Makefiles - Best practices and suggestions

• Laundry list
  • makefiles should remove all content that it will generate.
  • prefer library rules whenever possible or create library rules that are missing.
  • dependency builds - A second invocation of make within a sandbox should always be a NOP.
  • hardcoded values - avoid them like the plague.  Use variable assignments, autogen paths and dynamic file gathering.
  • platform specific - avoid this logic whenever possible.
    • Code as little platform specific logic within a makefile as possible, a conditional and compiler flags at most should be needed.  For ex use defines within sources to shift platform logic into your source code.
    • If a unit test is platform specific write your makefile so it can be used on all platforms but only do work on the specific hardware or class of hardware.
    • For classes of hardware (unix/windows) place your makefile in a subdirectory, unix/Makefile.in
• Always include dependencies when creating a target
  • Initial make call should always be the workhorse: build, generate, deploy, install, etc.
  • All subsequent make calls must become a NOP unless sources or dependencies change or have been removed.
  • Incorrect dependencies will contribute to wasted cycles and can contribute to circular dependencies.
• Directory dependencies
  • Do not use directories as a dependency for generated targets, ever.  Any activity within a directory will alter inodes forcing targets to always be stale.
    • For ex, individual unit tests would invalidate all prior test activity whenever a test touched a timestamp file in the directory to signal success.
  • Parallel make: add an explicit timestamp dependency (.done) that make can synchronize threaded calls on to avoid a race condition.

# Transient directory for storing timestamps
TS=.ts

#####################################################
## Extra dep needed to synchronize parallel execution
#####################################################
$(TS): $(TS)/.done
$(TS)/.done:
  $(MKDIR) -p $(dir $@)
  touch $@ 

#  "clean" target
GARBAGE_DIRS += $(TS)

• Maintain clean targets - makefiles should be able to remove all content that is generated so "make clean" will return the sandbox/directory back to a clean state.

targets = foo bar tans
timestampDIR = .ts

all: $(timestampDIR) $(targets)

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

$(timestampDIR):
    $(MKDIR) $@

# "clean" target
GARBAGE += $(targets)
GARBAGE_DIRS += $(timestampDIR)

• Wrapper check/unit tests with a ENABLE_TESTS conditional so tests can be disabled on demand.

ifdef ENABLE_TESTS 
    ifeq ($(NULL),$(filter WINNT OS2,$(OS_ARCH)))
        DIRS += test 
    endif # WIN
endif # ENABLE_TESTS