Wayland の wl_shm とファイルディスクリプタ送信

 /**
  * @ingroup iface_wl_shm
  *
  * Create a new wl_shm_pool object.
  *
  * The pool can be used to create shared memory based buffer
  * objects.  The server will mmap size bytes of the passed file
  * descriptor, to use as backing memory for the pool.
  */
 static inline struct wl_shm_pool *
 wl_shm_create_pool(struct wl_shm *wl_shm, int32_t fd, int32_t size)

static inline struct wl_shm_pool *
wl_shm_create_pool(struct wl_shm *wl_shm, int32_t fd, int32_t size)
{
	struct wl_proxy *id;

	id = wl_proxy_marshal_flags((struct wl_proxy *) wl_shm,
			 WL_SHM_CREATE_POOL, &wl_shm_pool_interface, wl_proxy_get_version((struct wl_proxy *) wl_shm), 0, NULL, fd, size);

	return (struct wl_shm_pool *) id;
}

/** Prepare a request to be sent to the compositor
 * ...
 * Translates the request given by opcode and the extra arguments into the
 * wire format and write it to the connection buffer.  This version takes an
 * array of the union type wl_argument.
 * ...
 */
WL_EXPORT struct wl_proxy *
wl_proxy_marshal_array_flags(struct wl_proxy *proxy, uint32_t opcode,
			     const struct wl_interface *interface, uint32_t version,
			     uint32_t flags, union wl_argument *args)
{
    struct wl_closure *closure;
    const struct wl_message *message;
    ...
    message = &proxy->object.interface->methods[opcode];
    ...
    closure = wl_closure_marshal(&proxy->object, opcode, args, message);
    ....
    if (wl_closure_send(closure, proxy->display->connection)) {
        wl_log("Error sending request: %s\n", strerror(errno));
    		 display_fatal_error(proxy->display, errno);
    }
    ....
}

struct wl_closure *
wl_closure_marshal(struct wl_object *sender, uint32_t opcode,
		   union wl_argument *args,
		   const struct wl_message *message)
{
    const char *signature;
    struct argument_details arg;
    int fd, dup_fd;
    ...
    signature = message->signature;
    for (i = 0; i < count; i++) {
        signature = get_next_argument(signature, &arg);
        
        switch (arg.type) {
            ...
            case 'h':
                fd = args[i].h;
                dup_fd = wl_os_dupfd_cloexec(fd, 0);
                if (dup_fd < 0) {
                    wl_closure_destroy(closure);
                    wl_log("error marshalling arguments for %s: dup failed: %s\n",
                           message->name, strerror(errno));
                    return NULL;
                }
               closure->args[i].h = dup_fd;
               break;
        }
    }
}

int
wl_closure_send(struct wl_closure *closure, struct wl_connection *connection)
{
	int size;
	uint32_t buffer_size;
	uint32_t *buffer;
	int result;

	if (copy_fds_to_connection(closure, connection))
		return -1;

	buffer_size = buffer_size_for_closure(closure);
	buffer = zalloc(buffer_size * sizeof buffer[0]);
	if (buffer == NULL)
		return -1;

	size = serialize_closure(closure, buffer, buffer_size);
	if (size < 0) {
		free(buffer);
		return -1;
	}

	result = wl_connection_write(connection, buffer, size);
	free(buffer);

	return result;
}

static int
copy_fds_to_connection(struct wl_closure *closure,
		       struct wl_connection *connection)
{
	const struct wl_message *message = closure->message;
	uint32_t i, count;
	struct argument_details arg;
	const char *signature = message->signature;
	int fd;

	count = arg_count_for_signature(signature);
	for (i = 0; i < count; i++) {
		signature = get_next_argument(signature, &arg);
		if (arg.type != 'h')
			continue;

		fd = closure->args[i].h;
		if (wl_connection_put_fd(connection, fd)) {
			wl_log("request could not be marshaled: "
			       "can't send file descriptor\n");
			return -1;
		}
		closure->args[i].h = -1;
	}

