[incise.org] simple unix shared memory interface

If you want to use multiple blocks of shared memory in a single process then you might need to pass a different id to ftok() for each one. I used the same block every time so I had no need for that.

In any case, the main entry point is open_shm(). It returns a block of shared memory of the specified size. If justreading is 1 and there's been data written to the block of shared mem, then you access what has already been written. Otherwise, you create a new one with undefined contents.

Once you get your memory block successfully returned to you by open_shm(), you can treat it like any other block of memory that you'd get from e.g. malloc().

PDIE() is left as an exercise to the reader...

/* This code written by Nick Welch , 2006.
 *
 * This code is in the public domain
 * and is provided AS IS, with NO WARRANTY. */

static key_t get_shm_key(void)
{
    /* both arbitrary */
    const char * KEY_PATH = "/dev/null";
    const char KEY_ID = 0xc4; 

    key_t key = ftok(KEY_PATH, KEY_ID);
    if (key == (key_t)-1)
        PDIE("ftok");

    return key;
}

static int get_shm_min(void)
{
    struct shminfo info;
    if((shmctl(0, IPC_INFO, (struct shmid_ds *)(void *)&info)) == -1)
        PDIE("shmctl (shminfo)");
    return info.shmmin;
}

static void * open_shm(size_t bytes, int justreading)
{
    void * shm;
    key_t key = get_shm_key();

    int id; 

    if(!justreading)
    {
        /* we need to delete it in case we need to allocate more memory this
         * time than last time.  if we try that without first deleting it, we
         * will get EINVAL.
         */

        id = shmget(key, get_shm_min(), 0644|IPC_CREAT);

        if(id == -1)
            PDIE("shmget (first call)");

        shm = shmat(id, NULL, 0);
        if (shm == NULL)
            PDIE("shmat(first call)");

        if(shmctl(id, IPC_RMID, NULL) == -1)
            PDIE("shmctl (marking shm segment for removal)");

        if(shmdt(shm) == -1)
            PDIE("shmdt (detaching shm segment to delete it)");
    }

    id = shmget(key, bytes, justreading ? 0 : 0644|IPC_CREAT);
    if(id == -1)
        PDIE("shmget (second call)");

    shm = shmat(id, NULL, 0);
    if (shm == NULL)
        PDIE("shmat (second call)");

    return shm;
}