Makefile：循环 - 依赖项已删除

Question

I have designed a Makefile that compiles all the .c files individually and produces a .o respectively (I think this happens Implicitly and works perfectly fine).

The executable (.out) is not being generated from the .o files.

Makefile:

TARGET = all.out
OBJS = file1.o file2.o file3.o
CC = gcc
CFLAGS = -g -Wall
all : $(TARGET)
$(TARGET) : $(OBJS)
#   gcc $^ -o $@
run : $(TARGET)
    ./$<
clean :
    rm -rf *.o $(TARGET)

Output:

$ make 
make: Circular all.out <- all dependency dropped.
gcc -g -Wall    -c -o file1.o file1.c
gcc -g -Wall    -c -o file2.o file2.c
gcc -g -Wall    -c -o file3.o file3.c
cp file1.o a.out

Note: The Makefile works perfectly and produces the perfect results if the line no. 7 present in it is uncommented.

line no.7:

#   gcc $^ -o $@

Output when line no. 7 is uncommented (Works perfectly as intended):

gcc -g -Wall    -c -o file1.o file1.c
gcc -g -Wall    -c -o file2.o file2.c
gcc -g -Wall    -c -o file3.o file3.c
gcc file1.o file2.o file3.o -o a.out

I am new to Makefiles.

Queries:

1. why does commenting line no. 7 causing this issue and uncommenting it works perfectly?
2. What is cp in the first output when line no.7 was commented?
3. What does circular - dependency dropped error occur?

Answer 1

I can't explain how you are seeing the problem you showed to us. Either what you wrote above is not actually what you're using, or you have a buggy version of GNU make. I can't reproduce the behavior you're seeing.

But, I'm sure it's related to this: GNU make has a built-in rule that knows how to build an xx.out file from a file xx for any xx :

# make -p -f/dev/null
  ...
%.out: %
#  recipe to execute (built-in):
        @rm -f $@
        cp $< $@

If you comment out your own recipe as an explicit rule, then make will search for one among the pattern rules it knows about and it will find this built-in pattern rule.

However this rule shouldn't match based on what you've shown us: in order for it to match with a target of a.out , make would have to find or know how to build a target a and that doesn't seem to be available. Also, knowing how to build a would show a circular dependency on a.out .

If your makefile was:

TARGET = all.out

THEN it would all make perfect sense because you would have:

all : all.out
all.out : file1.o file2.o file3.o

and after the implicit rule match %.out: % it would expand like this:

all : all.out
all.out : all file1.o file2.o file3.o
        @rm -f all.out
        cp all all.out

So I assume that when you copied the output into your question you changed it: best to not do that. You should post exactly the problem you have (and verify you still have the problem with what you posted).