====== Makefile ======

Quick reference [[https://www.gnu.org/software/make/manual/html_node/Quick-Reference.html]]

[[http://aegis.sourceforge.net/auug97.pdf|Recursive Make Considered Harmfull]]

[[http://make.mad-scientist.net/papers/rules-of-makefiles/]]


=== one shell ===

NOTE: read carefully issues introduced with "one shell":
[[https://www.gnu.org/software/make/manual/html_node/One-Shell.html|5.3.1 Using One Shell]]

<code make>
.ONESHELL:
foo : bar/lose
        cd $(<D)
        gobble $(<F) > ../$@
</code>

alternatives w/o ''.ONESHELL''

<code make>
target:
	$(Q)( \
		set -eo pipefail ; \
		if [ -x "/opt/SEGGER/JLink/JLinkExe" ]; then \
			echo "Skipping JLINK intallation (reason: already installed)."; \
			exit 0; \
		fi; \
		\
		wget -c -P /tmp --post-data "accept_license_agreement=accepted" --progress=dot:giga \
			"https://www.segger.com/downloads/jlink/$(JLINK_VERSION).deb"; \
		\
		$(SUDO) apt-get update; \
		$(SUDO) apt-get -y --fix-broken install "/tmp/$(JLINK_VERSION).deb"; \
		\
		rm "/tmp/${JLINK_VERSION}.deb"; \
		make .jlink-workaround; \
		make jlink-notice; \
	)

</code>

=== disable built-in rules ===

<code makefile>
# Disable built-in rules and variables
MAKEFLAGS += --no-builtin-rules
MAKEFLAGS += --no-builtin-variables
</code>

=== VPATH ===

[[https://www.gnu.org/software/make/manual/html_node/General-Search.html]]

VPATH tells makefile where to search for prerequisities:

''VPATH = .:$(OBJDIR):$(LIBDIR)''

=== order only prereq ===

Special pipe "|" character is used. 
If any ''$(objects)'' has to be build, then ''obj'' has to be build first.
But if ''obj'' is out of date or missing, this doesn't force ''$(objects)'' to built.


<code make>
$(objects): | obj

obj:
  @mkdir -p $@
</code>

<code make>
targets : normal-prerequisites | order-only-prerequisites
</code>

[[https://www.gnu.org/software/make/manual/html_node/Prerequisite-Types.html]]

=== double colon rules ===

[[https://www.chemie.fu-berlin.de/chemnet/use/info/make/make_4.html#SEC40]]

It can be useful, when target needs to be updated and command to update target depends on which prerequisite file caused the update.

<code make>
all::
</code>

Another scenario is to find source .c file in different dirs

<code make>
build/%.o:: test/%.c
        $(COMPILE) $(CFLAGS) $< -o $@
        
build/%.o:: src/%.c
        $(COMPILE) $(CFLAGS) $< -o $@
        
build/%.o:: lib/src/%.c lib/src/%.h
        $(COMPILE) $(CFLAGS) $< -o $@
</code>

  * The same target can appear in more than one rule
  * each rule is executed if target is older than any prerequisite of that rule
    * if no prereq, command is always executed
  * Order of execution is not guaranteed

====== Parallel make ======

===== Disabling parallel =====

For whole file: <code makefile>.NOTPARALLEL:</code>
For single target:

<code make>
.NOTPARALLEL: %.a 
.NOTPARALLEL: foo.c bar.c 
</code>

[[https://stackoverflow.com/questions/8496135/parallel-makefile-requires-dependency-ordering|Parallel makefile requires dependency ordering]]

===== Prevent one target from parallel run =====