undefined reference to pthread_create in linux (c programming)

Question

#include<stdio.h>
#include<pthread.h>
#include<semaphore.h>

sem_t e,n,s;
int a[10];
int flag=0;
int sizeb=10;

void take()
{
    int out;
    if(flag==0)
    {
        printf("the consumer is waiting\n");
    }
    else
    {
    printf("\n the consumer has entered the cs:\n");
    for(out=0;out<10;out++)
    {

        printf("\t%d",a[out]);
    }
}
}
void consumer()
{
    sem_wait(&n);
    sem_wait(&s);
    printf("\n Consumer Unit\n");
    take();
    if(flag==1)
    {
        printf("\n the consumer has consumed\n");
    }
    sem_post(&s);
    sem_post(&e);
}
void append()
{
    int in;
    printf("\n the producer has entered the cs\n");
    for(in=0;in<10;in++)
    {
        a[in]=in+1;
        printf("\t%d",a[in]);
    }
}
void producer()
{
    sem_wait(&e);
    sem_wait(&s);
    printf("\n producer unit\n");
    append();
    printf("\n the producer has produced\n");
    sem_post(&s);
    sem_post(&n);
    flag=1;
}
int main()
{
    sem_init(&s,0,1);
    sem_init(&n,0,1);
    sem_init(&e,0,20);
    pthread_t p1,p2,p3;
    pthread_create(&p1,NULL,(void *)consumer,NULL);
    pthread_join(p1,NULL);
     pthread_create(&p2,NULL,(void *)producer,NULL);
        pthread_join(p2,NULL);
     pthread_create(&p3,NULL,(void *)consumer,NULL);
        pthread_join(p3,NULL);
    return 0;
}

Answer 1

Using -pthread is preferred to -lpthread in this specific case.

Answer 2

how are you compiling it?? compile using gcc program_name -lpthread

Answer 3

Add -lpthread in the command line when compiling to search for the pthreads library when linking.