-#define DEFAULT_CHANNEL_SUBBUF_NUM 8
-#define DEFAULT_CHANNEL_SWITCH_TIMER 0 /* usec */
-#define DEFAULT_CHANNEL_READ_TIMER 200 /* usec */
-/* See lttng.h enum lttng_kernel_output for channel output */
-#define DEFAULT_KERNEL_CHANNEL_OUTPUT LTTNG_KERNEL_SPLICE
-#define DEFAULT_UST_CHANNEL_OUTPUT LTTNG_UST_MMAP
+#define DEFAULT_CHANNEL_SUBBUF_NUM 8
+#define DEFAULT_CHANNEL_SWITCH_TIMER 0 /* usec */
+#define DEFAULT_CHANNEL_READ_TIMER 200 /* usec */
+#define DEFAULT_CHANNEL_OUTPUT LTTNG_EVENT_MMAP
+
+#define DEFAULT_METADATA_SUBBUF_SIZE 4096
+#define DEFAULT_METADATA_SUBBUF_NUM 2
+
+/* Kernel has different defaults */
+
+/* DEFAULT_KERNEL_CHANNEL_SUBBUF_SIZE must always be a power of 2 */
+#define DEFAULT_KERNEL_CHANNEL_SUBBUF_SIZE 262144 /* bytes */
+/* DEFAULT_KERNEL_CHANNEL_SUBBUF_NUM must always be a power of 2 */
+#define DEFAULT_KERNEL_CHANNEL_SUBBUF_NUM 4
+/* See lttng-kernel.h enum lttng_kernel_output for channel output */
+#define DEFAULT_KERNEL_CHANNEL_OUTPUT LTTNG_EVENT_SPLICE
+
+/* User space defaults */
+
+/* Must be a power of 2 */
+#define DEFAULT_UST_CHANNEL_SUBBUF_SIZE 4096 /* bytes */
+/* Must be a power of 2 */
+#define DEFAULT_UST_CHANNEL_SUBBUF_NUM 4
+/* See lttng-ust.h enum lttng_ust_output */
+#define DEFAULT_UST_CHANNEL_OUTPUT LTTNG_UST_MMAP
+
+/*
+ * Takes a pointer x and transform it so we can use it to access members
+ * without a function call. Here an example:
+ *
+ * #define GET_SIZE(x) LTTNG_REF(x)->size
+ *
+ * struct { int size; } s;
+ *
+ * printf("size : %d\n", GET_SIZE(&s));
+ *
+ * For this example we can't use something like this for compatibility purpose
+ * since this will fail:
+ *
+ * #define GET_SIZE(x) x->size;
+ *
+ * This is mostly use for the compatibility layer of lttng-tools. See
+ * poll/epoll for a good example. Since x can be on the stack or allocated
+ * memory using malloc(), we must use generic accessors for compat in order to
+ * *not* use a function to access members and not the variable name.
+ */
+#define LTTNG_REF(x) ((typeof(*x) *)(x))