cross platform application development
I believe the only sensible answer here is to use some kind of a web application as HTML is relatively the most universal/cross platform-friendly presentation method.
What I would do is I would create a front end first (let's say using ASP .NET) which would look similar on all platforms you're interested in, then I would compile your c code to a dll and just hook up this dll to your front end exposing its selected functions. Alternatively you can use that dll to create a web service exposing desired functions of your code.
I've seen a few such 100% cross platform applications and to be honest none of them was something more than just decent. I believe the whole effort you're going to put in making your app really platform friendly you could invest better in creating a web service unifying the business logic for all of them and creating native UI for each environment so the app feels friendly for all kinds of users.
Framework similar to electronJS for developing cross-platform mobile apps
You may want to have a look at React Native
which is used to build cross-platform mobile applications. It is developed by Facebook.
https://facebook.github.io/react-native/
Framework for cross platform desktop apps
Did you try ELECTRON ?
If you can build a website, you can build a desktop app. Electron is a framework for creating native applications with web technologies like JavaScript, HTML, and CSS. It takes care of the hard parts so you can focus on the core of your application.
PS: Skype and Visual Studio Code are built using Electron.
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