Common reasons why 'Linux make' always build target although not needed

Introduction

Layman explanation

If you are Linux programmer and you have used make command, you might have great respect for make command. You think that make command will ensure to avoid duplicate compilation effort automatically. Suddenly, if you see that make is compiling a file always even though there is no edit in this file, you might be surprised. And it is not uncommon to witness such surprises. This document helps to understand why it could be happening.

Technical explanation

gnu make tool empower user to automate compilation. To execute make in the right way, Makefile must be written correctly. make tool uses Makefile to create dependency graph. Makefile provides input to make tool which target should be built and when it should be built. If make is building target always although not needed, then it can be error in Makefile programming.

Finding reasons in Makefile

Check if the specified target is created 

If target file is not created(symbolic target), then make falsely assumes that target recipe must be executed. 

Example where target is built always due to non-existent target file

Before:

CC=gcc

CFLAGS=-Wall -I. -c

EXEC_FILE=program1

all: program

hellofunc.o: hellofunc.c

hellofunc.c: hellofunc.h

hellomake.o: hellomake.c

hellomake.c: hellomake.h

objects: hellofunc.c hellomake.c

    $(CC) hellofunc.c hellomake.c $(CFLAGS)

program: hellofunc.o hellomake.o

    $(CC) hellofunc.o hellomake.o -o $(EXEC_FILE)

=================================

[root@ubuntu /personal/testcode/makefile_learning]# make -f Makefile_1

gcc hellofunc.o hellomake.o -o program1

After:

CC=gcc

CFLAGS=-Wall -I. -c

EXEC_FILE=program1

all: $(EXEC_FILE)

hellofunc.o: hellofunc.c

hellofunc.c: hellofunc.h

hellomake.o: hellomake.c

hellomake.c: hellomake.h

objects: hellofunc.c hellomake.c

    $(CC) hellofunc.c hellomake.c $(CFLAGS)

$(EXEC_FILE): hellofunc.o hellomake.o

    $(CC) hellofunc.o hellomake.o -o $(EXEC_FILE)

=============================================

[root@ubuntu /personal/testcode/makefile_learning]# make -f Makefile_2

make: Nothing to be done for `all'.

Check if all dependencies have corresponding specified target created

If any dependency doesn't create target(symbolic target), then make falsely assumes that dependency is out-dated and all targets associated to this dependency must be re-executed.

Example where a dependency is not creating target

Before:

convert: devel/bar

 touch convert

init: devel/foo

 echo 'init'

devel/foo:

 mkdir -p devel

 touch devel/foo



devel/bar: init

 touch devel/bar

==============================

[root@ubuntu /personal/testcode/makefile_learning]# make -f Makefile_1

mkdir -p devel

touch devel/foo

echo 'init'

init

touch devel/bar

touch convert

[root@ubuntu /personal/testcode/makefile_learning]# make -f Makefile_1

echo 'init'

init

touch devel/bar

touch convert



After:

convert: devel/bar

    touch convert



init: devel/foo

    echo 'init'

    touch init



devel/foo:

    mkdir -p devel

    touch devel/foo



devel/bar: init

    touch devel/bar

==========================

[root@ubuntu /personal/testcode/makefile_learning]# make -f Makefile_2

echo 'init'

init

touch init

touch devel/bar

touch convert

[root@ubuntu /personal/testcode/makefile_learning]# make -f Makefile_2

make: `convert' is up to date.

Check if any dependency of target is marked 'always build'

.PHONY makefile option ensures that target is always built. All targets dependent on this target will be rebuilt everytime.

Example where a dependency is marked as .PHONY

Before:

.PHONY: init

convert: devel/bar

    touch convert



init: devel/foo

    echo 'init'

    touch init



devel/foo:

    mkdir -p devel

    touch devel/foo



devel/bar: init

    touch devel/bar

==========================================

[root@ubuntu /personal/testcode/makefile_learning]# make -f Makefile_1

echo 'init'

init

touch init

touch devel/bar

touch convert

[root@ubuntu /personal/testcode/makefile_learning]# make -f Makefile_1

echo 'init'

init

touch init

touch devel/bar

touch convert



After:



.PHONY: init

convert: devel/bar

    touch convert



init: devel/foo

    echo 'init'

    touch init



devel/foo:

    mkdir -p devel

    touch devel/foo



devel/bar: init

    touch devel/bar

=============================

[root@ubuntu /personal/testcode/makefile_learning]# make -f Makefile_2

echo 'init'

init

touch init

touch devel/bar

touch convert

[root@ubuntu /personal/testcode/makefile_learning]# make -f Makefile_2

make: `convert' is up to date.

See if directory timestamp change is falsely triggering rebuild

Consider an example where your targets are to be placed in a separate directory, and that directory might not exist before make is run. In this situation, you want the directory to be created before any targets are placed into it but, because the timestamps on directories change whenever a file is added, removed, or renamed, we certainly don’t want to rebuild all the targets whenever the directory’s timestamp changes. One way to manage this is with order-only prerequisites: make the directory an order-only prerequisite on all the targets: 

OBJDIR := objdir
OBJS := $(addprefix $(OBJDIR)/,foo.o bar.o baz.o)

$(OBJDIR)/%.o : %.c
        $(COMPILE.c) $(OUTPUT_OPTION) $<

all: $(OBJS)

$(OBJS): | $(OBJDIR)

$(OBJDIR):
        mkdir $(OBJDIR)

Now the rule to create the objdir directory will be run, if needed, before any ‘.o’ is built, but no ‘.o’ will be built because the objdir directory timestamp changed.

What if none of above method works for me

If none of above method helps you, then you may need to use make debugging tool. make provides -d option to get debugging capability. It gives detail output about why a target is builfd (for example, reason related to pre-requisite and dependency detail) . If you found any new reason, please share with me in this comment section. All the very best.