[blob.c] What is BLOB_COOKIE used for?

For Developers
1

Hi!

I can't understand how the constant BLOB_COOKIE is used (c.f. blob.c).

static struct blob_attr *
blob_add(struct blob_buf *buf, struct blob_attr *pos, int id, int payload)
{
	int offset = attr_to_offset(buf, pos);
	int required = (offset - BLOB_COOKIE + sizeof(struct blob_attr) + payload) - buf->buflen;
    ...
}

static inline int
attr_to_offset(struct blob_buf *buf, struct blob_attr *attr)
{
	return (char *)attr - (char *) buf->buf + BLOB_COOKIE;
}

It seems to me that the function attr_to_offset calculates the offset between the start of the buffer and the position in the buffer of the new blob attribute. For some unknown reason, the integer constant BLOB_COOKIE is added to the returned value of attr_to_offset (offset). However, BLOB_COOKIE is again subtracted from the next value (offset - BLOB_COOKIE). So it's as if we never added the value of BLOB_COOKIE, is it?