Question

Right now I’m working on my project and I have a question about initialization of semaphores. Actually I’m programming on Mac OS X, but I tried to compile me project on Linux and it doesn’t compile. On OS X it compiles but every time just crashing at the moment of initialization.

sem_t *mutex_1, *mutex_2, *mutex_3, *reader, *writer;

int initialization_semaphores (void) 
{
    int ERROR = EOK;
    if ((mutex_1 = mmap(NULL, sizeof(sem_t), PROT_READ | PROT_WRITE, MAP_ANON | MAP_SHARED, 0, 0)) == MAP_FAILED)
        ERROR = ESEM;
    if ((mutex_2 = mmap(NULL, sizeof(sem_t), PROT_READ | PROT_WRITE, MAP_ANON | MAP_SHARED, 0, 0)) == MAP_FAILED)
        ERROR = ESEM;
    if ((mutex_3 = mmap(NULL, sizeof(sem_t), PROT_READ | PROT_WRITE, MAP_ANON | MAP_SHARED, 0, 0)) == MAP_FAILED)
        ERROR = ESEM;
    if ((reader = mmap(NULL, sizeof(sem_t), PROT_READ | PROT_WRITE, MAP_ANON | MAP_SHARED, 0, 0)) == MAP_FAILED)
        ERROR = ESEM;
    if ((writer = mmap(NULL, sizeof(sem_t), PROT_READ | PROT_WRITE, MAP_ANON | MAP_SHARED, 0, 0)) == MAP_FAILED)
        ERROR = ESEM;

    if (ERROR == EOK) {
        if (sem_init(mutex_1, 1, 1) == -1)
            ERROR = ESEM;
        if (sem_init(mutex_2, 1, 1) == -1)
            ERROR = ESEM;
        if (sem_init(mutex_3, 1, 1) == -1)
            ERROR = ESEM;
        if (sem_init(reader, 1, 1) == -1)
            ERROR = ESEM;
        if (sem_init(writer, 1, 1) == -1)
            ERROR = ESEM;
    }
}

When I tried to compile it on linux I see this:

/tmp/ccmkN9G7.o: In function `initialization_semaphores':
readerWriter.c:(.text+0x1a2): undefined reference to `sem_init'
readerWriter.c:(.text+0x1cb): undefined reference to `sem_init'
readerWriter.c:(.text+0x1f4): undefined reference to `sem_init'
readerWriter.c:(.text+0x21d): undefined reference to `sem_init'
readerWriter.c:(.text+0x246): undefined reference to `sem_init'
readerWriter.c:(.text+0x275): undefined reference to `shm_open'

Is it right? :

int ERROR = EOK;
mutex_1 = sem_open("mutex1", O_CREAT, S_IRUSR | S_IWUSR, 1);   
mutex_2 = sem_open("mutex2", O_CREAT, S_IRUSR | S_IWUSR, 1);  
mutex_3 = sem_open("mutex3", O_CREAT, S_IRUSR | S_IWUSR, 1); 
reader = sem_open("reader", O_CREAT, S_IRUSR | S_IWUSR, 1); 
writer = sem_open("writer", O_CREAT, S_IRUSR | S_IWUSR, 1);   

Answer 1

Mac OSX is non-conformant and does not support sem_init. The function exists but it silently fails or worse, leaving you with a non-working semaphore.

I’d encourage you to file a bug with Apple, since this is a real, long-standing problem that’s affecting application portability badly. The more people who complain, the more hope there is of getting it fixed.

As for working around it, you can try to find/write a replacement implementation of all the POSIX semaphore functions and link your program to that, or you could switch to using sem_open instead of mmap and sem_init.

(As long as you’re already going through the overhead of mapping a whole page for each semaphore, sem_open doesn’t really cost you anything more. Where this bug is really a show-stopper is when you want to include your semaphores inside an existing struct.)