gcc make dependency and object simultaneously

Question

I am using gcc 4.8.2, and I am trying to build both the object file and the dependency file concurrently.

This works:

$ g++ -std=c++11 -MP -MD -c foo.cxx -o foo.o
$ [ -s foo.d ] && [ -s foo.o ] && echo yay
yay

However, instead of generating foo.d, I want to generate foo.D, so I tried:

$ rm foo.d foo.o
$ g++ -std=c++11 -MP -M -MF foo.D -c foo.cxx -o foo.o
$ [ -s foo.D ] && [ -s foo.o ] && echo yay
$

That successfully generates foo.D, but makes an empty foo.o. Why? The documentation for -MD starts with:

-MD is equivalent to -M -MF file, except that -E is not implied.

Have you tried just adding -MF foo.D to the original call? — Etan Reisner
– Etan Reisner, Commented Jun 18, 2015 at 14:49
@EtanReisner Nope! Guess that's the answer :) Didn't think of that for some reason. — Barry
– Barry, Commented Jun 18, 2015 at 14:54
@EtanReisner Want to post that as an answer so I can accept it? — Barry
– Barry, Commented Jun 18, 2015 at 15:20
The docs for -MF explicitly mention it that's why I suggested it. I'm not sure why -M -MF doesn't work though. That strikes me as odd. — Etan Reisner
– Etan Reisner, Commented Jun 18, 2015 at 16:09

Etan Reisner · Accepted Answer · 2015-06-18 16:10:40Z

2

I don't know why -M -MF foo.D isn't working for you my reading of the documentation is the same as yours. I think it should be working. An strace of the compilation might tell you something interesting about what is going on.

But as a solution you can just add the -MF foo.D argument to the original command line and that should do what you want.

As the documentation for -MF says:

-MF file When used with -M or -MM, specifies a file to write the dependencies to. If no -MF switch is given the preprocessor sends the rules to the same place it would have sent preprocessed output

When used with the driver options -MD or -MMD, -MF overrides the default dependency output file.

answered Jun 18, 2015 at 16:10

Etan Reisner

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