title: What are C macros useful for?
body: <p>I have written a little bit of C, and I can read it well enough to get a general idea of what it is doing, but every time I have encountered a macro it has thrown me completely. I end up having to remember what the macro is and substitute it in my head as I read. The ones that I have encountered that were intuitive and easy to understand were always like little mini functions, so I always wondered why they weren't just functions.</p>
  
  <p>I can understand the need to define different build types for debug or cross platform builds in the preprocessor but the ability to define arbitrary substitutions seems to be useful only to make an already difficult language even more difficult to understand.</p>
  
  <p>Why was such a complex preprocessor introduced for C? And does anyone have an example of using it that will make me understand why it still seems to be used for purposes other than simple if #debug style conditional compilations?</p>
  
  <p>Edit:</p>
  
  <p>Having read a number of answers I still just don't get it. The most common answer is to inline code. If the inline keyword doesn't do it then either it has a good reason to not do it, or the implementation needs fixing. I don't understand why a whole different mechanism is needed that means "really inline this code" (aside form the code being written before inline was around). I also don't understand the idea that was mentioned that "if its too silly to be put in a function". Surely any piece of code that takes an input and produces an output is best put in a function. I think I may not be getting it because I am not used to the micro optimisations of writing C, but the preprocessor just feels like a complex solution to a few simple problems.</p>