why sem_wait doesn't wait semaphore on mac OSX?

Question

The following code shows a producer-consumer example: Once a product is produced, the consumer will get this product.

But I'm surprised that the consumer will sill get a product when there is no product.

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

#define NUM 5
int queue[NUM];
int i;
sem_t *blank_number, *product_number;

void *producer(void *arg) {
    int p = 0;
    while (1) {
        sem_wait(blank_number);
        queue[p] = rand() % 1000 + 1;
        printf("Produce queue[%d]:%d\n", p, queue[p]);
        i = sem_post(product_number);
        //printf("i_p=%d\n", i);
        p = (p+1)%NUM;
        sleep(rand()%5);
    }
}

void *consumer(void *arg) {
    int c = 0;
    while (1) {
        sem_wait(product_number);
        printf("Consume queue[%d]:%d\n", c, queue[c]);
        queue[c] = 0;
        i = sem_post(blank_number);
        //printf("i_c=%d\n", i);
        c = (c+1)%NUM;
        sleep(rand()%5);
    }
}

int main(int argc, char *argv[]) {
    pthread_t pid, cid;  

    //set blank_number to NUM
    blank_number = sem_open("blank_number", O_CREAT, S_IRWXU, NUM);
    if(blank_number == SEM_FAILED){
        perror("open blank_number");
        return 1;
    }
    //set product_number to 0
    product_number = sem_open("product_number", O_CREAT, S_IRWXU, 0);
    if(product_number == SEM_FAILED){
        perror("open product_number");
        return 1;
    }

    pthread_create(&pid, NULL, producer, NULL);
    pthread_create(&cid, NULL, consumer, NULL);
    pthread_join(pid, NULL);
    pthread_join(cid, NULL);
    sem_close(blank_number);
    sem_close(product_number);
    return 0;
}

In my test result, there is only one product : 808, but the consumer gets two products : 808 and 0;

$ sudo ./a.out
Produce queue[0]:808
Consume queue[0]:808
Consume queue[1]:0

Is there any wrong in my code?

Answer 1

Your problem is that you never deleted your semaphores. So when you open them you recover some old/bad state. Try to open with O_EXCL you will be able to observe the problem.

Write a simple command to delete them with sem_unlink() or initialize them before using them with semctl .

You also need to set the appropriate values in sem_open not 022...

Alos note that POSIX named semaphores should have a name starting with / .

Change the beginning of your main to :

sem_unlink("blank_number");
sem_unlink("product_number");
//set blank_number to 1
blank_number = sem_open("blank_number", O_CREAT|O_EXCL, S_IRWXU, 1);
if(blank_number == SEM_FAILED){
    perror("open blank_number");
    return 1;
}
//set product_number to 0
product_number = sem_open("product_number", O_CREAT|O_EXCL, S_IRWXU, 0);
if(product_number == SEM_FAILED){
    perror("open product_number");
    return 1;
}

Answer 2

Maybe try to use sem_init with an unnamed semaphore instead of sem_open :

sem_t semaphore;
int ret = sem_init(&semaphore, 0, 0);