Kernel compatibility : d_off?

Now, I would like to ask you a few questions about some stuff I've not fully understood. Let's start with the d_off field in the dirent structure of the getdents(2) and getdents64(2) syscalls. What is its meaning?

Personnaly, I've implemented it as documented in the manpage, but since it doesn't seems to correspond to the actual behaviour of the Linux kernel, there might be an incompatibility here, so let me know if this is an issue.