pthread_create的使用方法

pthread_create的使用方法.txt人生重要的不是所站的位置，而是所朝的方向。

不要用自己的需求去衡量别人的给予，否则永远是抱怨。

1：pthread_create 函数的安全使用问题做过linux多线程开发的人都会用过pthread_create函数，但是很少人会注意到：主线程在使用pthread_create创建线程时，如果pthread_create 函数的第四个参数（指针参数）传入的值会被主线程随时修改时，这时我们的线程从pthread_create 函数的第四个参数获取的值可能就不是我们想要传入的值了。

看看下面代码：在 main 函数的 for 循环中创建10个线程；每创建一个线程时都把循环变量 i 的值通过 pthread_create 函数的第四个参数传入线程函数中；但是线程获取的值可能不并不是我们想传入的值。

#include <pthread.h>#include <stdio.h>void * threadFunc( void * pm_iPthreadId ){unsigned int iId = * ( unsigned int * ) pm_iPthreadId;printf( "### No.%d pthread start exec; pthread id = %u !\n", iId, pthread_self() );time_t iStartTime = time( NULL );while(1){time_t iTempTime = time( NULL );if ( iTempTime - iStartTime > 2 ){printf( "$$$ NO.%d pthread cirle exec !\n", * ( unsigned int * ) pm_iPthreadId );iStartTime = time( NULL );}}}int main(){pthread_t * pthreadId;pthreadId = new pthread_t[10];for( unsigned int i = 0; i < 10; i++ ){int iRet = pthread_create( pthreadId + i, NULL, threadFunc, &i ); if ( iRet == 0 ){printf( "@@@ create No.%d pthread success; pthreadId = %u !\n", i, pthreadId + i );}}while(1){}return 0;}程序运行结果如下（结果是随机的，每次运行可能都不一样）：@@@ create No.0 pthread success; pthreadId = 138174472 ! ### No.1 pthread start exec; pthread id = 3076803472 ! ### No.1 pthread start exec; pthread id = 3087293328 ! @@@ create No.1 pthread success; pthreadId = 138174476 ! @@@ create No.2 pthread success; pthreadId = 138174480 ! ### No.3 pthread start exec; pthread id = 3055823760 ! ### No.3 pthread start exec; pthread id = 3066313616 ! @@@ create No.3 pthread success; pthreadId = 138174484 ! ### No.4 pthread start exec; pthread id = 3045333904 ! @@@ create No.4 pthread success; pthreadId = 138174488 ! @@@ create No.5 pthread success; pthreadId = 138174492 ! ### No.6 pthread start exec; pthread id = 3024354192 ! ### No.6 pthread start exec; pthread id = 3034844048 ! @@@ create No.6 pthread success; pthreadId = 138174496 ! @@@ create No.7 pthread success; pthreadId = 138174500 ! ### No.8 pthread start exec; pthread id = 3003374480 ! ### No.8 pthread start exec; pthread id = 3013864336 ! @@@ create No.8 pthread success; pthreadId = 138174504 ! @@@ create No.9 pthread success; pthreadId = 138174508 ! ### No.10 pthread start exec; pthread id = 2992884624 ! $$$ NO.10 pthread cirle exec !$$$ NO.10 pthread cirle exec !$$$ NO.10 pthread cirle exec !$$$ NO.10 pthread cirle exec !$$$ NO.10 pthread cirle exec !$$$ NO.10 pthread cirle exec !$$$ NO.10 pthread cirle exec !$$$ NO.10 pthread cirle exec !$$$ NO.10 pthread cirle exec !$$$ NO.10 pthread cirle exec !$$$ NO.10 pthread cirle exec !$$$ NO.10 pthread cirle exec !$$$ NO.10 pthread cirle exec !$$$ NO.10 pthread cirle exec !$$$ NO.10 pthread cirle exec !从以“###”开头的字符结果可以看出：以“###No.0”、“###No.2”、“###No.5、“###No.7”、“###No.7”的字符没有；从这可以看出，我们从pthread_create 函数的第四个参数传入的值是不安全的，应该受到安全保护。

2：此问题原因pthread_create 函数是非阻塞函数；会立即返回，主线程不会在创建线程后阻塞到线程函数执行完后返回，导致线程函数获取到循环变量 i 的值被主线程立即改变。

3：此问题解决方法利用条件变量的保护，在线程函数中把 pthread_create 第四个参数传入的值保存为局部变量。

#include <pthread.h>#include <stdio.h>pthread_cond_t m_Cond;pthread_mutex_t m_Locker;void * threadFunc( void * pm_iPthreadId ){pthread_mutex_lock(&m_Locker);unsigned int iId = * ( unsigned int * ) pm_iPthreadId;printf( "### No.%d pthread start exec; pthread id = %u !\n", iId, pthread_self() );pthread_cond_signal(&m_Cond);pthread_mutex_unlock(&m_Locker);time_t iStartTime = time( NULL );while(1){time_t iTempTime = time( NULL );if ( iTempTime - iStartTime > 2 ){printf( "$$$ NO.%d pthread cirle exec !\n", * ( unsigned int * ) pm_iPthreadId );iStartTime = time( NULL );}}}int main(){pthread_mutex_init(&m_Locker, NULL);pthread_cond_init(&m_Cond, NULL);pthread_t * pthreadId;pthreadId = new pthread_t[10];for( unsigned int i = 0; i < 10; i++ ){pthread_mutex_lock(&m_Locker);int iRet = pthread_create( pthreadId + i, NULL, threadFunc, &i ); if ( iRet == 0 ){printf( "@@@ create No.%d pthread success; pthreadId = %u !\n", i, pthreadId + i );}pthread_cond_wait(&m_Cond, &m_Locker);pthread_mutex_unlock(&m_Locker);}while(1){}return 0;}上面代码运行结果：@@@ create No.0 pthread success; pthreadId = 147902472 !### No.0 pthread start exec; pthread id = 3087563664 !@@@ create No.1 pthread success; pthreadId = 147902476 !### No.1 pthread start exec; pthread id = 3077073808 !@@@ create No.2 pthread success; pthreadId = 147902480 !### No.2 pthread start exec; pthread id = 3066583952 !@@@ create No.3 pthread success; pthreadId = 147902484 !### No.3 pthread start exec; pthread id = 3056094096 !@@@ create No.4 pthread success; pthreadId = 147902488 !### No.4 pthread start exec; pthread id = 3045604240 !@@@ create No.5 pthread success; pthreadId = 147902492 !### No.5 pthread start exec; pthread id = 3035114384 !@@@ create No.6 pthread success; pthreadId = 147902496 !### No.6 pthread start exec; pthread id = 3024624528 !@@@ create No.7 pthread success; pthreadId = 147902500 !### No.7 pthread start exec; pthread id = 3014134672 !@@@ create No.8 pthread success; pthreadId = 147902504 !### No.8 pthread start exec; pthread id = 3003644816 !@@@ create No.9 pthread success; pthreadId = 147902508 ! ### No.9 pthread start exec; pthread id = 2993154960 ! $$$ NO.10 pthread cirle exec !$$$ NO.10 pthread cirle exec !$$$ NO.10 pthread cirle exec !$$$ NO.10 pthread cirle exec !$$$ NO.10 pthread cirle exec !$$$ NO.10 pthread cirle exec !$$$ NO.10 pthread cirle exec !$$$ NO.10 pthread cirle exec !$$$ NO.10 pthread cirle exec !$$$ NO.10 pthread cirle exec !$$$ NO.10 pthread cirle exec !$$$ NO.10 pthread cirle exec !$$$ NO.10 pthread cirle exec !。