Thread: Undefined reference to semaphore functions even though semaphore header is included

Can't seem to figure out this error. I have the semaphore.h file included, but for some reason I keep getting compiler errors stating it can't identify a reference to functions sem_init(), sem_post(), and sem_wait(). Here's the code I have:

Code:

#include <stdlib.h>
#include <semaphore.h>
#include <pthread.h>
#include "buffer.h"

/* POSITION MARKERS (CONCURRENCY ISSUE BECAUSE GLOBAL DATA SHARED AMONG THREADS)*/
int in_mrkr;                    /* in marker points to the next free position in the buffer */
int out_mrkr;                   /* out marker points to the first full position in the buffer*/

buffer_item buffer[BUFFER_SIZE];    /* buffer used by producer/consumer pair */

/* SEMAPHORES */
sem_t empty;                        /* counter indicating all positions of the buffer are available */
sem_t full;                         /* counter indicating all positions of the buffer are unavailable */

/* MUTEXES */
pthread_mutex_t mutex;              /* lock for allowing consumer or producer to access buffer exclusively */

void buffer_init(void) {
    /* initialize markers */
    in_mrkr = 0;
    out_mrkr = 0;

    /* initialize semphores */
    if (sem_init(&empty, 0, BUFFER_SIZE))
        print_err();

    if (sem_init(&full, 0, 0))
        print_err();

    /* initialize mutex lock */
    if (pthread_mutex_init(&mutex, NULL))
        print_err();
}

int insert_item(buffer_item item) {
    /* decrement empty semaphore for producer */
    sem_wait(&empty);

    /* lock thread so producer can add item to the buffer */
    pthread_mutex_lock(&mutex);

    //--- PRODUCER CRITICAL SECTION ---
    /* add item to buffer and move the in marker to next free position */
    buffer[in_mrkr] = item;
    in_mrkr = (in_mrkr + 1) % BUFFER_SIZE;

    /* release the lock on the producer thread */
    pthread_mutex_unlock(&mutex);

    /* increment full semaphore to inidicate buffer has a filled slot */
    sem_post(&full);

    return 0;       // NEED ERROR CHECKING -- indicate successfull return for now
}

int remove_item(buffer_item *item) {
    /* decrement full semaphore for consumer */
    sem_wait(&full);

    /* lock thread so consumer can pull the item off the buffer */
    pthread_mutex_lock(&mutex);

    //--- CONSUMER CRITICAL SECTION ---
    /* remove an item from the buffer and move the out marker to next full position */
    *item = buffer[out_mrkr];
    out_mrkr = (out_mrkr + 1) % BUFFER_SIZE;

    /* release the lock on the consumer thread */
    pthread_mutex_unlock(&mutex);

    /* increment empty semaphore to inidicate buffer has a empty slot */
    int a = sem_post(&empty);

    return 0;       // NEED ERROR CHECKING -- indicate successfull return for now
}

I checked the semaphore header and it contains the prototype definitions as follows:

Code:

/* Initialize semaphore object SEM to VALUE.  If PSHARED then share it
   with other processes.  */
extern int sem_init (sem_t *__sem, int __pshared, unsigned int __value)
     __THROW;

/* Wait for SEM being posted.

   This function is a cancellation point and therefore not marked with
   __THROW.  */
extern int sem_wait (sem_t *__sem);

/* Post SEM.  */
extern int sem_post (sem_t *__sem) __THROW;

I don't understand why I'm getting errors like this from the compiler

In function `buffer_init':
/home/jshowa/NetBeansProjects/producer_consumer_prob/buffer.c:40: undefined reference to `sem_init'
/home/jshowa/NetBeansProjects/producer_consumer_prob/buffer.c:43: undefined reference to `sem_init'

There's error for the insert_item and remove_item functions that contain sem_wait() and sem_post(), but the error is the same.

If anyone could shed some light on this, I'd appreciate it, thanks.

Wow, didn't even notice that. I was checking the description of sem_init() in the Open Group Base Specification document and it says nothing about having to link it with pthreads on compilation.

The Open Group Base Specifications Issue 7

It worked thought. Thanks for the help Codeplug.