pthread_once的pthread_once

Posted 2023-05-17

参考技术A

POSIX的只执行一次的pthread_once#ifdefWIN32#include<windows.h>#defineSLEEP(ms)Sleep(ms)#elseifdefined(LINUX)#include<stdio.h>#defineSLEEP(ms)sleep(ms)#endif#include<cassert>#include<pthread.h>//取出线程的IDintGetThreadID()#ifdefWIN32return(int)pthread_getw32threadhandle_np(pthread_self());#elseifdefined(LINUX)return(int)pthread_self();#endif//该静态变量被所有线程使用staticints_nThreadResult=0;staticpthread_once_tonce=PTHREAD_ONCE_INIT;//该初始化函数，我在多线程下只想执行一次voidthread_init()s_nThreadResult=-1;printf([Child%0.4x]loopingi(%0.8x)\\n,GetThreadID(),s_nThreadResult);void*theThread(void*param)//通过once的控制，thread_init只会被执行一次pthread_once(&once,&thread_init);printf([Child%0.4x]loopingi(%0.8x)\\n,GetThreadID(),s_nThreadResult);s_nThreadResult++;pthread_exit(&s_nThreadResult);returnNULL;intmain(intargc,char*argv[])pthread_ttid1,tid2;pthread_create(&tid1,NULL,&theThread,NULL);pthread_create(&tid2,NULL,&theThread,NULL);//无论是否休眠，两个子线程最终都会被主线程join到//因为两个子线程都是默认的PTHREAD_CREATE_JOINABLE类型//SLEEP(3);void*status=NULL;intrc=pthread_join(tid1,&status);assert(rc==0&&pthread_join1,rc);if(status!=PTHREAD_CANCELED&&status!=NULL)printf(Returnedvaluefromthread:%d\\n,*(int*)status);rc=pthread_join(tid2,&status);assert(rc==0&&pthread_join2,rc);if(status!=PTHREAD_CANCELED&&status!=NULL)printf(Returnedvaluefromthread:%d\\n,*(int*)status);return0;

线程的创建，pthread_create，pthread_self，pthread_once

typedef unsigned long int pthread_t;

//come from /usr/include/bits/pthreadtypes.h

int pthread_create(pthread_t *thread, const pthread_attr_t *attr,void *(*start_routine) (void *), void *arg);创建新的线程

 

pthread_t pthread_self(void);获取本线程的线程ID

 

int pthread_equal(pthread_t t1, pthread_t t2);判断两个线程ID是否指向同一线程

 

int pthread_once(pthread_once_t *once_control, void (*init_routine) (void))；用来保证init_routine线程函数在进程中只执行一次。

 

#include <stdio.h>
#include <pthread.h>
#include <unistd.h>
#include <stdlib.h>



int* thread_func(void* arg)
{
    pthread_t new_thid;
    new_thid = pthread_self();//打印线程自己的线程ID
    printf("the new thread, ID is %lu\n", new_thid);

    return NULL;
}


int main()
{
    pthread_t thid;

    printf("main thread, ID is %lu\n", pthread_self());//打印主线程自己的线程ID

    if (pthread_create(&thid, NULL, (void*)thread_func, NULL) != 0)
    {
        printf("create thread failed\n");
        exit(0);
    }

    
    sleep(5);

    return 0;
}

 

某些情况下，函数执行次数要被限制为1次，这种情况下，要使用pthread_once，代码示例：

#include <stdio.h>
#include <pthread.h>
#include <unistd.h>
#include <stdlib.h>


pthread_once_t once = PTHREAD_ONCE_INIT;

void run(void)
{
    printf("function run is running in thread:%lu\n", pthread_self());
}


int* thread1(void* arg)
{
    pthread_t new_thid;
    new_thid = pthread_self();
    printf("current thread ID is %lu\n", new_thid);
    pthread_once(&once, run);
    printf("thread1 end\n");
    return NULL;
}

int* thread2(void* arg)
{
    pthread_t new_thid;
    new_thid = pthread_self();
    printf("current thread ID is %lu\n", new_thid);
    pthread_once(&once, run);
    printf("thread2 end\n");
    return NULL;
}

int main()
{
    pthread_t thid1, thid2;

    printf("main thread, ID is %lu\n", pthread_self());

    pthread_create(&thid1, NULL, (void*)thread1, NULL);
    pthread_create(&thid2, NULL, (void*)thread2, NULL);
    
    sleep(5);    
    printf("main thread exit\n");

    return 0;
}

运行结果：

main thread, ID is 3076200128
current thread ID is 3067804480
function run is running in thread:3067804480
thread2 end
current thread ID is 3076197184
thread1 end
main thread exit

虽然在thread1 跟thread2中都调用了run函数，但是run函数只执行了一次。