What is GNU Make?

• Tool for automating building software, documentation etc.
• Other tools with similar names, e.g. pmake (BSD), nmake (Microsoft).
• GNU Make sometimes shortened to gmake.
• This talk is GNU Make specific, though a lot applies to other Makes.

What does Make do?

• Builds things based on whether they are 'out of date'.
• Outdated is calculated based on file last modified timestamps.
• make followed immediately by make -> no build.
• Configured using a Makefile.

Makefile components

• Makefiles are a list of targets ('what to build').
• Targets have rules ('how to build').
• Targets have dependencies ('what to build first').
• Some dependencies are targets in their own right.

Makefile components

• Not all targets are dependencies.
• Not all dependencies are built by Make.
• End result: directed graph.

Make will build a target when...

• The target does not exist.
• The target is older than any of its dependencies (last modified time).

Make will not build a target when...

• The rules used to generate the target have changed.
• A configuration variable is set which affects the rules.

Make without a Makefile

• Make has some implicit rules.
• Compiling C source files to object files.
• Linking object files to an executable.
• cd examples/no-makefile
• make hello.o
• make hello

Example: slides

• Main target is slides.html.
• Dependency of slides.html is slides.md which must be built/exist first.
• slides.md is not a target, so not built by Make.
• If slides.html does not exist, build it.
• If (and only if) slides.md is newer than slides.html, rebuild slides.html.

Slides Makefile

slides.html: slides.md Makefile
    filepp slides.md | pandoc -t s5 -V s5-url=../s5/default -s -o slides.html

clean:
    rm -f *.html

.PHONY: clean

Phony targets and clean

• Common to have a clean target which removes intermediate files.
• clean can also be used to force a full rebuild.
• .PHONY: Stops Make being confused by file with same name as target.

Make variables

• Set: VAR = VALUE.
• Set conditionally: VAR ?= VALUE.
• Set environment: make VAR=VALUE.
• Access: $(VAR).

Make variables

• Built-in variables, e.g. SHELL.
• Common variables, e.g. CFLAGS.

Special variables

• $@: Full target filename (i.e. what will be built).
• $*: Target filename without the suffix, e.g. hello.o becomes hello.
• $<: The file which triggered this rule.

C example with explicit rules

CC = clang
CFLAGS = -Weverything -Werror
SOURCES = hello.c
OBJECTS = $(subst .c,.o,$(SOURCES))
EXECUTABLE = hello

$(EXECUTABLE): $(OBJECTS)
    $(CC) $(OBJECTS) -o $(EXECUTABLE)

%.o: %.c
    $(CC) $(CFLAGS) -c $< -o $@

clean:
    -rm -f $(OBJECTS) $(EXECUTABLE)

.PHONY: clean

Example: Configuration management workshop

• Two main targets: book and workshop notes.
• Two outputs for notes: print and screen.
• Files common to both book and notes.
• Configuration Management with Ansible

Debugging

• -n: Show commands which would be run.
• Caution: Rules which generate side-effects.

Parallel builds

• -j [number].
• --output-sync does what it says (>= 4.0).
• May cause problems with dependencies built out of order.
• Requires Makefiles to be written with this in mind.
• Example: Linux kernel.

Recursive Make

• Make can call itself recursively.
• cd src && $(MAKE) $@.
• Like parallel Make, use with caution.

Autotools

• Autoconf, Automake and Libtool.
• Works on Linux (good), macOS (usually), Windows (good luck!).
• End-user requires make and a POSIX shell (usually bash).
• ./configure && make && sudo make install.

Automake

• Makefile.am -> Makefile.in.
• ./configure turns Makefile.in into a platform-specific Makefile (via config.status).
• Automake is a talk in itself.

Gotchas

• Indent with tabs not spaces (watch out for editors).
• Changing C/C++ header files does not always rebuild dependent targets.
• Last modified behaviour means occasional unnecessary rebuilds.
• GNU Make uses /bin/sh by default (override with SHELL=).
• Changing Makefile does not cause targets to be rebuilt.
• Some variables are pre-set and ?= can have unintended behaviour.

Resources

• GNU Make manual.
• GNU Make vs nmake.
• The GNU Make Book by John Graham-Cumming (No Starch Press).
• Autotools by John Calcote (No Starch Press).