Initializing shared memory safely - c

Question

I have several processes communicating with each through POSIX shared memory on OS X.
My issue is these processes could spawn in any order, and try to initialize the shared memory segment at the same time.
I tried using advisory locks with fcntl and flock but both fail telling me I'm passing an invalid file descriptor (I'm positive the file descriptor is not invalid). So clearly that's out of the picture.
Are there any alternatives to this? Or is there any details about using locks with shared memory that I'm not aware of?
Edit:
My attempt at using locks looks like this:
// Some declarations...
struct Queue {
int index[ENTRIES_PER_QUEUE];
sem_t lock;
sem_t readWait;
sem_t writeSem;
struct Entry slots[ENTRIES_PER_QUEUE];
};
struct ipc_t {
int fd;
char name[512];
struct Queue* queue;
};
ipc_t ipc_create(const char* name, int owner) {
int isInited = 1;
struct Queue* queue;
struct flock lock = {
.l_type = F_WRLCK,
.l_whence = SEEK_SET,
.l_start = 0,
.l_len = 0
};
ipc_t conn = malloc(sizeof(struct ipc_t));
sprintf(conn->name, "/arqvenger_%s", name);
conn->fd = shm_open(conn->name, O_CREAT | O_RDWR, 0666);
if (conn->fd == -1) {
free(conn);
perror("shm_open failed");
return NULL;
}
if (fcntl(conn->fd, F_SETLKW, &lock) == -1) {
perror("Tanked...");
}
// Do stuff with the lock & release it
The output I get is:
Tanked...: Bad file descriptor

Answer 1

A common technique is to first call shm_open with O_CREAT|O_EXCL. This will succeed for only one process that then has to do the setup. The others then would have to do the open as before and wait a bit, probably polling, that the setup is finished.
Edit: To show how this could work as discussed in the comments.
struct head {
unsigned volatile flag;
pthread_mutex_t mut;
};
void * addr = 0;
/* try shm_open with exclusive, and then */
if (/* we create the segment */) {
addr = mmap(something);
struct head* h = addr;
pthread_mutex_init(&h->mut, aSharedAttr);
pthread_mutex_lock(&h->mut);
h->flag = 1;
/* do the rest of the initialization, and then */
pthread_mutex_unlock(&h->mut);
} else {
/* retry shm_open without exclusive, and then */
addr = mmap(something);
struct head* h = addr;
/* initialy flag is guaranteed to be 0 */
/* this will break out of the loop whence the new value is written to flag */
while (!h->flag) sched_yield();
pthread_mutex_lock(&h->mut);
pthread_mutex_unlock(&h->mut);
}

Answer 2

I have been able to reproduce the problem. Then I found a sad notice in the standard:
When the file descriptor fildes refers to a shared memory object, the
behavior of fcntl() shall be the same as for a regular file except the
effect of the following values for the argument cmd shall be
unspecified: F_SETFL, F_GETLK, F_SETLK, and F_SETLKW.
So it's likely it's not yet supported. For such a simple setup I would use a semaphore for signaling "the memory is ready".
EDIT
It seems I need to mention semaphores can be created using "O_EXCL" (so there are no races).