C Semaphore strange precedence behavior

Question

I'm practicing with concurrency in C, and I seem to encounter some problems with semaphores. I'm using Xcode 6.3.2 in MacOSX.

Here it is a sample program that seems to act strangely: the purpose of the example is to print either ABCD or BACD strings

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

void *thread1(void*);
void *thread2(void*);

sem_t *sem0, *sem1, *sem2;;

int main(int argc, const char * argv[]) {

    pthread_t t1, t2;

    sem0 = sem_open("sem0", O_CREAT, 0600, 2);
    sem1 = sem_open("sem1", O_CREAT, 0600, 0);
    sem2 = sem_open("sem2", O_CREAT, 0600, 0);

    // quick check
    if (sem0 == SEM_FAILED || sem1 == SEM_FAILED || sem2 == SEM_FAILED) {
        printf("Something went wrong\n");
        return 0;
    }

    pthread_create(&t1, NULL, thread1, NULL);
    pthread_create(&t2, NULL, thread2, NULL);

    pthread_join(t1, NULL);
    pthread_join(t2, NULL);

    sem_close(sem0);
    sem_close(sem1);
    sem_close(sem2);

    return 0;
}

void *thread1(void* arg) {
    int n=0;
    while (n<10) {
        sem_wait(sem0);
        printf("A");
        fflush(stdout);
        sem_wait(sem1);
        printf("C");
        fflush(stdout);
        sem_post(sem2);
        n++;
    }
    pthread_exit(NULL);
}

void *thread2(void* arg) {
    int n=0;
    while (n<10) {
        sem_wait(sem0);
        printf("B");
        fflush(stdout);
        sem_post(sem1);
        sem_wait(sem2);
        printf("D\n");
        fflush(stdout);
        sem_post(sem0);
        sem_post(sem0);
        n++;
    }
    pthread_exit(NULL);
}

If I implemented correctly the semaphores, the result would be either ABCD or BACD, but in reality I get a whole variety of strange output.

I will include part of that output here

ABCD
BAD
CABCD
BAD
CBAD
CBAD
CBAD
CBAD
CBAD
CBAD
C

Is someone able to help me? Thanks in advance

IMPORTANT EDIT: I downloaded Ubuntu, and the code is working smoothly there, no problems at all. So, to resume

• MacOSX 10.10.3 with Xcode 6.3.2 --> Not working properly
• Ubuntu 15.04 --> Working properly

Don't know why.

Answer 1

Your problem is probably that printf output is buffered and shared between threads. You'd have to ensure to empty the buffer with fflush before posting on the semaphores.

Answer 2

There's a typo when you call sem_open() for sem2 - the name passed in is "sem1":

sem1 = sem_open("sem1", O_CREAT, 0600, 0);
sem2 = sem_open("sem1", O_CREAT, 0600, 0);
//               ^^^^

So the pointers sem1 and sem2 would be referring to the same semaphore.