Using an external header file

You can use header files with an absolute name, like:

#include "c:\Temp\x.h"

Another way that might help is to make the include path like:

#include "q:\x.h"

And use Windows to map driver letter Q to the path you need.

I tried to do this, and looked into it, a few years ago, but it doesn't appear to be possible to do what you want to do. This guy was having the same issue, #include statement with relative path.

Seems like either Michel's suggestion or copying them (which is a pain) will work, from Can I include a header file that is not a library?:

If the include file is part of a single Sketch, instructions are here... http://www.arduino.cc/en/Hacking/BuildProcess [new link]

If the include file is meant to be shared by multiple Sketches then...

• Close the Arduino IDE
• Navigate to the {Arduino}\hardware\libraries directory
• Create a subdirectory. I suggest something like MyCommon.
• In the new subdirectory, create a header file. I suggest something like MyCommon.h.
• Open the new header file, edit it as you wish, and save it
• Open the Arduino IDE
• Create or open a Sketch
• Add a #include to the top of the Sketch that references your new include file

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This is why relative paths are not supported, from Re: how to include header file from previous folder?

Enable verbose mode when compiling. Navigate to the Testing.cpp file that is created from the Testing .ino file. Go up one level. Do you see the files you are trying to include?

and from Re: how to include header file from previous folder? (emphasis is mine)

The reason for having you go look at the something like/AppData/Local/Temp/build/sketch folder was to make you think about what the build process is doing, and maybe think that you could google Arduino + Build to get more details.

The IDE copies your sketch, and stuff #included by your sketch to a build directory.

If you #include , the file header.h, and everything else in the directory it is in, will be copied to the build directory, too.

If you #include "header.h", the file header.h, and everything else in the directory it is in, will be copied to the build directory, too.

The difference between <> and "" is where the header.h file is looked for. "" looks in the current directory. <> looks in the library directories.

If the token between the <> or the "" is not found in the appropriate folder, you are not told that; nothing gets copied, though.

But, that explains why relative names are useless. A sketch directory might contain a file called header.h. It will not contain a file called ..\header.h, because ..\header is NOT a valid file name.

Note that I did NOT say that ..\header.h is not a valid NAME. I said that it is not a valid FILE NAME.

Only file names between the <> or "" are possible, as far as the IDE is concerned.

If you do not like that, you are free to not use the IDE.

Finally from Re: How to specify a path to a specific header file?

The Arduino system has some severe limitations in its ability to work sensibly with the file system - can be a real PITA.