When I build my project and the main form is open (fairly large, dozens of components of all kind), the build process creates a file named 'c' in the project folder.
The file is empty.
If I close the main form in the IDE, the build process doesn't create that file.
How can I find out what causes this and why does the compiler care which files are open in the IDE at the moment a build is started?
I think one of your components (probably third party) has custom streaming code (defineproperties and the like), and generates this file as a non-standard tempfile.
If the form is open, the file is mutated, so when compiling the form is streamed from .dfm to .res and going through the designtime code of that component which creates the file.
Easiest way to figure out is to start removing components in a copy of the project till the behaviour goes away. Then try to dig in the corresponding designtime code if you can find the culprit.
If you have CnWizard installed, then this tool is creating the empty c files. Regarding there homepage, this is already fixed in the beta version. Need to wait until that version is released.