AtmelStudio 7.0.634
Arduino IDE for Atmel Studio 7

Open the Blink example
Add a .c and header from Visual Micro menu
Declare void DoNothing() in .h
Define void DoNothing() {} in .c
Try to link

Linker tells me it can't find DoNothing()

I've attached the sample project.   

Can anyone get it to build?

Hi Tim,

I've been developing C/C++ for over 25 years on numerous platforms including Visual Studio.  To answer your general question, have I included C sources in projects before? Yes, quite a few times  This is the first time I've included files in a VM project.  If I select New Arduino Class and Header, works like a charm. 

I turned on avr-gcc -Q flag and I see the symbol being generated and the linker is including the .o file.

Doesn't make any sense.  Can email you a zip of the solution if you have minute to take a look.

Thanks

Figured it out.

Guess avr-gcc is set for cpp by default so got to wrap pure C header files in extern "C" { } when including in .ino file (which is compiled as .cpp).

I've attached a copy of the Blink sketch that builds.