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How does linux kernel Makefile understands .config? Does it have a parser of defconfig file? It has to produce a lot of #defines for each enabled option from defconfig and also maintain a lot of minor Makefiles which are compiled or not, based on directives in .config file.

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The syntax of the .config file is compatible with make; for example a line like CONFIG_CRC16=m sets the make variable CONFIG_CRC16 to the value m. It's parsed by make and included indirectly in the toplevel Makefile:

  • Makefile contains -include include/config/auto.conf
  • include/config/auto.conf is built by recursively calling the toplevel Makefile on the silentoldconfig target.

Conditional compilation of files is done mostly by playing with target names: makefiles include rules like

obj-$(CONFIG_CRC16) += crc16.o

The target obj-y thus builds all objects that are enabled as built-ins by a configuration option, and obj-m builds all objects that are enabled as modules. There are also conditional directives in the makefiles for more complex cases.

For conditional compilation in the C language, C source files include include/generated/autoconf.h which contains lines like #define CONFIG_CRC16_MODULE 1. This file is generated from include/config/auto.conf by the programs invoked by the xxxconfig targets (scripts/kconfig/conf for batch targets like oldconfig, scripts/kconfig/qconf for xconfig, etc.); the source code for that is scripts/kconfig/confdata.c which does some very simple text processing.

  • +1. The only trickiness really is the stuff in the top-level Makefile designed to ensure that all the config targets are run before any of the build targets. – Stephen Kitt Nov 18 '16 at 9:46

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