关于 Linux 下的线程函数未定义问题

#include<pthread.h>
#include<stdio.h>
#include<stdlib.h>
static int counter=0;
#define loops 10000000
static void* thread(void *unused)
{
    int i;
    for(i=0;i<loops;++i)
    {
        ++counter;
    }
    return NULL;
}
int main()
{
    pthread_t t1,t2;
    pthread_create(&t1,NULL,thread,NULL);
    pthread_create(&t2,NULL,thread,NULL);
    pthread_join(t1,NULL);
    pthread_join(t2,NULL);
    printf("counter is %d by thread\n",counter);
    return 0;
}