Re: Questions on the result of dependency generation

[Xi Ruoyao <ryxi at stu dot xidian dot edu dot cn>]

On 2017-08-19 02:05 -0700, chran wang wrote:
> Hello Ruoyao,
> 
> For (B), I'm sorry that I forgot to say that I changed '#include
> "c/c.h" ' to '#include "c.h" ' in that case. For others, I did not
> encounter any errors on my machine as you showed.
> 
> I'm using ubuntu 16.04. At first, I tried with g++ 5.4.0 since it is
> shipped with ubuntu. Then I compiled a gcc 7.1 from its source code. I
> got the same result under g++ 7.1 (and no error output).
> 
For (A) there should be error output.

Try g++ -E -v c/c.cpp in root.  Then you can see which file "c/c.h" really
is.

Note that you may have a "c/c.h" in system header directories (-isystem).
-MM doesn't mention system headers.  By default your include directive can
not find your root/c/c.h.  So maybe you have "-isystem root" or likewise,
or maybe "/usr/include/c/c.h" (weird name collision).

Check you environment CPLUS_INCLUDE_PATH since it may add system header
directories unintentionally.
> 
> On Sat, Aug 19, 2017 at 1:32 AM, Xi Ruoyao <ryxi@stu.xidian.edu.cn> wrote:
> > On 2017-08-18 15:52 -0700, chran wang wrote:
> > > Hello,
> > > 
> > > I observed some weird results about the dependency generation.
> > > 
> > > Suppose the source code structure is as follows:
> > > 
> > > root
> > >   |-- c
> > >   |   |-- c.cpp
> > >   |   |-- c.h
> > > 
> > > 'c.cpp' includes the header file 'c.h' by
> > >       #include "c/c.h"
> > > 
> > > Now I try to generate the dependencies for c/c.cpp.
> > > 
> > > (A) If I'm now in 'root' folder and run command
> > >       g++ -MM -MT c/c.o c/c.cpp
> > > the compiler outputs
> > >       c/c.o : c/c.cpp    #  which is wrong because it lacks c/c.h
> > 
> > Your #include directive is bad.  The compiler gives:
> > 
> > LANG= g++ -MM -MT c/c.o c/c.cpp
> > c/c.cpp:1:10: fatal error: c/c.h: No such file or directory
> >  #include "c/c.h"
> >           ^~~~~~~
> > compilation terminated.
> > 
> > > (B) If I'm in folder 'c' and run command
> > >       g++ -MM -MT c.o c.cpp
> > > the compiler can ouput the correct results
> > >       c.o : c.cpp c.h
> > 
> > Ditto.
> > 
> > > (C) If I'm in the parent folder of folder 'root' and run command
> > >       g++ -I./root -MM -MT root/c/c.o root/c/c.cpp
> > > the results is also correct:
> > >       root/c/c,o : root/c/c.cpp root/c/c.h
> > 
> > With -I./root the include directive is correct.
> > 
> > > (D) Now, if I'm in folder 'root' again and include the head file 'c.h'
> > > for 'c.cpp' by
> > >      #include "c.h"  instead of #include "c/c.h"
> > > the result is also correct.
> > 
> > Ditto.
> > 
> > > I'm wondering why (A) cannot give a correct dependency output but (B),
> > > (C) and (D) can.
> > 
> > Which version of GCC are you using?  It seems like a preprocessor bug
> > and I can't reproduce it with GCC 4.6, 4.7, 5.x, 6.x and 7.x.
> > --
> > Xi Ruoyao <ryxi@stu.xidian.edu.cn>
> > School of Aerospace Science and Technology, Xidian University
-- 
Xi Ruoyao <ryxi@stu.xidian.edu.cn>
School of Aerospace Science and Technology, Xidian University