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I want to make a program which includes source files and header files from a sub-directory in addition to the main directory. The name of the sub-directory, and parts of the filenames themselves need to be chosen with a compiler constant. Here's an example file tree.

=main.h
=main.c
=anotherfile.h
=anotherfile.c
=A
---= A_file1.h
---= A_file1.c
---= A_file2.h
=B
---= B_file1.h
---= B_file1.c
---= B_file2.h

Every compilation involves main and anotherfile, which reference X_file1 and X_file2 where "X" is chosen at built time or with a constant. So the project can either be built with A files or B files.

So how does one (and what is the "best" way to) implement this? Can one put a reference to a compiler constant in an #include statement? (something like #include X+"/"+"x+"_file1.h) Or is there another way?

I'm very new to C build systems & the preprocessor so apologies if this is a poor question. Search engines have not been much help.

Eric Postpischil
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CatOnAKeyboard
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  • If it's defined at built time you can consider compiling with `gcc ... -D path="/your/path"` and then `#include path"/XfileX.h"` in your units – David Ranieri Mar 21 '20 at 10:16
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    Actually I remember with Makefile you can choose which files you want to compile. Just have to set them in a Makefile rule and it does the job. https://www.gnu.org/software/make/manual/make.html – SidoShiro92 Mar 21 '20 at 10:32
  • With `gcc` command line you may indicates some include paths using the -I options, the path may be also relative paths.But better way is to use `make` with makefile. You may also use (in Linux, but I think also in Windows) environment variables. In bash you may use: `export MYINCLUDE=whatyounedd` and E.G. `gcc -I$MYINCLUDE.` also in make/makefile you may use the environ variables (see a make manual). – Sir Jo Black Mar 21 '20 at 11:24
  • This question is too broad - there is no "best" way. C is an old language and there are [a lot of build systems](https://en.wikipedia.org/wiki/List_of_build_automation_software). I advise to use [cmake](https://cmake.org/), there are many tutorials available online and it became really popular. The "some constant" should be configured by the user during `cmake` [configuration stage](https://cmake.org/runningcmake/) and then `cmake` can easily choose what library to compile. There is no need to name each file unique - I would name files in `A` and `B` the same. – KamilCuk Mar 21 '20 at 12:32
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    `something like #include X+"/"+"x+"_file1.h` - You can do something like that, but that would be (in my opinion) unreadable and unmaintainable - it will take new programmers real time to find what is going on. I would just have the build system setup proper include directory and have a constant `#include ` in your source. Finding files is the job for your compiler and tools around it, the source should have the logic. – KamilCuk Mar 21 '20 at 12:35
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    @DavidRanieri: string concatenation doesn't work in an `#include` statement, so that won't work. You can use [this technique](https://stackoverflow.com/questions/32066204/construct-path-for-include-directive-with-macro/32077478#32077478) but crafting a makefile is much cleaner. – rici Mar 21 '20 at 13:17
  • @rici oops, you're right. Thanks for the link – David Ranieri Mar 21 '20 at 22:11

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