创建N个线程 [英] Creating N number of threads

问题描述

我编写了以下代码来创建N个线程并打印每个线程的线程ID.

I wrote the following code to create N number of threads and print the thread ID of each thread.

#include<stdio.h>
#include<pthread.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/syscall.h>
#include <unistd.h>


void *threadFunction (void *);

int main (void)
{

   int n=0,i=0,retVal=0;
   pthread_t *thread;

   printf("Enter the number for threads you want to create between 1 to 100 \n");
   scanf("%d",&n);

   thread = (pthread_t *) malloc (n*sizeof(pthread_t));

   for (i=0;i<n;i++){
       retVal=pthread_create(&thread[i],NULL,threadFunction,(void *)&i);
       if(retVal!=0){
           printf("pthread_create failed in %d_th pass\n",i);
           exit(EXIT_FAILURE);        
       }
   }

   for(i=0;i<n;i++){
        retVal=pthread_join(thread[i],NULL);
            if(retVal!=0){
               printf("pthread_join failed in %d_th pass\n",i);
               exit(EXIT_FAILURE);        
            }
   }

}

void *threadFunction (void *arg)
{
    int threadNum = *((int*) arg);

    pid_t tid = syscall(SYS_gettid);

    printf("I am in thread no : %d with Thread ID : %d\n",threadNum,(int)tid);


}

我传递给每个线程的参数是一个计数器i，对于每个新线程，该计数器从0递增到n-1. 但是在输出中我看到我所有线程的值均为零，无法理解，有人可以解释一下.

the Argument i am passing to each thread is a counter i which increments from 0 to n-1 for each new thread. But while at the output i see i has the value zero for all the threads, not able to undestand , can somebody please explain.

  Enter the number for threads you want to create between 1 to 100 
  5
  I am in thread no : 0 with Thread ID : 11098
  I am in thread no : 0 with Thread ID : 11097
  I am in thread no : 0 with Thread ID : 11096
  I am in thread no : 0 with Thread ID : 11095
  I am in thread no : 0 with Thread ID : 11094

推荐答案

问题出在以下几行:

retVal=pthread_create(&thread[i],NULL,threadFunction,(void *)&i);

请勿传递i的地址，因为i在主函数中会不断变化.而是传递i的值，并在线程函数中适当地进行类型转换并使用.

Don't pass the address of i, as i keeps changing in the main function. Instead pass the value of i and typecast it appropriately in the thread function and use.

例如，传递如下值:

retVal=pthread_create(&thread[i],NULL,threadFunction,(void *)i);

在线程函数访问中，如下所示:

In the thread function access as below:

void *threadFunction (void *arg)
{
    int threadNum = (int)arg;

    pid_t tid = syscall(SYS_gettid);

    printf("I am in thread no : %d with Thread ID : %d\n",threadNum,(int)tid);


}

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