Makefile Learning

GitHub: mmdjiji/makefile-learning.

Introduction

What is makefile? Most Windows developers don’t know this, and it is useful in Linux development. Most often, the makefile directs Make on how to compile and link a program.

Processes of compile

Make sure you have installed gcc before learning.

1. Preprocessing

Try the following command:

gcc -E hello.c -o hello.i

It means, preprocess only with hello.c; do not compile, assemble or link. Then output to hello.i.

You can read the output file hello.i and try to understand this file.

2. Compile

Compile is the most important step, it transform the preprocessing file into an assembly file.

gcc -S hello.i -o hello.s

It means, compile only hello.i; do not assemble or link. Then output to hello.s, which is an assembly file.

3. Assemble

After compiling, you need to assemble it to an object file.

gcc -c hello.s -o hello.o

It means, compile and assemble hello.s, but do not link. Then output to hello.o, which is an object file.

4. Link

The final step is link.

gcc hello.o -o hello

Now you can run this program by the following command:

./hello

Convenient Compile

You can run the following command without the generation of intermediate file, but the actual process will not change.

gcc hello.c -o hello

Basic Grammar

Example

# This is a comment.
CC = gcc # variable CC

main: main.c # main rely on main.c
	$(CC) main.c -o main # compile, use variable CC

Symbols

= is the most basic assignment;
:= is the value before the override;
?= is the value after the sign if it has not been assigned;
+= is the value after adding the sign.

Build Rules

target: dependencies
	system command(s)

• target could be an object file, executable file or just a target.
• dependencies (also called prerequisite) are the file used in the make process. If nothing used, keep empty.
• system command(s) (also called recipe) is an action that make carries out, usually use shell command.

Example:

hello: hello.c
	gcc hello.c -o hello

And the example could be also described as follow:

hello: hello.o
	gcc hello.o -o hello

hello.o: hello.s
	gcc -c hello.S -o hello.o

hello.s: hello.i
	gcc -S hello.i -o hello.s

hello.i: hello.c
	gcc -E hello.c -o hello.i

The order of makefile does not affect the execution results, but for the readability, it is best to use the top-down order.

Phony Target

There are many advantages to use phony target. One is to avoid the name conflict between the command only target defined in the makefile and the actual file in the working directory; the other is the efficiency of submitting and executing the makefile.

Use .PHONY to define the phony target, such as following:

.PHONY: clean

The .PHONY can define more than one target:

.PHONY: clean install

clean:
	rm -rf *.o

install:
	cp hello /usr/local/bin

Multi Build

If you have two targets or more to build, you may try to write as follows:

main_a: main_a.c
	gcc main_a.c -o main_a

main_b: main_b.c
	gcc main_b.c -o main_b

This cannot help you compile two targets or more, if you check the built files, you won’t see main_b. The reason is that makefile only processes the top one, and main_a doesn’t rely on main_b, so main_b won’t be built.

The solution is use all:, like follows:

.PHONY: all

all: main_a main_b

main_a: main_a.c
	gcc main_a.c -o main_a

main_b: main_b.c
	gcc main_b.c -o main_b

Now all become the top one, and it rely on main_a and main_b, so all targets will be built.

Abstract Build

If the compiler now instead of gcc, we use g++, clang or clang++, and we don’t want to change every gcc because it is too trouble. We can use abstract build to solve the problem.

We can define a variable CC = gcc, and use it by using $(CC) later.

CC = gcc

main: main.c
	$(CC) main.c -o main

Then, we can use more and more variable to abstract this file.

TAR = abstract
OBJ = main.o
CC = gcc

$(TAR): $(OBJ)
	$(CC) $(OBJ) -o $(TAR)

main.o: main.c
	$(CC) -c main.c -o main.o

.PHONY: clean

clean:
	${RM} $(OBJ)

Batch Build

Use the pattern rules can help you batch build:

• $@: Target files.
• $^: Dependencies files.
• $<: First dependency file.
• $%: When the target file is a static library file, it represents a member name of the static library.
• $?: A list of all dependent files that are newer than the target file, separated by spaces. If the target file is a static library file, it represents a library file(.o).

Example:

TAR = batch
OBJ = alpha.o bravo.o charlie.o delta.o main.o
CC = gcc

$(TAR): $(OBJ)
	$(CC) $^ -o $@

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

VPATH

If the source code in the folder but not the current directory, you may use VPATH:

VPATH := src

It means the source code in the src folder.

If you want to define more than one VPATH, use follows:

VPATH := src1 src2

vpath

Selective search files from a directory.

Usage:

vpath PATTERN DIRECTORIES

Example:

vpath %.c src tester/src

Clean the vpath for %.c:

vpath %.c

Clean the vpath for all:

vpath

Include

Include other makefile:

include <filenames>

When the included file doesn’t exist or there are no rules to create it, if you want make will continue to execute the program (Only when the ultimate goal reconstruction can’t be completed, our program will prompt error save and exit.), you can use follows:

-include <filenames>

License

All documents use CC BY-SA 4.0 license, all codes use GNU GPLv3.