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.

Answer 3

Printf is accessing one resource, the standard out file descriptor. You need to request a lock to allow access to stdout by only one thread at a time. Otherwise you may get interleaved text. I'm not sure why you're creating three semaphores, but consider that if you have multiple semaphores blocking the same resource, you have multiple windows open to that resource. If you only close one at a time, then you're not really blocking the resource at all.

edit: An example. thread 1 locks sem 0, thread 2 waits. thread 1 unlocks sem0 and thread 2 can continue. Thread 1 now locks sem1. They're both writing to standard out though. That means that they both, even obeying your locks, are allowed write to stdout.

Why it doesn't happen earlier is just due to thread contention.

edit 2: I think John's right in that my original explanation doesn't cover your question. I can't reproduce your output, and I can't imagine a path to your output. Even with the fflush example someone else mentioned, I still don't see how that particular string would be formed even using the same buffer with the way your semaphores should be working. I can only guess to this:

1. Your semaphores are named and you don't destroy them. That means they persist between invocations of your program. If you ended your program at just the right time, the semaphore values may be in a state you wouldn't expect(they ignore the value parameter of sem_open if the semaphore exists). Maybe that could lead to your output.
2. You're not checking return values of your sem functions, they can return errors. Calling subsequent functions on invalid semaphores should return immediately, thereby leaving your printfs only subject to thread contention.