Several of the tasks I’ve worked on recently have involved looking at some of the kernel’s build infrastructure. This is all fairly well documented which makes it nice to work with.
The kernel automatically generates some files at build time. This is mostly
set up to be transparent to developers unless they are looking for them.
The majority of these files are headers at
include/generated. A good example
of something which needs to be generated is the
#define representing the
kernel version (e.g. 4.15.12). The header file
#defines for several enum constants calculated at build time.
Cleverly, most of these files are only actually replaced if the generated
output changes to avoid unnecessary recompile. Most of this work is handled
The C preprocessor is typically used on C files, as one might obviously expect.
It’s not actually limited to C files though. Each architecture has to define
a linker script which meets the architectural requirements. The linker language
is common across architectures so it’s beneficial to have common definitions
for typical sections such as initcalls and rodata. There’s a global
to run the pre-processor on any
.lds.S file. Devicetree files also get
preprocessed which avoids a lot of copy and pasting of numerical defines.
The compiler flags are typically set in the top level
Makefile and named
as you might expect (
CXXFLAGS etc.). The process of building the
kernel requires building a number of smaller programs. The c flags for
these programs are controlled by a different set of variables (
It sounds incredibly obvious but I’ve lost time from my day trying to figure
out why setting options in
CFLAGS weren’t being picked up by the host
compiler. For more fun, it’s possible to use environment variables to set
different flags for compiling built-in vs. module code. The moral of
the story is know what you’re setting.
Debugging build infrastructure isn’t always pleasant but the kernel build system isn’t too bad overall. I’m at least beginning to understand more parts of it as I find increasingly more obscure things to modify.