Thinking about makefile "Automatically Processing the dependency of header files"

Original article: automatically handles the dependency between header files

This article explains in detail how makefile automatically handles header file dependencies. The MAKEFILE file is as follows:

all: mainmain: main.o stack.o maze.ogcc $^ -o $@clean:-rm main *.o.PHONY: cleansources = main.c stack.c maze.cinclude $(sources:.c=.d)%.d: %.cset -e; rm -f $@; \$(CC) -MM $(CPPFLAGS) $< > $@.$$$$; \sed 's,\($*\)\.o[ :]*,\1.o $@ : ,g' < $@.$$$$ > $@; \rm -f $@.$$$$

The explanation of the original article is clear, and I will not analyze it in detail. What I don't understand is in this sentence:

sed 's,\($*\)\.o[ :]*,\1.o $@ : ,g' < $@.$$$$ > $@; \

Can it be changed:

sed 's,\($*\)\.o[ :]*,\1.o : ,g' < $@.$$$$ > $@; \

I think $ @ is redundant. After careful consideration, I went into the test and found that $ @ is definitely indispensable and I checked my testing program.

My dependencies are as follows:

Main. O: Main. c Foo. h

Now I Add the following sentence to foo. h:

#include “barrrrrr.h”

The barrrrrrr. h file does not exist.

When $ @ is not added to the MAKEFILE file, compile the program and output the result:

make: Nothing to be done for 'all'

This is obviously not as expected. Main. c relies on barrrrr. H through Foo. H, and barrrrrr. h does not exist. This should be true only when an error is reported.

View the main. d file:

main.o: main.c foo.h

The original main. d file simply adds the dependency on barrrrr. H!

I found the counterexample and looked back at the role of $ @, so I can understand it very well.

Main. d also needs to have the same dependency as main. O.