Symbol referencing errors - c++

Question

I'm getting this error when I compile with g++ thread.cpp -o thread -lpthread and I can't seem to find the referencing error:
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
My header file just contains some global variables and function names:
#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_ */
This is my main file and check with an IDE there wasn't any basic syntax that I missed:
#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;
}
}

Answer 1

The linker is missing some symbols, that might indicate that some additional external library needs to be linked as well. On a unix platform a good way to figure out how to fix this is to have a look at the man pages:
man sem_init
This will usually tell you what you need to do. In this case it would specify that you need to link with -lrt