cannot use std in subthread

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C++

Linux

when I use gcc to compile this program, I get bunches of errors, like:"Undefined reference to std::cout","Undefined reference to std::basic_ostream",etc.
Anyone help me?

This is my program:

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

void *function1(void *arg);
void *function2(void *arg);

int main(int argc, char *argv[])
{
  pthread_t p1,p2;
  pthread_create(&p1,NULL,function1,NULL);
  pthread_create(&p2,NULL,function2,NULL);
  pthread_exit(NULL);
  return 0;
}

void *function1(void *arg)
{
  std::cout<<"thread1"<<std::endl;
}

void *function2(void *arg)
{
  std::cout<<"thread2"<<std::endl;
}