linux互斥锁pthread_mutex_t和条件变量pthread_cond_t的妙用

基于生产者消费者模式的MQ（msg queue）实现了线程间通信，在生产者消费者模型中通常都会用到互斥锁pthread_mutex_t来保护共享内存资源，多个线程访问共享内核空间之前都会尝试获取mutex，如果有其他线程正在使用则当前线程进入锁等待状态。这样的机制难免会带来两个问题：

1：如果两个线程同时获取mutex，则两个线程会进入死锁状态

2：如果多个线程依次获取mutex，那么这些线程都会进入锁等待，浪费了cpu资源

因此，我们可以使用互斥锁pthread_mutex_t和pthread_cond_t配合来避免上面的问题

简单来说这种方法的核心思路：就是调用pthread_mutex_lock(mutex)获取锁时再判断一下是否满足获取资源的条件，比如队列为空时执行get操作就是不满足条件，如果不满足条件则调用pthread_cond_wait(cond, mutex)这个函数使当前线程进入睡眠并且把mutex互斥锁释放掉，等待获取资源的条件满足时，比如队列新加了一条msg，此时其他线程会调用pthread_cond_signal()来唤醒这个线程并锁定mutex，这样就不会出现多个线程等待一个mutex或者死锁的情况了。

生产者和消费者模型中互斥锁和条件变量的使用流程图如下，其中蓝色代表消费者的执行流，红色是生产者的执行流

下面是一个简单的程序示例：

#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 ) ;

}