There's a single dot in a line between #include <sys/prcntl.h> and void my_sandbox(void). It is easy to miss, but makes the compile to fail, thus resulting in HAVE_LINUX_LANDLOCK to be never enabled.

Can someone explain to n00bs like me: what's "landlock" anyway and why is it significant here?

It's a Linux Security Module that allows to sandbox processes: https://docs.kernel.org/userspace-api/landlock.html

prctl, not prcntl

> How do you get "Jigar Kumar"'s email address?

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I think the period on the line above void my_sandbox.