符号引用错误

当我用g++ thread.cpp -o thread -lpthread编译时，我得到了这个错误，我似乎找不到引用错误:

Undefined                       first referenced
symbol                             in file
sem_destroy                         /var/tmp//ccfHWR7G.o
sem_init                            /var/tmp//ccfHWR7G.o
sem_post                            /var/tmp//ccfHWR7G.o
sem_wait                            /var/tmp//ccfHWR7G.o
ld: fatal: symbol referencing errors. No output written to thread
collect2: ld returned 1 exit status

我的头文件只包含一些全局变量和函数名:

#ifndef THREAD_H_
#define THREAD_H_
#define NUM_THREADS 6
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <iostream>
#include <semaphore.h>
#include <fstream>
#include <unistd.h>
using namespace std;
sem_t SEM;
fstream of;
pthread_t thread[NUM_THREADS];
pthread_attr_t attr;
int rc;
long t;
void *status;
void *Busy_Work(void *t);
void Open_Initialize_File();
void Initialize_Set_Attribute();
void Create_Thread();
void Free_Attricutes_Wait();


#endif /* THREAD_H_ */

这是我的主文件和检查与IDE没有任何基本的语法，我错过了:

#include "thread.h"
int main (int argc, char *argv[]) {
Open_Initialize_File();
Initialize_Set_Attribute();
Create_Thread();
Free_Attricutes_Wait();
sem_destroy(&SEM);
cout << "Main: program completed" << endl << "Exiting" << endl;
pthread_exit(NULL);
}
void Open_Initialize_File() {
of.open("SHARED.txt");
of << pthread_self() << "r" << endl;
of.close();
}
void Initialize_Set_Attribute() {
sem_init(&SEM, 0, 1);
pthread_attr_init(&attr);
pthread_attr_setdetachstate(&attr,PTHREAD_CREATE_JOINABLE);
}
void Create_Thread() {
for (int t = 0; t < NUM_THREADS; t++) {
    rc = pthread_create(&thread[t], &attr, Busy_Work, (void *)(t+1));
    if (rc) {
        cout << "ERROR: return code from pthread_create() is " << rc << endl;
        exit(-1);
    }
}
}
void *Busy_Work(void *t) {
int i;
long tid = (long)t;
for (i=0; i<10; i++)
{
    sem_wait(&SEM);
    cout << "Thread" << tid << "Thread id< " << pthread_self() << " >running..." << endl;
    of.open("SHARED.txt",std::fstream::app);//opening file in append mode
    of << pthread_self() << "r" << endl;
    of.close();
    sem_post(&SEM);
    if(tid%2==0)
        sleep(2);
    else
        sleep(3);
}
pthread_exit((void*) t);
}
void Free_Attricutes_Wait() {
pthread_attr_destroy(&attr);
for (t = 0; t < NUM_THREADS; t++) {
    rc = pthread_join(thread[t], &status);
    if (rc) {
        cout << "ERROR: return code from pthread_join() is" << rc << endl;
        exit(-1);
    }
    cout << "Thread " << t << " Thread id <" << pthread_self() << ">exited" << endl;
}
}