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what exactly is the process by which kernel include/config/ is populated?

  • Subject: what exactly is the process by which kernel include/config/ is populated?
  • From: "Robert P. J. Day" <rpjday [ at ] crashcourse [ dot ] ca>
  • Date: Wed, 18 Mar 2020 10:38:46 -0400 (EDT)
  never paid much attention to this until something came up this
morning, but starting with a current "git pull" of the kernel master
branch and, on x86-64, i run "make defconfig", i know that there is a
bunch of stuff installed under the new configuration directory
include/config/ ... pretty much all zero-length header files that act
as some sort of checkmark or timestamp or what have you to show what's
been "configured."

  i'm curious as to the exact algorithm used to populate that
directory and, in particular, how the header file names are chosen,
and here's why.

  a colleague this morning claimed that, in some kernel module code he
was trying to compile, the header file "xtables.h" was not being
found. i thought that was odd as, in the current kernel code base,
there *is* no such header file, but there is one named "x_tables.h",
so i suggested it was just a typo.

  no, he claimed, his kernel code has compiled before including
"xtables.h", and pointed to that file under include/config/netfilter/.
i was a bit confused as i'd never heard of explicitly including a
header file from the *generated* content under include/config/.

  that header file, like all the rest under include/config/, is empty,
so it's clearly a placeholder to mark a particular step in
configuration, but now i'd like to understand precisely what the
protocol is for generating that directory.

  thoughts? like i said, i've never paid much attention to it until
this morning, when someone tried to directly include one of its header
files.

rday

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