Question

我正在使用创建子线程的Pthreads编写C程序。创建子线程后，父线程应该输出两条消息：＆＃34; parent：begin＆＃34;然后它应该打印＆＃34; parent：done＆＃34;。儿童线程相同＆＃34;孩子：开始＆＃34;和孩子：完成＆＃34;。我必须确保主线程在生成（子）线程之前打印他的第二条消息。我必须遵循实现，但它只打印错误的顺序。我想我应该使用旗帜。任何帮助将不胜感激。

    <b:widget id='BlogArchive1' locked='false' title='' type='BlogArchive'>
              <b:includable id='main'>
  <b:if cond='data:title != &quot;&quot;'>
    <h2><data:title/></h2>
  </b:if>
  <div class='widget-content'>
  <div id='ArchiveList'>
  <div expr:id='data:widget.instanceId + &quot;_ArchiveList&quot;'>
    <b:include cond='data:style == &quot;HIERARCHY&quot;' data='data' name='interval'/>
    <b:include cond='data:style == &quot;FLAT&quot;' data='data' name='flat'/>
    <b:include cond='data:style == &quot;MENU&quot;' data='data' name='menu'/>
  </div>
  </div>
  <b:include name='quickedit'/>
  </div>
</b:includable>
              <b:includable id='flat' var='data'>
  <ul class='flat'>
    <b:loop values='data:data' var='i'>
      <li class='archivedate'>
        <a expr:href='data:i.url'><data:i.name/></a> (<data:i.post-count/>)
      </li>
    </b:loop>
  </ul>
</b:includable>
              <b:includable id='interval' var='intervalData'>
  <b:loop values='data:intervalData' var='interval'>
    <ul class='hierarchy'>
      <li expr:class='&quot;archivedate &quot; + data:interval.expclass'>
        <b:include cond='data:interval.toggleId' data='interval' name='toggle'/>
        <a class='post-count-link' expr:href='data:interval.url'>
          <data:interval.name/>
        </a>
        <span class='post-count' dir='ltr'>(<data:interval.post-count/>)</span>
        <b:include cond='data:interval.data' data='interval.data' name='interval'/>
        <b:include cond='data:interval.posts' data='interval.posts' name='posts'/>
      </li>
    </ul>
  </b:loop>
</b:includable>
              <b:includable id='menu' var='data'>
  <select expr:id='data:widget.instanceId + &quot;_ArchiveMenu&quot;'>
    <option value=''><data:title/></option>
    <b:loop values='data:data' var='i'>
      <option expr:value='data:i.url'><data:i.name/> (<data:i.post-count/>)</option>
    </b:loop>
  </select>
</b:includable>
              <b:includable id='posts' var='posts'>
  <ul class='posts'>
    <b:loop values='data:posts' var='post'>
      <li><a expr:href='data:post.url'><data:post.title/></a></li>
    </b:loop>
  </ul>
</b:includable>
              <b:includable id='toggle' var='interval'>
  <a class='toggle' href='javascript:void(0)'>
    <span expr:class='&quot;zippy&quot; + (data:interval.expclass == &quot;expanded&quot; ? &quot; toggle-open&quot; : &quot;&quot;)'>
      <b:if cond='data:interval.expclass == &quot;expanded&quot;'>
        &#9660;&#160;
      <b:elseif cond='data:blog.languageDirection == &quot;rtl&quot;'/>
        &#9668;&#160;
      <b:else/>
        &#9658;&#160;
      </b:if>
    </span>
  </a>
</b:includable>
            </b:widget>

Answer 1

如果您希望父级首先打印done，那么您应该让子线程旋转直到父级完成。（现在父进程在等待子进程旋转。）你还应该使用pthread_join来确保在主线程返回之前完成子进程：

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


volatile int done = 0;

void *child(void *arg) {
    printf("child: begin\n");
    while (done == 0); // spin
    printf("child: done\n");
    return NULL;
}

int main(int argc, char *argv[]) {
    printf("parent: begin\n");
    pthread_t c;
    pthread_create(&c, NULL, child, NULL); // create child
    printf("parent: done\n");
    done = 1;
    pthread_join(c, NULL);
    return 0;
}

在我的机器上，我得到了这个输出：

parent: begin
parent: done
child: begin
child: done

Answer 2

目前访问done而两个线程没有任何同步。正确的方法是通过条件变量向孩子发出父母已完成打印的信号。这导致数据竞赛。

此外，main（）线程在do之前完成执行。在这种情况下，整个过程将会死亡。您可以拨打pthread_join()或只使用pthread_exit()退出主线索。

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

    pthread_cond_t cond=PTHREAD_COND_INITIALIZER;
    pthread_mutex_t mutex=PTHREAD_MUTEX_INITIALIZER;

    volatile int done = 0;

    void *child(void *arg) {
      printf("child\n");
      pthread_mutex_lock(&mutex);
      while(done == 0)
        pthread_cond_wait(&cond, &mutex);

      pthread_mutex_unlock(&mutex);
      printf("child:done");
      return NULL;
   }

    int main(int argc, char *argv[]) {
      printf("parent: begin\n");
      pthread_t c;
      pthread_create(&c, NULL, child, NULL); // create child

      pthread_mutex_lock(&mutex);
      done = 1;
      printf("parent: end\n");
      pthread_cond_signal(&cond);
      pthread_mutex_unlock(&mutex);

      pthread_exit(0);
   }

Pthread in C基本打印

2 个答案: