Makefile handling auto-generated source and Makefile - c

Question

I have a problem with a Makefile that's using an auto-generated Makefile to compile .o files from auto-generated .c and .s (assembler) source files.
The context is embedded programming for STM32 microcontrollers, where a Makefile and hardware initialization source code is generated from a hardware configuration file (.ioc file).
Suppose, we have the following directory structure:
.
├── archive
│   ├── a
│   │   └── a.c
│   ├── b
│   │   └── b.s
│   └── Makefile
├── Makefile
├── source
│   └── generate.sh
The files archive/a/a.c and archive/b/b.s may be empty here. My main ./Makefile looks like this:
.PHONY: default all clean generate objects
BUILD_DIR := build
SOURCE_DIR := source
OBJECTS := build/source/a/a.o build/source/b/b.o
default: all
all: objects
objects: $(OBJECTS)
$(BUILD_DIR)/%.o: %.c $(SOURCE_DIR)/Makefile
mkdir -p $(shell dirname $#)
$(MAKE) -C $(SOURCE_DIR) BUILD_DIR=../$(shell dirname $#) VPATH=../$(shell dirname $<) ../$#
$(BUILD_DIR)/%.o: %.s $(SOURCE_DIR)/Makefile
mkdir -p $(shell dirname $#)
$(MAKE) -C $(SOURCE_DIR) BUILD_DIR=../$(shell dirname $#) VPATH=../$(shell dirname $<) ../$#
$(SOURCE_DIR)/%.c :: generate
$(SOURCE_DIR)/%.s :: generate
$(SOURCE_DIR)/Makefile :: generate
generate: $(SOURCE_DIR)/generate.sh
cd $(SOURCE_DIR) ; bash generate.sh
clean:
$(RM) -r build $(SOURCE_DIR)/Makefile $(SOURCE_DIR)/a $(SOURCE_DIR)/b
The source/generate.sh looks like this:
cp -r ../archive/. ./
In reality, this process much more complicated and takes a few minutes.
Finally, the archive/Makefile looks like this:
BUILD_DIR := build
$(BUILD_DIR)/%.o: %.c
cp $^ $#
$(BUILD_DIR)/%.o: %.s
cp $^ $#
In reality, we use gcc instead of a simple cp to compile the C and ASM source files.
The auto-generated Makefile uses VPATH to find the required input file and is actually meant to compile everything in a flat build folder (which I don't want).
I have no control over the contents of the auto-generated Makefile! The source/generate.sh should be treated as a black-box that I have no influence on.
Therefore, the superordinate Makefile has to slightly abuse the auto-generated Makefile by overwriting the BUILD_DIR and VPATH accordingly.
Now, if I first run make generate and then, e.g. make build/source/a/a.o, everything works as planned:
First, the source directory is populated with the auto-generated stuff, and then build/source/a/a.o is "compiled" from source/a/a.c.
However, if I don't run make generate first, I get:
$ make build/source/a/a.o
make: *** No rule to make target 'build/source/a/a.o'. Stop.
My assumption was that the recipes would have been resolved in the following manner:
$(BUILD_DIR)/%.o: $(SOURCE_DIR)/%.c $(SOURCE_DIR)/Makefile
-> $(SOURCE_DIR)/%.c: generate
-> generate: $(SOURCE_DIR)/generate.sh
-> $(SOURCE_DIR)/Makefile: generate
-> generate: $(SOURCE_DIR)/generate.sh
After a bit of trial and error, I figured that I could somewhat fix this behaviour by defining the recipes as follows:
$(SOURCE_DIR)/%.c: generate
true
$(SOURCE_DIR)/%.s: generate
true
$(SOURCE_DIR)/Makefile: generate
true
This yields:
$ make build/source/a/a.o
cd source ; bash generate.sh
true
true
mkdir -p build/source/a
make -C source BUILD_DIR=../build/source/a VPATH=../source/a ../build/source/a/a.o
make[1]: Entering directory '/[...]/source'
cp ../source/a/a.c ../build/source/a/a.o
make[1]: Leaving directory '/[...]/source'
rm source/a/a.c
However, this leads to the generate step being run every time I run make build/source/a/a.o. Since the generate step is expensive in real life, this is not an option.
Also, source/a/a.c is treated as a temporary file that is meant to be deleted which I don't want.
How can I design my superordinate ./Makefile so that the generate step is being run automatically, but only if necessary?
I also need to be able to build non-auto-generated source files from the root directory.
A few more strange make behaviours:
Simply running make (i.e. make objects) with this Makefile yields
cd source ; bash generate.sh
true
true
mkdir -p build/source/a
make -C source BUILD_DIR=../build/source/a VPATH=../source/a ../build/source/a/a.o
make[1]: Entering directory '/[...]/source'
cp ../source/a/a.c ../build/source/a/a.o
make[1]: Leaving directory '/[...]/source'
true
mkdir -p build/source/b
make -C source BUILD_DIR=../build/source/b VPATH=../source/b ../build/source/b/b.o
make[1]: Entering directory '/[...]/source'
cp ../source/b/b.s ../build/source/b/b.o
make[1]: Leaving directory '/[...]/source'
rm source/a/a.c
Why does make remove source/a/a.c, but not source/b/b.c? For the record: I want neither of these files to be auto-removed.
An issue that I am sadly unable to reproduce with my example is that make basically outputs the following line at the start (adpated to the example):
make: Circular source/generate.sh.c <- generate dependency dropped.
Changing the recipe to the following
$(SOURCE_DIR)/%.c: generate
true
$(SOURCE_DIR)/%.s: generate
true
$(SOURCE_DIR)/Makefile: generate
yields:
$ make build/source/a/a.o
cd source ; bash generate.sh
true
cc -c -o source/Makefile.o source/Makefile.c
cc1: fatal error: source/Makefile.c: No such file or directory
compilation terminated.
make: *** [<builtin>: source/Makefile.o] Error 1
Why does make think that it needs to build a Makefile.o here?