I think this may apply to C in general, but am I allowed to have default values in both the forward declaration and implementation? Because my code compiles if I only declare default values once. My question is, which is best practice? Should I have default values in my header file (forward declaration), or in my implementation (.c file)? And should I specify the default value with comments? Or would that be difficult to maintain?

Is it good practice to ask about best practices?

Only if you practice first...

In my code I tend to either assign a value (usually 0) to normal types or use memset() to 'zero out' array types. I don't like leaving variables unassigned.

= Xander

Sorry, now I see how that was a really bad question...
What I meant was, default values for functions. For example,

Currently that would be illegal. Where should I put the `x=15`?

You put it in the prototype only. It makes the argument optional as well. I use this in my suite, quite a bit.

= Xander

Ok. Thanks! Should I have the value in comments next to the implementation? Or is that unnecessary?

Only if you think they can't read the code

= Xander