c/c++: 多线程编程基础讲解（三）

http://blog.csdn.net/lzx_bupt/article/details/6910632

线程会创建了，如何在线程调用函数时，传入参数呢？则应如下所示：

#include <iostream>
#include <pthread.h>

using namespace std;

#define NUM_THREADS 5

void* say_hello(void* args)
{
    int i = *((int*)args);//对传入的参数进行强制类型转换，由无类型指针变为整形数指针，然后再读取；
    cout << "hello in " << i << endl;
}

int main()
{
    pthread_t tids[NUM_THREADS];
    cout << "hello in main..." << endl;
    for(int i = 0; i < NUM_THREADS; ++i)
    {
        int ret = pthread_create(&tids[i], NULL, say_hello, (void *)&i);//传入的时候必须强制转换为void* 类型，即无类型指针
        cout << "Current pthread id =" << tids[i] << endl;//这里学会使用tids数组打印创建的进程id信息；
        if (ret != 0)
        {
           cout << "pthread_create error: error_code=" << ret << endl;
        }
    }

    pthread_exit(NULL);
}

编译、运行，结果如下：

Current pthread id =139671233451792
Current pthread id =139671222961936
Current pthread id =139671212472080
Current pthread id =139671201982224
Current pthread id =139671191492368
hello in 4196496
hello in 4196496
hello in 4196496
hello in 4196496
hello in 4196496

是否发现了问题？对，i的值没有输出预想的结果，这是因为多线程造成的，主进程在i还未赋值时，线程已经开始跑啦！~

那么下面代码是正确的：

#include <iostream>
#include <pthread.h>

using namespace std;

#define NUM_THREADS 5

void* say_hello(void* args)
{
    cout << "hello in thread " << *((int *)args) << endl;
}

int main()
{
    pthread_t tids[NUM_THREADS];
    int indexes[NUM_THREADS];//用个数组来保存i的值，就不会变了

    for(int i = 0; i < NUM_THREADS; ++i)
    {
        indexes[i] = i;//先保存i的值，在调用线程就不会出现问题了
        int ret = pthread_create( &tids[i], NULL, say_hello, (void *)&(indexes[i]) );
        if (ret != 0)
        {
           cout << "pthread_create error: error_code=" << ret << endl;
        }
    }
    for (int i = 0; i < NUM_THREADS; ++i)
        pthread_join(tids[i], NULL);
}

编译、运行：（源程序去掉了打印线程id的废话)

[cpp@node2 pthread]$ ./ex_create_args_ok
hello in thread 3
hello in thread 4
hello in thread 2
hello in thread 1
hello in thread 0