= Basics of Makefile =
[[http://en.wikipedia.org/wiki/Make_(software)|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:
{{{#!make
#
# 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 ==
{{{#!make
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)
}}}

[[http://www.gnu.org/software/make/manual/make.html|GNU make web pape]]