《linux程序设计》多线程学习

thread8.c

在同一个程序中创建多个线程

#include 
#include 
#include 
#include 

#define NUM_THREADS 6

void *thread_function(void *arg);

int main()
{
    int res;
    pthread_t a_thread[NUM_THREADS];
    void *thread_result;
    int lots_of_threads;

    for(lots_of_threads = 0; lots_of_threads < NUM_THREADS; lots_of_threads++)
    {
        res = pthread_create(&(a_thread[lots_of_threads]), NULL, thread_function, (void *)&lots_of_threads);
        if (res != 0)
        {
            perror("Thread creation failed");
            exit(EXIT_FAILURE);
        }
    //  sleep(3);
    }
    printf("Waiting for threads to finish...\n");
    for(lots_of_threads = NUM_THREADS - 1; lots_of_threads >= 0; lots_of_threads--)
    {
        res = pthread_join(a_thread[lots_of_threads], &thread_result);
        if (res != 0)
        {
            perror("Thread join failed");
        }
        printf("Pick up a thread\n");
    }
    printf("All done\n");
    exit(EXIT_SUCCESS);
}

void *thread_function(void *arg)
{
    int my_number = *(int *)arg;
    int rand_num;

    printf("thread_function is running. Argument was %d\n", my_number);
    rand_num = 1 + (int) (9.0 * rand() / (RAND_MAX + 1.0));
    sleep(rand_num);
    printf("Bye from %d\n", my_number);
    pthread_exit(NULL);
}

运行结果：

奇怪的是一些线程以相同的参数被启动
原因是线程函数的参数是一个局部变量，这个变量在循环中被更新
引起问题的代码行：

    for(lots_of_threads = 0; lots_of_threads < NUM_THREADS; lots_of_threads++)
    {
        res = pthread_create(&(a_thread[lots_of_threads]), NULL, thread_function, (void *)&lots_of_threads);

原因：
如果主线程运行的足够快，就有可能改变某些线程的参数（即lots_of_threads)

解决:
直接传递这个参数的值

    for(lots_of_threads = 0; lots_of_threads < NUM_THREADS; lots_of_threads++)
    {
        res = pthread_create(&(a_thread[lots_of_threads]), NULL, thread_function, (void *)lots_of_threads);

同时修改thread_function函数

void *thread_function(void *arg) {
    int my_number = (int)arg;