C ++ Makefile未创建必要的.o文件

Question

The following is my makefile for compiling a few basic C++ files into an executable:

OBJ= node.o link.o trie.o testtrie.o
OPTS= -g -c -Wall -Werror

testtrie: $(OBJ)
        g++ -o testtrie $(OBJ)

testtrie.o: prog3.cc trie.h trie.cc link.h link.cc node.h node.cc
        g++ $(OPTS) prog3.cc

trie.o: trie.cc trie.h
        g++ $(OPTS) trie.cc

link.o: link.cc link.h
        g++ $(OPTS) link.cc

node.o: node.cc node.h
        g++ $(OPTS) node.cc

clean:
        rm -f *.o *~

However, when I run make , upon trying to compile prog3.cc, it says:

g++: testtrie.o: No such file or directory.

I cannot for the life of me figure out why it doesn't make testtrie.o...

Thanks in advance for your help!

Answer 1

Your testtrie.o rule:

testtrie.o: prog3.cc trie.h trie.cc link.h link.cc node.h node.cc
    g++ $(OPTS) prog3.cc

Does not actually build testtrie.o , it builds prog3.o .

(It also has several prerequisite source files which it doesn't actually use, unless you're doing some unhealthy things with, eg, #include .)

Answer 2

That is because your executable ( testtrie ) depends on testtrie.o .

By default, a compiler (g++ in our case), when compiling a source file (eg src.cc ) if -o is not given the output object file will match the source file name, so it will be src.o . Your testtrie.o target is never generated; instead prog3.o is:

testtrie.o: prog3.cc trie.h trie.cc link.h link.cc node.h node.cc
    g++ $(OPTS) prog3.cc

Modify the command to:

    g++ $(OPTS) -o $@ prog3.cc

Answer 3

The rule for the target testtrie.o should build a testtrie.o file like this:

testtrie.o: prog3.cc trie.h trie.cc link.h link.cc node.h node.cc
    g++ $(OPTS) prog3.cc -o testtrie.o

You can also add an automatic variable $@ to make it recognize the output filename based on the target name:

testtrie.o: prog3.cc trie.h trie.cc link.h link.cc node.h node.cc
    g++ $(OPTS) prog3.cc -o $@

Both version will have the same effect. Also, remember to correct the remaining targets in the same way .