当调用pthread_create 第三个和第四个参数不为空时，要注意第四个参数的传递方法。
一段流行的代码：
#include <iostream>
#include <pthread.h>
using namespace std;
#define NUM_THREADS 10
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);
}
程序运行结果：
hello in main...
Current pthread id =140210171029248
Current pthread id =140210162636544
Current pthread id =140210154243840
Current pthread id =140210145851136
Current pthread id =140210137458432
Current pthread id =140210129065728
Current pthread id =140210120673024
Current pthread id =140210112280320
hello in 6
hello in 6
hello in 6
hello in 6
hello in hello in 6
hello in 6
7
Current pthread id =140210103887616
Current pthread id =140210095494912
hello in 10
hello in 10
hello in 10
显然与预期不一致，我猜想原因：我们 pthread_create的时候，传入i的方式是 取i的地址指针 再强制转换为 无类型指针，然后在sayhello函数里，再把无类型指针转换为int类型指针（这样做是为了满足phtread_create的参数设定），再通过 * 号取值。这意味着，我们是通过指向 i 的指针来获取i的值的，但是在多个线程切换间，i 的值已经改变了，但是指向 i 的地址并没有改变，多个线程随机竞争执行，所以通过 i 的地址 取值，显然取到的结果和预期的很难一致。
流行的一种做法：把 i 赋值给一个数组保存，这样就避免了在同一地址取值。
#include <iostream>
#include <pthread.h>
using namespace std;
#define NUM_THREADS 10
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);
}
运行结果：
hello in main..
Current pthread id = 140457201362688
Current pthread id = 140457192969984
Current pthread id = 140457184577280
Current pthread id = 140457176184576
Current pthread id = 140457167791872
Current pthread id = 140457159399168
Current pthread id = 140457151006464
Current pthread id = 140457142613760
Current pthread id = 140457134221056
Current pthread id = 140457125828352
hello in 3
hello in 4
hello in 5
hello in 6
hello in 7
hello in 8
hello in 9
hello in 2
hello in 1
hello in 0
还有一种写法也可以： 直接把 i 的值 强制转换成 无类型指针 传过去，然后再 在 被调函数里 把无类型指针参数 强制转换回来，注意，在64 位Linux机里，指针是8个字节，int是4个字节，long也是8个字节，所以必须转换成long型，否则会因丢失精度而报错。从这里也可以看出来，第一个程序结果和预期不一致 不是因为 i 的值和预期不一致，而是 i 的地址出问题了。
#include <iostream>
#include <pthread.h>
using namespace std;
#define NUM_THREADS 10 //线程数
void* say_hello( void* args )
{
cout << "hello in " << long(args) << endl;
}
int main()
{
pthread_t tids[NUM_THREADS]; //线程id
cout << "hello in main.." << endl;
for( int i = 0; i < NUM_THREADS; ++i )
{
int ret = pthread_create( &tids[i], NULL, say_hello, (void*)i );
//直接把i的值传过去
cout << "Current pthread id = " << tids[i] << endl;
//用tids数组打印创建的进程id信息
if( ret != 0 ) //创建线程成功返回0
{
cout << "pthread_create error:error_code=" << ret << endl;
}
}
for( int i = 0; i < NUM_THREADS; ++i )
{
pthread_join(tids[i],NULL);
}
}
运行结果：
hello in main..
Current pthread id = 140406320125696
Current pthread id = 140406311732992
Current pthread id = 140406303340288
Current pthread id = 140406294947584
Current pthread id = 140406286554880
Current pthread id = 140406278162176
Current pthread id = 140406269769472
Current pthread id = 140406261376768
Current pthread id = 140406252984064
Current pthread id = 140406244591360
hello in 6
hello in 7
hello in 8
hello in 9
hello in 5
hello in 4
hello in 3
hello in 2
hello in 1
hello in 0