Interactive Programming in C (2014)

Summary

December 23, 2014 nullprogram.com/blog/2014/12/23/ I’m a huge fan of interactive programming (see: JavaScript, Java, Lisp, Clojure). That is, modifying and extending a program while it’s running. For certain kinds of non-batch applications, it takes much of the tedium out of testing and tweaking during development. Until last week I didn’t know how to apply interactive programming to C. How does one go about redefining functions in a running C program? Last week in Handmade Hero (days 21-25), Casey Muratori added interactive programming to the game engine. This is especially useful in game development, where the developer might want to tweak, say, a boss fight without having to restart the entire game after each tweak. Now that I’ve seen it done, it seems so obvious. The secret is to build almost the entire application as a shared library. This puts a serious constraint on the design of the program: it cannot keep any state in global or static variables, though this should be avoided anyway. Global state will be lost each time the shared library is reloaded. In some situations, this can also restrict use of the C standard library, including functions like malloc(), depending on how these functions are implemented or linked. For example, if the C standard library is statically linked, functions with global state may introduce global state into the shared library. It’s difficult to know what’s safe to use. This works fine in Handmade Hero because the core game, the part loaded as a shared library, makes no use of external libraries, including the standard library. Additionally, the shared library must be careful with its use of function pointers. The functions being pointed at will no longer exist after a reload. This is a real issue when combining interactive programming with object oriented C. An example with the Game of Life To demonstrate how this works, let’s go through an example. I wrote a simple ncurses Game of Life demo that’s easy to modify. You can get the ...