Linux 线程代码问题

jackseans

#include <stdio.h>
#include <pthread.h>

pthread_key_t key;

void echomsg(int t)
{
printf("destructor excuted in thread %d,param=%d\n",pthread_self(),t);
}

void * child1(void *arg)
{
int tid=pthread_self();
printf("thread %d enter\n",tid);
pthread_setspecific(key,(void *)tid);
sleep(2);
printf("thread %d returns %d\n",tid,pthread_getspecific(key));
sleep(5);
}

void * child2(void *arg)
{
int tid=pthread_self();
printf("thread %d enter\n",tid);
pthread_setspecific(key,(void *)tid);
sleep(1);
printf("thread %d returns %d\n",tid,pthread_getspecific(key));
sleep(5);
}

int main(void)
{
int tid1,tid2;

printf("hello\n");
pthread_key_create(&key,echomsg);
pthread_create(&tid1,NULL,child1,NULL);
pthread_create(&tid2,NULL,child2,NULL);
sleep(10);
pthread_key_delete(key);
printf("main thread exit\n");
return 0;
}  

这个程序 有些地方不懂 请高手解释下
pthread_key_create(&key,echomsg); 这行代码的意思
定义这个echomsg函数的意思，新创建的两个线程会执行这个函数吗  这个程序主要用处是想干什么