11. Writing rules - pseudo target

1. PHONY

perform ：

make [ The goal is ]

If there is no target , Default to the first target .

.PHONY : clean

clean:

rm *.o temp

Like in our previous example “clean” equally , That's to say, we generate a lot of compiled files , We should also provide a way to clear them “ The goal is ” For complete recompilation . （ With “make clean” To use the target ）

because , We don't generate “clean” This file .“ Fake target ” It's not a document , It's just a label , because “ Fake target ” It's not a document , therefore make Unable to generate its dependencies and determine whether it will execute . We can only indicate this by showing “ The goal is ” To make it work . however ,“ Fake target ” The name of cannot be the same as the file name , Otherwise, it will lose “ Fake target ” That's the meaning of .

Of course , In order to avoid the situation of duplicate name with file , We can use a special tag “.PHONY” To show that a goal is “ Fake target ”, towards make explain , Whether or not there's this file , The goal is “ Fake target ”.

.PHONY : clean

As long as there's this statement , Whether or not there is “clean” file , To run “clean” This goal , Only “makeclean” such . So the whole process can be written like this ：

.PHONY: clean
clean:
rm *.o temp

2. Specify the dependent file for the pseudo target

Pseudo targets generally have no dependent files . however , We can also specify the file that the pseudo target depends on . Pseudo targets can also be used as “ The default target ”, Just put it first . An example is , If your Makefile You need to generate several executables at one time , But you just want to simply tap one make Finished , also , All the target files are written in a Makefile in , Then you can use “ Fake target ” This feature ：

.PHONY : all

all : prog1 prog2 prog3

prog1 : prog1.o utils.o
cc -o prog1 prog1.o utils.o

prog2 : prog2.o
cc -o prog2 prog2.o

prog3 : prog3.o sort.o utils.o
cc -o prog3 prog3.o sort.o utils.o

We know ,Makefile The first target in will be taken as its default target . We declared a “all” False target , It depends on three other goals . Because the characteristic of false target is , Always executed , Therefore, the three goals it depends on are always inferior to “all” This goal is new . therefore , The rules of the other three goals are always decided . This will achieve our goal of generating multiple goals in one breath .“.PHONY : all” The statement “all” This goal is “ Fake target ”.

3. Make pseudo targets dependent

As can be seen from the above example , Goals can also become dependencies . therefore , Pseudo targets can also become dependencies . See the following example ：

.PHONY: cleanall cleanobj cleandiff

cleanall : cleanobj cleandiff
rm program

cleanobj :
rm *.o

cleandiff :
rm *.diff

“make clean” All files to be cleared will be cleared .“cleanobj” and “cleandiff” These two pseudo targets are a bit like “ Subroutines ” It means . We can type in “make cleanall” and “make cleanobj” and “make cleandiff” Command to clear different kinds of files .