Re: Questions on the result of dependency generation

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

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