創建N個線程

Question

我編寫了以下代碼來創建N個線程，並打印每個線程的線程ID。

#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);


}

我傳遞給每個線程的參數是一個計數器i，每個新線程的計數器從0遞增到n-1。 但是在輸出中我看到所有線程的值均為零，無法理解，有人可以解釋一下。

  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

Answer 1

問題出在以下幾行：

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

不要傳遞i的地址，因為i在main函數中不斷變化。 而是傳遞i的值，並在線程函數中適當地類型轉換並使用。

例如，傳遞如下值：

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

在線程函數訪問如下：

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);


}