c语言多线程

#include

#include

#include

#include

#include

#define NUM_THREADS 8

void *PrintHello(void *args)

{

int thread_arg;

sleep(1);

thread_arg = (int)(*((int*)args));

printf("Hello from thread %d\n", thread_arg);

return NULL;

}

int main(void)

{

int rc,t;

pthread_t thread[NUM_THREADS];

for( t = 0; t < NUM_THREADS; t++ )

{

printf("Creating thread %d\n", t);

rc = pthread_create(&thread[t], NULL, PrintHello, &t);

if (rc)

{

printf("ERROR; return code is %d\n", rc);

return EXIT_FAILURE;

}

}

sleep(5);

for( t = 0; t < NUM_THREADS; t++ )

pthread_join(thread[t], NULL);

return EXIT_SUCCESS;

}
这段程序为什么都是输出Hello from thread 8，而不是0-7

因为您传入的是t的地址：rc = pthread_create(&thread[t], NULL, PrintHello, &t);

所以在PrintHello函数中thread_arg = (int)(*((int*)args));读取时会读取到t的最新值，即8.
这是因为主线程一直在执行，for( t = 0; t < NUM_THREADS; t++ )很快执行完，跳出循环t的值为8，而在PrintHello中sleep(1);肯定会读取到t的最新值8.
直接传t的值即可，如下：
#include
#include 
#include 
#include  // 加入这个头文件
#include 
#include 

#define    NUM_THREADS     8

void *PrintHello(void *args)
{
          int thread_arg;
          sleep(1);
          thread_arg = (int)(uintptr_t)args; //这样读取传来的t的值
          printf("Hello from thread %d\n", thread_arg);
          return NULL;

}

int main(void)
{
        int rc,t;
        pthread_t thread[NUM_THREADS];
        for( t = 0; t < NUM_THREADS; t++ )
        { 
            printf("Creating thread %d\n", t);
            rc = pthread_create(&thread[t], NULL, PrintHello, (void *)(uintptr_t)t); // 这样传t的值
            if (rc)
            {
                printf("ERROR; return code is %d\n", rc);
                return EXIT_FAILURE;
            }
        }
        sleep(5);
        for( t = 0; t < NUM_THREADS; t++ )
            pthread_join(thread[t], NULL);
        return EXIT_SUCCESS;
}