c - How do I synchronize access to shared memory in LynxOS/POSIX?

Question

I am implementing two processes on a LynxOS SE (POSIX conformant) system that will communicate via shared memory.

One process will act as a "producer" and the other a "consumer". In a multi-threaded system my approach to this would be to use a mutex and condvar (condition variable) pair, with the consumer waiting on the condvar (with pthread_cond_wait) and the producer signalling it (with pthread_cond_signal) when the shared memory is updated.

How do I achieve this in a multi-process, rather than multi-threaded, architecture?

Is there a LynxOS/POSIX way to create a condvar/mutex pair that can be used between processes?
Or is some other synchronization mechanism more appropriate in this scenario?

Answer 1

Credit goes to @nos, but I'd like to expand a little bit on his answer.
In the end (excluding error handling for clarity) I did as follows:

1. Define the shared memory structure

This contains the inter-process sync objects and the data to be shared.

typedef struct
{
    // Synchronisation objects
    pthread_mutex_t ipc_mutex;
    pthread_cond_t  ipc_condvar;
    // Shared data
    int number;
    char data[1024];
} shared_data_t;

2. Create the shared memory and set size (Master process)

On the Master process create a new shared memory object:

fd = shm_open(SHAREDMEM_FILENAME, O_CREAT|O_EXCL|O_RDWR, S_IRUSR|S_IWUSR);
ftruncate(fd, sizeof(shared_data_t));

2. OR Open the shared memory (Slave process)

On the Slave just attach to existing object:

fd = shm_open(SHAREDMEM_FILENAME, O_RDWR, S_IRUSR|S_IWUSR);

3. Mmap into process space

// Specify addr of calling address, mostly use NULL is most portable way
shared_data_t* sdata = (shared_data_t*)mmap(NULL, sizeof(shared_data_t), PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
close(fd);

4. Init the sync variables (Master process only)

pthread_mutexattr_t mutex_attr;
pthread_mutexattr_init(&mutex_attr);
pthread_mutexattr_setpshared(&mutex_attr, PTHREAD_PROCESS_SHARED);
pthread_mutex_init(&sdata->ipc_mutex, &mutex_attr);

pthread_condattr_t cond_attr;
pthread_condattr_init(&cond_attr);
pthread_condattr_setpshared(&cond_attr, PTHREAD_PROCESS_SHARED);
pthread_cond_init(&sdata->ipc_condvar, &cond_attr);

That's it.

Mutex and cond can now be used as normal to control access to the shared data.

The only real gotchas are making sure the Master process has created the shared memory and initialised the sync variables before the Slave process is started. And making sure you tidy up with munmap() and shm_unlink() as required.

Note: XSI Alternative

The POSIX:XSI extension has other functions for sharing memory (shmget(), shmat() etc) which may be more useful if they are available, but they are not on the version of LynxOS-SE I am using.