Makefile: Avoid compiling Fortran modules for new folder

fortran, makefile, module

Solution

I would expect that (most?) compilers would specify an option to change the module file path. with `gfortran`, the option is `-J` or `-M` (from the man page):

-Jdir
-Mdir
       This option specifies where to put .mod files for compiled modules.  It is also
       added to the list of directories to searched by an "USE" statement.

       The default is the current directory.

       -M is deprecated to avoid conflicts with existing GCC options.

I think that most compilers look for .mod files in directories included with `-I`

EDIT

As of gfortran 4.6, `-M` is no longer supported. Use `-J`

based on one of my configure scripts, the flag is `-module` for `ifort` and `pgf90` although I almost never use those compilers these days so somebody else should confirm that ...

Problem

I have a Fortran program that uses modules, i.e. it creates `.mod`-files during compilation. I also wrote a `Makefile` that uses all the `.f90`-files from `src/` puts all created `.o`-files in `obj/` and the binary in the current folder, and everything works fine. I now recompile my program in different folders for different calculations (say `calc1/`), i.e. I copy the `Makefile` into `calc1/`, type `make all` in `calc1/` and all it does is the linking, because the object-files already exist. However, if the program includes any modules the compiler needs the corresponding `.mod`-files to be present in `calc1/`. Until now, I recompiled everything (`make clean all`), but with the program growing this takes too much time! A possible solution I came up with is to have one specific folder for the binaries (`bin/`). But this is not a viable option because I have jobs in the queue, which obviously need a stable binary, while I also try new features in the meantime. So, I'm looking for a solution that somehow treats the `.mod`-files similar to `.o`-files, e.g. places them in `obj/`.

Original source