	return 0;
}

static int
wl_connection_put_fd(struct wl_connection *connection, int32_t fd)
{
	if (ring_buffer_size(&connection->fds_out) == MAX_FDS_OUT * sizeof fd) {
		connection->want_flush = 1;
		if (wl_connection_flush(connection) < 0)
			return -1;
	}

	return ring_buffer_put(&connection->fds_out, &fd, sizeof fd);
}

struct wl_ring_buffer {
	char data[4096];
	uint32_t head, tail;
};

struct wl_connection {
	struct wl_ring_buffer in, out;
	struct wl_ring_buffer fds_in, fds_out;
	int fd;
	int want_flush;
};


static int
ring_buffer_put(struct wl_ring_buffer *b, const void *data, size_t count)
{
	uint32_t head, size;

	if (count > sizeof(b->data)) {
		wl_log("Data too big for buffer (%d > %d).\n",
		       count, sizeof(b->data));
		errno = E2BIG;
		return -1;
	}

	head = MASK(b->head);
	if (head + count <= sizeof b->data) {
		memcpy(b->data + head, data, count);
	} else {
		size = sizeof b->data - head;
		memcpy(b->data + head, data, size);
		memcpy(b->data, (const char *) data + size, count - size);
	}

	b->head += count;

	return 0;
}

static int
serialize_closure(struct wl_closure *closure, uint32_t *buffer,
		  size_t buffer_count)
{
	const struct wl_message *message = closure->message;
	unsigned int i, count, size;
	uint32_t *p, *end;
	struct argument_details arg;
	const char *signature;

	p = buffer + 2;
	end = buffer + buffer_count;

	signature = message->signature;
	count = arg_count_for_signature(signature);
	for (i = 0; i < count; i++) {
		signature = get_next_argument(signature, &arg);

		if (arg.type == 'h')
			continue;

		switch (arg.type) {
        ...
		case 'i':
			*p++ = closure->args[i].i;
			break;
		default:
			break;
		}
	}

	size = (p - buffer) * sizeof *p;

	buffer[0] = closure->sender_id;
	buffer[1] = size << 16 | (closure->opcode & 0x0000ffff);

	return size;
}

int
wl_connection_flush(struct wl_connection *connection)
{
	struct iovec iov[2];
	struct msghdr msg = {0};
	char cmsg[CLEN];
	int len = 0, count;
	size_t clen;
	uint32_t tail;

	if (!connection->want_flush)
		return 0;

	tail = connection->out.tail;
	while (connection->out.head - connection->out.tail > 0) {
		ring_buffer_get_iov(&connection->out, iov, &count);

		build_cmsg(&connection->fds_out, cmsg, &clen);

		msg.msg_iov = iov;
		msg.msg_iovlen = count;
		msg.msg_control = (clen > 0) ? cmsg : NULL;
		msg.msg_controllen = clen;

		do {
			len = sendmsg(connection->fd, &msg,
				      MSG_NOSIGNAL | MSG_DONTWAIT);
		} while (len == -1 && errno == EINTR);

		if (len == -1)
			return -1;

		close_fds(&connection->fds_out, MAX_FDS_OUT);

		connection->out.tail += len;
	}

	connection->want_flush = 0;

	return connection->out.head - tail;
}

static void
build_cmsg(struct wl_ring_buffer *buffer, char *data, size_t *clen)
{
	struct cmsghdr *cmsg;
	size_t size;

	size = ring_buffer_size(buffer);
	if (size > MAX_FDS_OUT * sizeof(int32_t))
		size = MAX_FDS_OUT * sizeof(int32_t);

	if (size > 0) {
		cmsg = (struct cmsghdr *) data;
		cmsg->cmsg_level = SOL_SOCKET;
		cmsg->cmsg_type = SCM_RIGHTS;
		cmsg->cmsg_len = CMSG_LEN(size);
		ring_buffer_copy(buffer, CMSG_DATA(cmsg), size);
		*clen = cmsg->cmsg_len;
	} else {
		*clen = 0;
	}
}