A target given in the
Makefile is a file which will be created or updated when any of its source files are modified. The command(s) given in the subsequent line(s) (which
must be preceded by a
tab character) are executed in order to create the target file.
Note that in the
Makefile shown above, the
.h files are listed, but there are no references in their corresponding commands. This is because the
.h files are referred within the corresponding
.c files through the
#include "file.h". If you do not explicitly include these in your
Makefile, your program will not be updated if you make a change to your header (
.h) files.
The order in which dependencies are listed is important. If you simply type
make and then return,
make will attempt to create or update the first dependency listed.
You can also specify one of the other targets listed in the
Makefile, and only that target (and its corresponding source files) would be made.
===========================================================================
这里要说明一点的是，clean不是一个文件，它只不过是一个动作名字，有点像C语言中的lable一样，其冒号后什么也没有，那么，make就不会自动去找文件的依赖性，也就不会自动执行其后所定义的命令。要执行其后的命令，就要在make命令后明显得指出这个lable的名字。这样的方法非常有用，我们可以在一个makefile中定义不用的编译或是和编译无关的命令，比如程序的打包，程序的备份，等等。
If you just indicate just the
.h files in the dependency line of the
Makefile that the current target is dependent on,
make will know that the corresponding
.c file is already required. You don't even need to include the command for the compiler.
Macros in make
The make program allows you to use macros, which are similar to variables, to store names of files. The format is as follows:
OBJECTS = data.o io.o main.o
Whenever you want to have make expand these macros out when it runs, type the following corresponding string $(OBJECTS).
Here is our sample Makefile again, using a macro.
OBJECTS = data.o main.o io.o
project1: $(OBJECTS)
cc $(OBJECTS) -o project1
data.o: data.c data.h
cc -c data.c
main.o: data.h io.h main.c
cc -c main.c
io.o: io.h io.c
cc -c io.c
You can also specify a macro's value when running
make, as follows:
make 'OBJECTS=data.o newio.o main.o' project1
This overrides the value of
OBJECTS in THE
Makefile 一、显示命令
通常，make会把其要执行的命令行在命令执行前输出到屏幕上。当我们用“@”字符在命令行前，那么，这个命令将不被make显示出来，最具代表性的例子是，我们用这个功能来像屏幕显示一些信息。如：
@echo 正在编译XXX模块......
当make执行时，会输出“正在编译XXX模块......”字串，但不会输出命令，如果没有“@”，那么，make将输出：
echo 正在编译XXX模块......
正在编译XXX模块......
如果make执行时，带入make参数“-n”或“--just-print”，那么其只是显示命令，但不会执行命令，这个功能很有利于我们调试我们的Makefile，看看我们书写的命令是执行起来是什么样子的或是什么顺序的。
