Basics of Makefile

​Wiki

Subprogram/Module/Class

• functions can be defined in separated files.
  • subprogram/module (Fortran, C…)
  • class (C++, python, Objective-C,Java…)
• subprogram/module/class is defined for some specific purpose. one can use it without looking in the detail, just know what inputs and outputs / functions…
  • C module
  • .c files: source code / functions
  • .h files: header files, define functions and etc…

Makefile

• A common way of automating software builds and other complex tasks with dependencies.
• A Makefile is itself a program in a special language.
• The command 'make' reads 'Makefile' and do some tasks.
• Can specify file name 'make -f mymakefile'.
• The default name is "Makefile"

Structure of Makefile

Typical element in the simple Makefile:

target: dependencies
[TAB] command to make target

It is important to use tab character, not spaces.

command 'make target' means:

• Make sure all the dependencies are up to date
• If target is older than any dependency, recreate it using the specified commands.

For example:

#
# Large Scale Computing
# Heat / Mass Transfer
#
ALL: ex20 ex21
#
CC = gcc
CFLAGS = -g
#
SRC_EX20 = ex20.c cg.c
SRC_EX21 = ex21.c cg.c
#
OBJ_EX20 = $(SRC_EX20:.c=.o)
OBJ_EX21 = $(SRC_EX21:.c=.o)
#
ex20 : $(OBJ_EX20)
        $(CC) $(CFLAGS) -o $@ $(OBJ_EX20)
 
ex21 : $(OBJ_EX21)
        $(CC) $(CFLAGS) -o $@ $(OBJ_EX21)
#
%.o : %.c
        $(CC) $(CFLAGS) -c $<
cg.o : cg.c cg.h
        $(CC) $(CFLAGS) -c cg.c 
clean:  
        rm -f *.o ex20

lines begin with # is comments

Age of dependency

The last modification time of the file is used.

$ ls -l
-rw-r--r--  1 fuji  staff    388 Sep 21 22:37 Makefile
-rw-r--r--  1 fuji  staff   2030 Sep 21 19:31 cg.c
-rw-r--r--  1 fuji  staff    838 Sep 21 19:31 cg.h
-rw-r--r--  1 fuji  staff   4768 Sep 21 22:37 cg.o
-rwxr-xr-x  1 fuji  staff  13308 Sep 22 09:44 ex20
-rw-r--r--  1 fuji  staff   3035 Sep 21 22:39 ex20.c
-rw-r--r--  1 fuji  staff   7728 Sep 22 09:44 ex20.o

• cg.c and cg.h to generate cg.o
• ex20.c to generate ex20.o
• ex20.o and cg.o to generate ex20

Since all target files are newer than dependencies,

$ make
make: Nothing to be done for `ALL'.

make command does not do anything.

$ touch cg.h
$ ls -l
-rw-r--r--  1 fuji  staff    388 Sep 21 22:37 Makefile
-rw-r--r--  1 fuji  staff   2030 Sep 21 19:31 cg.c
-rw-r--r--  1 fuji  staff    838 Sep 22 09:51 cg.h
-rw-r--r--  1 fuji  staff   4768 Sep 21 22:37 cg.o
-rwxr-xr-x  1 fuji  staff  13308 Sep 22 09:44 ex20
-rw-r--r--  1 fuji  staff   3035 Sep 21 22:39 ex20.c
-rw-r--r--  1 fuji  staff   7728 Sep 22 09:44 ex20.o

Now cg.h is newer than cg.o,

$ make
gcc -g -c cg.c
gcc -g -o ex20 ex20.o cg.o

make command generates cg.o then generates ex20.

implicit rules

General rule to make the *.o files from *.c files:

%.o : %.c %.h
        gcc  -c $<

To make target

ex20 : $(OBJ_EX20)
        gcc -o $@ $(OBJ_EX20)

variables / macros

CC = gcc
CFLAGS = -g
#
SRC_EX20 = ex20.c cg.c
SRC_EX21 = ex21.c cg.c
#
OBJ_EX20 = $(SRC_EX20:.c=.o)
OBJ_EX21 = $(SRC_EX21:.c=.o)

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