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

You can achive this with the shared memory/semaphore IPC.

Here's an article with a nice example:

http://www-personal.washtenaw.cc.mi.us/~chasselb/linux275/ClassNotes/ipc/shared_mem.htm

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.

This is done with unnamed POSIX semaphores, i.e. the semaphores themselves are placed in shared memory.

The standard way to create a process shared mutex/cond. variable is to initialize them with an attribute where you set pthread_mutexattr_setpshared/pthread_condattr_setpshared. Check if LynxOS supports that.

You'll naturally need to place such mutexes/cond. variables in shared memory somehow, so all processes can use it.