On Wed, 23 Jul 2025 10:46:12 -0700
Thorsten Blum [off-list ref] wrote:

Your commit fca8300f68fe3 changed it from __dynamic_array() to __array()
and __string() seems to be just a special version of __dynamic_array()
with a length of -1.

In the commit description you wrote: "Since the size of the name is at
most 16 bytes (defined by IFNAMSIZ), it is not worth spending the effort
to determine the size of the string."

So the original had:

	__dynamic_array(char,  name,   IFNAMSIZ )

Which is not dynamic at all. A dynamic_array (like __string) saves the
size in meta data within the event. So basically the above is wasting
bytes to save a fixed size. If you are going to use a dynamic array,
might as well make it dynamic!

I was doing various clean ups back then so I didn't look too deeply
into this event when I made that change. I just saw the obvious waste
of space in the ring buffer.

Just to explain it in more detail. A dynamic_array has in the ring buffer:

	short offset;
	short len;
	[..]
	char  name[len];

That is, 4 bytes are used to know the size of the array and where in
the event it is located. Thus the __dynamic_array() usage basically had:

	short offset;
	short len = IFNAMSIZ;
	[..]
	char name[IFNAMSIZ];

Why have the offset and length? with just __array(char, name, IFNAMSIZ}
it would be just:

	char name[IFNAMSIZ];

See why I changed it?

Now, the change I'm suggesting now would make the __string() be dynamic!

	short offset;
	short len = strlen(res->nh && res->nh->fib_nh_dev ? res->nh->fib_nh_dev->name : "-") + 1;
	[..]
	char name[len];

As IFNAMSIZ is 16, and the above adds 4 bytes to the name, if the name
is less than 7 bytes or less, you save memory on the ring buffer.

	2 bytes: offset
	2 bytes: len;
	7 bytes + '\0'

total: 12 bytes

Note, if there's only one dynamic value, it is always at least 4 bytes aligned.

-- Steve