compile_commands.mk

#example for generating compile_commands.json for a simple Makefile
# variables:
# CURDIR (make builtin) the CWD of the make process (build directory)
# /                     the project root - we're assuming the Makefile lives there
# CC                    the C compiler
# CFLAGS                C compiler flags
# C_SRCS                list of .c files that are built

/ := $(dir $(lastword $(MAKEFILE_LIST)))

$(CURDIR)/compile_commands.json:
	@echo Generating $@
	$(file >$@,$(compile_commands))

define compile_commands_1
{
	"directory": "$(CURDIR)",
	"command": "$(CC) $(CFLAGS) -c $1 -o $1.o",
	"file": "$1"
}
endef

# since commpile_commands_1 does not emit a "," after each element, we need to add it.
# For this, we're using make's subst function. Since function arguments are delimited by
# commas, we can't use a literal , in the function call. For this, we define the _comma
# variable and use that instead.
# We'll be relacing "} {" by "},{"
# Another possibility would be to emit the comma in compile_commands_1 so we'll end up with
# with an invalid JSON array like : [{}, {}, ]
# The solution is to add a dummy element at the end: [{}, {}, {dummy-element}]
# Which brings us the the next problem: that dummy element can't be empty, otherwise clang tools will
# choke on the .json. So the dummy element will need to contain "directory", "command" and "file" fields
# whose contents will not really matter. I don't quite like this approach, so we'll do the 
# 's/} {/},{'  trick...
_comma=,
define compile_commands
[
$(subst } {,}$(_comma){,$(foreach s,$(C_SRCS),$(call compile_commands_1,$(realpath $/$s))))
]
endef