf( "consumer %d begin wait a condition...\n", num ) ;
// put a thread blocked ont a condition variable( here is g_cond),
// and unlock the mutex( here is g_mutex )
pthread_cond_wait( &g_cond, &g_mutex ) ;
}
// here means n != 0 and consumer can goes on
// consumer consumed shared variable, so the number of shared variable shell minus
printf( "consumer %d end wait a condition...\n", num ) ;
printf( "consumer %d begin consume product\n", num ) ;
-- share_variable ;
pthread_mutex_unlock( &g_mutex ) ;
/******** critial section end *********/
sleep( 1 ) ;
}
return NULL ;
}
void* producer( void* arg )
{
int num = (int)arg ;
while ( 1 )
{
/******* critical section begin *******/
pthread_mutex_lock( &g_mutex ) ;
// produce a shared variable
printf( "producer %d begin produce product...\n", num ) ;
++ share_variable ;
printf( "producer %d end produce product...\n", num ) ;
// unblock threads blocked on a condition variable( here is g_cond )
pthread_cond_signal( &g_cond ) ;
printf( "producer %d notified consumer by condition variable...\n", num ) ;
pthread_mutex_unlock( &g_mutex ) ;
/******** critial section end *********/
sleep( 5 ) ;
}
return 1 ;
}
int main( void )
{
// initiate mutex
pthread_mutex_init( &g_mutex, NULL ) ;
// initiate condition
pthread_cond_init( &g_cond, NULL ) ;
// initiate consumer threads
for ( int i = 0; i < CONSUMERS_COUNT; ++ i )
{
pthread_create( &g_thread[i], NULL, consumer, (void*)i ) ;
}
sleep( 1 ) ;
// initiate producer threads
for ( int i = 0; i < PRODUCERS_COUNT; ++ i )
{
pthread_create( &g_thread[i], NULL, producer, (void*)i ) ;
}
for ( int i = 0; i < CONSUMERS_COUNT + PRODUCERS_COUNT; ++ i )
{
pthread_join( g_thread[i], NULL ) ;
}
pthread_mutex_destroy( &g_mutex ) ;
pthread_cond_destroy( &g_cond ) ;
}
5、執行結果
1. 第一個框,消費者 1 和0 發現share_variable == 0,於是先後等待在條件變量上;
2. 第二個框,生產者 0 開始生產共享變量,即 ++ share_variable,然後通知等待在條件變量上的消費者;
3. 第三個框,消費者 1 被生產者喚醒,開始消費共享變量,即– share_variable;
4. 第四個框,生產者 0 繼續生產共享變量,++ share_variable,然後通知等待在條件變量上的消費者;
5. 第五個框,消費者 0 被喚醒,開始消費共享變量,即– share_variable;
以此類推,以上描述簡化了拿鎖和釋放鎖的過程,可以結合上面的流程圖來理解代碼。