pthread_mutex 应用

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本文详细介绍了pthread互斥锁的使用方法，包括初始化、销毁、上锁与解锁等操作，并通过示例代码展示了如何正确使用互斥锁来保护共享资源。

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一、 pthread_mutex 家庭成员：

1.int pthread_mutex_init (pthread_mutex_t *mutex , pthread_mutexattr_t * attr );

Description

The pthread_mutex_init function initializes the given mutex with the given attributes. Ifattr is null, then the default attributes are used.

2. int pthread_mutex_destroy (pthread_mutex_t *mutex );

Description

The pthread_mutex_destroy function destroys the given mutex. If the mutex is already destroyed, thenerrno is set to EINVAL. If the mutex is locked, then errno is set to EBUSY.

3.int pthread_mutex_lock (pthread_mutex_t *mutex );

Description

The pthread_mutex_lock function locks the given mutex. If the mutex is already locked, then the calling thread blocks until the thread that currently holds the mutex unlocks it.

4.int pthread_mutex_trylock (pthread_mutex_t *mutex );

Description

The pthread_mutex_trylock function tries to lock the given mutex. If the mutex is already locked, the function returns without waiting for the mutex to be unlocked.

Returns

The pthread_mutex_trylock function returns zero if the call is successful, otherwise it setserrno to EINVAL and returns -1.

5.int pthread_mutex_unlock (pthread_mutex_t *mutex );

Description

The pthread_mutex_unlock function unlocks the given mutex.

二、成员作用

1. 互斥锁的初始化和销毁

.初始化有两种初始化方式:

a. 对于静态分配的互斥锁一半用宏赋值的方式初始化

eg:static pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;

b. 对于动态分配的互斥锁（如调用malloc）或分配在共享内存中，则必须调用

pthread_mutex_init(pthread_mutex*mutex, pthread_mutexattr_t *mutexattr)函数来进行初始化。

动态分配释放例子：
pthread_mutex_init(*mutex ,*attr);
pthread_mutex_destory(*mutex)
pthread_mutexattr_init(*attr)
pthread_mutexattr_destory(*attr)动态分配的代码：
pthread_mutex_t *lock;
lock=(pthread_mutex_t *)malloc(sizeof(szieof(pthread_mutex_t)));
pthread_mutex_init(lock ,NULL);

. 互斥锁摧毁
pthread_mutex_destory(lock);
free(lock);

销毁一个互斥锁即意味着释放它所占用的资源，且要求锁当前处于开放状态。由于在Linux中，互斥锁并不占用任何资源，因此LinuxThreads中的 pthread_mutex_destroy()除了检查锁状态以外（锁定状态则返回EBUSY）没有其他动作。

2. 互斥锁上锁、尝试上锁和解锁

要对公共变量进行上锁来进行保护，必须最少要上两个锁以上，换句话说就得调用pthread_mutex_lock()函数两次以上，否则，只是上一个锁会达不到相应的效果。尝试上锁pthread_mutex_trylock()和pthread_mutex_lock()的区别是：后者会阻塞等待另外一个锁被解锁；前者尝试去加锁，如果不成功就返回非0，如果成功返回0，不会产生阻塞。

每次上锁之后，一定要解锁，否则会造成程序一直在阻塞状态。

三、代码讲解：

代码说明1：互斥锁基本应用

#include<stdio.h>

#include<pthread.h>

#include"unistd.h"

pthread_mutex_tmutex = PTHREAD_MUTEX_INITIALIZER;

intcount = 0;

void*consume( void *arg)

{

while(1 )

{

pthread_mutex_lock(&mutex );

printf("************************consumebegin lock\n");

printf("************************consumed%d\n",count );

sleep(2);

count++;

printf("************************consumed%d\n",count);

printf("************************consumeover lock\n");

pthread_mutex_unlock(&mutex );

printf("************************I'mout of pthread_mutex\n");

sleep(1);

}

void* produce( void * arg )

{

while(1 )

