?????? Implicit rule for linking multiple object files

[ljh]

Hi Paul,

I don't know if this is related to gcc support of c++20. 

But mentioning target.o ("x.o") and recipe or not in my Makefile, does have different result. My test follows.

Thanks

---

1. error: with target.o ("x.o"), without recipe

$ ls

 a.cpp   b.cpp   c.cpp   d.cpp   main.cpp   Makefile

$

$ cat Makefile

CXXFLAGS = -Wall -Wextra -std=c++2a -fmodules-ts -g # -O3 -fPIC

main : c.o b.o a.o d.o main.o

#       $(CXX) $^ -o $@

$ make

g++ -Wall -Wextra -std=c++2a -fmodules-ts -g    -c -o main.o main.cpp

In module imported at main.cpp:1:1:

A: error: failed to read compiled module: No such file or directory

A: note: compiled module file is ??gcm.cache/A.gcm??

A: note: imports must be built before being imported

A: fatal error: returning to the gate for a mechanical issue

compilation terminated.

make: *** [<builtin>: main.o] Error 1

$ 

2. ok: without target.o and recipe

$ rm -fr *.o gcm.cache main

$ cat Makefile

CXXFLAGS = -Wall -Wextra -std=c++2a -fmodules-ts -g # -O3 -fPIC

main : c.o b.o a.o d.o # main.o

#       $(CXX) $^ -o $@

$ make

g++ -Wall -Wextra -std=c++2a -fmodules-ts -g    -c -o c.o c.cpp

g++ -Wall -Wextra -std=c++2a -fmodules-ts -g    -c -o b.o b.cpp

g++ -Wall -Wextra -std=c++2a -fmodules-ts -g    -c -o a.o a.cpp

g++ -Wall -Wextra -std=c++2a -fmodules-ts -g    -c -o d.o d.cpp

g++ -Wall -Wextra -std=c++2a -fmodules-ts -g     main.cpp c.o b.o a.o d.o   -o main

$ 

3. ok: with target.o and recipe

$ rm -fr *.o gcm.cache main

$ cat Makefile

CXXFLAGS = -Wall -Wextra -std=c++2a -fmodules-ts -g # -O3 -fPIC

main : c.o b.o a.o d.o main.o

        $(CXX) $^ -o $@

$ make

g++ -Wall -Wextra -std=c++2a -fmodules-ts -g    -c -o c.o c.cpp

g++ -Wall -Wextra -std=c++2a -fmodules-ts -g    -c -o b.o b.cpp

g++ -Wall -Wextra -std=c++2a -fmodules-ts -g    -c -o a.o a.cpp

g++ -Wall -Wextra -std=c++2a -fmodules-ts -g    -c -o d.o d.cpp

g++ -Wall -Wextra -std=c++2a -fmodules-ts -g    -c -o main.o main.cpp

g++ c.o b.o a.o d.o main.o -o main

$ 

$ make --version

GNU Make 4.3

Built for x86_64-pc-linux-gnu

Copyright (C) 1988-2020 Free Software Foundation, Inc.

License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>

This is free software: you are free to change and redistribute it.

There is NO WARRANTY, to the extent permitted by law.

$ 

------------------ ???????? ------------------

??????: "psmith"<psmith@gnu.org>;

????????: 2022??8??11??(??????) ????2:31

??????: "ljh"<ljhm@qq.com>; "bug-make"<bug-make@gnu.org>;

????: Re: Implicit rule for linking multiple object files

On Thu, 2022-08-11 at 01:58 +0800, ljh wrote:
> I have three c source files: x.c, y.c, z.c and I name x as the target
> on left. Can I put x.o in the prerequisites on the right too? Are
> they the same, with or without x.o in the prerequisites on the right?
> ?0?2 ?0?2
> ?0?2 ?0?2 x: y.o z.o x.o ?0?2# with x.o

It is correct to do this.

These two rules do not behave exactly the same:

  x: y.o z.o

versus

  x: y.o z.o x.o

(you can see the difference for yourself by running "make" both ways)
but the result of both of these will give you the same working program.

> Is it correct for me to use patsubst function to include all object
> files?
> ?0?2 ?0?2 x: $(patsubst %.c,%.o,$(wildcard *.c))

This is fine too.

> In my test the rule with patsubst works on most cases. But if my code
> uses C++20 modules, I need to omit x.o if I want to omit the recipe:
> ?0?2 ?0?2 x: y.o z.o ?0?2# without x.o and recipe
> 
> if I include x.o, I can't omit the recipe:
> ?0?2 ?0?2 ` x: y.o z.o x.o ` # with x.o?0?2
> ?0?2 ?0?2 ` ?0?2 ?0?2$ (CXX) $ (LDFLAGS) $^ $(LDLIBS) -o $@ ` # with recipe

I don't know why you keep referring to C++20 modules.  Make doesn't
know anything about C++20 modules, it doesn't even know what version of
C++ the compiler is building with.  It barely even knows that there is
such a thing as C++: all it knows is "some source files end in .cpp or
.cc or .cxx and those should be built with a recipe that uses variables
CXX and CXXFLAGS".

In any event, I see no reason why an implicit rule without a recipe and
with x.o as a prerequisite wouldn't work.  In fact, it works fine for
me:

  $ ls
  Makefile  x.c  y.c  z.c

  $ cat Makefile
  x: $(patsubst %.c,%.o,$(wildcard *.c))

  $ make
  cc    -c -o x.o x.c
  cc    -c -o y.o y.c
  cc    -c -o z.o z.c
  cc   x.o y.o z.o   -o x