[英]Creating N number of threads
I wrote the following code to create N number of threads and print the thread ID of each thread. 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。
#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);
}
the Argument i am passing to each thread is a counter i which increments from 0 to n-1 for each new thread. 我传递给每个线程的参数是一个计数器i,每个新线程的计数器从0递增到n-1。 But while at the output i see i has the value zero for all the threads, not able to undestand , can somebody please explain. 但是在输出中我看到所有线程的值均为零,无法理解,有人可以解释一下。
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
The problem lies in the below line: 问题出在以下几行:
retVal=pthread_create(&thread[i],NULL,threadFunction,(void *)&i);
Don't pass the address of i
, as i
keeps changing in the main function. 不要传递i
的地址,因为i
在main函数中不断变化。 Instead pass the value of i and typecast it appropriately in the thread function and use. 而是传递i的值,并在线程函数中适当地类型转换并使用。
For Example, pass the value like below: 例如,传递如下值:
retVal=pthread_create(&thread[i],NULL,threadFunction,(void *)i);
In the thread function access as below: 在线程函数访问如下:
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);
}
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