Problems initializing pthreads

Question

Hi,

I'm debugging a program using pthreads for a class I have to solve a producer consumer problem. What I'm running into is that I get an error saying that I have a undefined reference to the create a join functions. After that I put the initialization functions in but now I get the error that none of them were declared within the scope, and removing the using namespace std does not help. I have already included pthread.h and have only made global the variables that need to be shared. I have check with the text book and I'm doing everything that he did (although he leaves out lines in favor of a single comment saying what was done a lot) and can't find and problems online ether. What should I do to fix this error. Here's my code:

#include <iostream>
#include <pthread.h>
#include <time.h>
#include <queue>
using namespace std;

#define NUM_THRDS  128
#define MAX  64
queue<int> q;
int inq=0;
pthread_mutex_t q_mutex;
int num_items=0;

void *produce(void *);
void *consume(void *);

int main()
{
  pthread_t thrds [NUM_THRDS];
  pthread_attr_t attr;
  time_t sec,sec2;

  pthread_init();
  ptread_attr_init(&attr);
  pthread_attr_setscope(&attr, PTHREAD_SCOPE_SYSTEM);
  pthread_mutex_init(&q_mutex,NULL);

  sec = time(NULL);
  cout<<sec<<endl;

  for(int i=0; i<NUM_THRDS; i++)
    {
      if(i%2==0)
    pthread_create(&thrds[i], &attr, produce, NULL);
      else
    pthread_create(&thrds[i], &attr, consume, NULL);
    }

  for(int i=0; i<NUM_THRDS; i++)
    pthread_join(thrds[i],NULL);

  sec2=time(NULL);
 cout<<sec2<<endl<<"Total time "<<sec2-sec<<" seconds."<<endl;

 cout<<endl<<endl;
  return 0;
}

void *produce(void * n)
{

  while(true)
  {
    pthread_mutex_lock(&q_mutex);
    if(num_items >= 1000)
      {
        pthread_mutex_unlock(&q_mutex);
        pthread_exit(0);
      }

    if(inq < MAX)
      {
        q.push(1);
        inq++;
        num_items++;

      }
    pthread_mutex_unlock(&q_mutex);
  }
}

void *consume(void * n)
{
  while(true)
    {
      pthread_mutex_lock(&q_mutex);
      if(num_items >= 1000 && inq == 0)
    {
      pthread_mutex_unlock(&q_mutex);
      pthread_exit(0);
    }
      if(inq > 0)
    {
      q.pop();
      inq--;
    }
      pthread_mutex_unlock(&q_mutex);
    }
}

The last time I compiled I got the following error (copy pasted out of the bash terminal):

queue_program.cpp: In function ‘int main()’: queue_program.cpp:23: error: ‘pthread_init’ was not declared in this scope queue_program.cpp:24: error: ‘ptread_attr_init’ was not declared in this scope

Note: please don't leave comment telling me that the progrm does nothing. The point of this homework was to implement producer and consumer threads to a queue and provide mutex, not to do anything with what is actually going in or out of the queue.

Answer 1

It would be helpful if you were to post the exact error message that you're getting.

But my guess is that it's a linker error. You need to link against the pthread library, by adding -lpthread to the linker flags (e.g. in your makefile).

Answer 2

Most likely, you either:

Forgot to declare the header as extern "C".extern "C" { #include <pthreads.h> }
Didn't link against the static library.

Answer 3

queue_program.cpp:23: error: ‘pthread_init’ was not declared in this scope 

There is no function called pthread_init

queue_program.cpp:24: error: ‘ptread_attr_init’ was not declared in this scope

You spelled pthread wrong.

Answer 4

The first error is that you're calling a non-existant function, pthread_init(). Remove that line. (There are no standard function called pthread_init , where did you learn about it ?)

The second error is that you misspelled pthread_attr_init , change pthread_attr_init.