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Subject[PATCH v2 8/9] kbuild: wire up the build rule of compile_commands.json to Makefile
Currently, you need to manually run scripts/
to create compile_commands.json. It parses all the .*.cmd files found
under the specified directory.

If you rebuild the kernel over again without 'make clean',
.*.cmd files from older builds will create stale entries in

This commit wires up the compile_commands.json rule to Makefile, and
makes it parse only the .*.cmd files involved in the current build.

Pass $(KBUILD_VMLINUX_OBJS), $(KBUILD_VMLINUX_LIBS), and modules.order
to the script. The objects or archives linked to vmlinux are listed in
listed in modules.order.

You can create compile_commands.json from Make:

$ make -j$(nproc) CC=clang compile_commands.json

You can also build vmlinux, modules, and compile_commands.json all
together in a single command:

$ make -j$(nproc) CC=clang all compile_commands.json

It works for M= builds as well. In this case, compile_commands.json
is created in the top directory of the external module.

This is convenient, but it has a drawback; the coverage of the
compile_commands.json is reduced because only the objects linked to
vmlinux or modules are handled. For example, the following C files are
not included in the compile_commands.json:

- Decompressor source files (arch/*/boot/)
- VDSO source files
- C files used to generate intermediates (e.g. kernel/bounds.c)
- Standalone host programs

I think it is fine for most developers because our main interest is
the kernel-space code.

If you want to cover all the compiled C files, please build the kernel
then run the script manually as before:

$ make clean # if you want to delete stale .cmd files [optional]
$ make -j$(nproc) CC=clang
$ scripts/gen_compile_commands.json

Here is a note for out-of-tree builds. 'make compile_commands.json'
works with O= option, but please notice compile_commands.json is
created in the object tree instead of the source tree.

Some people may want to have compile_commands.json in the source tree
because Clang Tools searches for it through all parent paths of the
first input source file.

However, you cannot do this for O= builds. Kbuild should never generate
any build artifact in the source tree when O= is given because the
source tree might be read-only. Any write attempt to the source tree
is monitored and the violation may be reported. See the commit log of

So, the only possible way is to create compile_commands.json in the
object tree, then specify '-p <build-path>' when you use clang-check,
clang-tidy, etc.

Signed-off-by: Masahiro Yamada <>

(no changes since v1)

Makefile | 29 +++++++++++++++++++++++++----
1 file changed, 25 insertions(+), 4 deletions(-)

diff --git a/Makefile b/Makefile
index 9cac6fde3479..65ed336a6de1 100644
--- a/Makefile
+++ b/Makefile
@@ -635,7 +635,7 @@ endif
# in addition to whatever we do anyway.
# Just "make" or "make all" shall build modules as well

-ifneq ($(filter all modules nsdeps,$(MAKECMDGOALS)),)
+ifneq ($(filter all modules nsdeps %compile_commands.json,$(MAKECMDGOALS)),)

@@ -1464,7 +1464,8 @@ endif # CONFIG_MODULES

# Directories & files removed with 'make clean'
CLEAN_FILES += include/ksym vmlinux.symvers \
- modules.builtin modules.builtin.modinfo modules.nsdeps
+ modules.builtin modules.builtin.modinfo modules.nsdeps \
+ compile_commands.json

# Directories & files removed with 'make mrproper'
MRPROPER_FILES += include/config include/generated \
@@ -1698,9 +1699,12 @@ KBUILD_MODULES := 1

build-dirs := $(KBUILD_EXTMOD)
PHONY += modules
-modules: descend
+modules: $(MODORDER)
$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.modpost

+$(MODORDER): descend
+ @:
PHONY += modules_install
modules_install: _emodinst_ _emodinst_post

@@ -1714,8 +1718,12 @@ PHONY += _emodinst_post
_emodinst_post: _emodinst_
$(call cmd,depmod)

+compile_commands.json: $(extmod-prefix)compile_commands.json
+PHONY += compile_commands.json
clean-dirs := $(KBUILD_EXTMOD)
-clean: rm-files := $(KBUILD_EXTMOD)/Module.symvers $(KBUILD_EXTMOD)/modules.nsdeps
+clean: rm-files := $(KBUILD_EXTMOD)/Module.symvers $(KBUILD_EXTMOD)/modules.nsdeps \
+ $(KBUILD_EXTMOD)/compile_commands.json

PHONY += help
@@ -1828,6 +1836,19 @@ nsdeps: export KBUILD_NSDEPS=1
nsdeps: modules
$(Q)$(CONFIG_SHELL) $(srctree)/scripts/nsdeps

+# Clang Tooling
+# ---------------------------------------------------------------------------
+quiet_cmd_gen_compile_commands = GEN $@
+ cmd_gen_compile_commands = $(PYTHON3) $< -a $(AR) -o $@ $(filter-out $<, $(real-prereqs))
+$(extmod-prefix)compile_commands.json: scripts/ \
+ $(call if_changed,gen_compile_commands)
+targets += $(extmod-prefix)compile_commands.json
# Scripts to check various things for consistency
# ---------------------------------------------------------------------------

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