{

pthread_mutex_lock(&mutex );

printf("productbegin lock\n");

printf("Produced %d\n", count );

printf("productover lock\n");

pthread_mutex_unlock(&mutex );

printf("I'mout of pthread_mutex\n");

sleep(1);

}

intmain( void )

{

pthread_tthread1,thread2;

pthread_create(&thread1, NULL, &produce, NULL );

pthread_create(&thread2, NULL, &consume, NULL );

pthread_join(thread1,NULL);

pthread_join(thread2,NULL);

return0;

}

结果显示：

[elbort@elborttest1]$ gcc -Wall -lpthread -o test test.c

[elbort@elborttest1]$ ./test

productbegin lock

Produced0

productover lock

I'mout of pthread_mutex

************************consumebegin lock

************************consumed0

/*中间等待了2秒但是product线程没有执行|*/

************************consumed1

************************consumeover lock

************************I'mout of pthread_mutex

productbegin lock

Produced1

productover lock

I'mout of pthread_mutex

************************consumebegin lock

************************consumed1

************************consumed2

************************consumeover lock

************************I'mout of pthread_mutex

productbegin lock

Produced2

productover lock

I'mout of pthread_mutex

/************************************************************************/

代码说明2：单一调用互斥锁没效果

void*consume( void *arg)

{

while(1 )

{

pthread_mutex_lock(&mutex );

printf("************************consumebegin lock\n");

printf("************************consumed%d\n",count );

sleep(2);

count++;

printf("************************consumed%d\n",count);

printf("************************consumeover lock\n");

pthread_mutex_unlock(&mutex );

printf("************************I'mout of pthread_mutex\n");

sleep(1);

}

void* produce( void * arg )

{

while(1 )

{

count=count+5;

printf("Produced %d\n", count );

sleep(1);

}

intmain( void )

{

pthread_tthread1,thread2;

pthread_create(&thread1, NULL, &produce, NULL );

pthread_create(&thread2, NULL, &consume, NULL );

pthread_join(thread1,NULL);

pthread_join(thread2,NULL);

return0;

}

结果说明：

[elbort@elborttest1]$ ./test

Produced5

************************consumebegin lock

************************consumed5

Produced10 //线程comsume被中断，conut的值被线程product修改

************************consumed11

************************consumeover lock

************************I'mout of pthread_mutex

/********************************************************/

代码说明3：pthread_mutex_trylock作用

intcount = 0;

void*consume( void *arg)

{

while(1 )

{

pthread_mutex_lock(&mutex );

printf("************************consumebegin lock\n");

printf("************************consumed%d\n",count );

sleep(2);

count++;

printf("************************consumed%d\n",count);

printf("************************consumeover lock\n");

pthread_mutex_unlock(&mutex );

printf("************************I'mout of pthread_mutex\n");

sleep(1);

}

void* produce( void * arg )

{

while(1 )

{

if(pthread_mutex_trylock(&mutex ) == 0)

{

printf("productbegin lock\n");

count++;

printf("Produced %d\n", count );

printf("productover lock\n");

pthread_mutex_unlock(&mutex );

printf("I'mout of pthread_mutex\n");

sleep(1);

}

else

{

printf("Ihave try!But i can lock the mutex!\n");

sleep(1);

}

intmain( void )

{

pthread_tthread1,thread2;

pthread_create(&thread1, NULL, &produce, NULL );

pthread_create(&thread2, NULL, &consume, NULL );

pthread_join(thread1,NULL);

pthread_join(thread2,NULL);

return0;

}

结果显示：

[elbort@elborttest1]$ ./test

************************consumebegin lock

************************consumed0

Ihave try!But i can lock the mutex!

************************consumed1

************************consumeover lock

************************I'mout of pthread_mutex

productbegin lock

Produced2

productover lock

I'mout of pthread_mutex

************************consumebegin lock

************************consumed2

Ihave try!But i can lock the mutex!

************************consumed3

************************consumeover lock

************************I'mout of pthread_mutex

productbegin lock

Produced4

productover lock

I'mout of pthread_mutex