Trying to make sense of the coding style in Arduino libraries. (Particularly the Radiohead library)

Question

I've been using the Arduino IDE for some time now, and I want to shift to coding for the bare AVR microcontroller. So I wanted to begin by porting required arduino libraries to C/C++. I started with the Radiohead library, but I've noticed a lot of variables/tokens that are directly used without a definition for them. For example:

 // path :RadioHead/RH_ASK.cpp
#if (RH_PLATFORM == RH_PLATFORM_GENERIC_AVR8)
 #ifdef RH_ASK_PTT_PIN 
 RH_ASK_PTT_DDR |= (1<<RH_ASK_PTT_PIN); 
 RH_ASK_TX_DDR |= (1<<RH_ASK_TX_PIN);
 RH_ASK_RX_DDR &= ~(1<<RH_ASK_RX_PIN);
 #else
 RH_ASK_TX_DDR |= (1<<RH_ASK_TX_PIN);
 RH_ASK_RX_DDR &= ~(1<<RH_ASK_RX_PIN);
 #endif
#else
 // Set up digital IO pins for arduino
 pinMode(_txPin, OUTPUT);
 pinMode(_rxPin, INPUT);
 pinMode(_pttPin, OUTPUT);
#endif

None of the tokens in this snippet like "RH_ASK_PTT_PIN" or "RH_ASK_TX_PIN" were mentioned anywhere in the code before or any other file that was included.

I understand what the code does, but how does the compiler know what these tokens mean? I tried to compile this code using Atmel Studio but it gives me an error (As it should): "RH_ASK_PTT_PIN" not declared in this scope So how does this library work with the Arduino IDE?

Link to the library : https://github.com/PaulStoffregen/RadioHead

Also can someone suggest tags for this post? Its my first post here.

Answer 1

They're not defined. If you notice that whole block is wrapped in a #ifdef which will be false unless you're on some specific platform, where the platform defines those macros.

Everything else uses the last little bit:

pinMode(_txPin, OUTPUT);
pinMode(_rxPin, INPUT);
pinMode(_pttPin, OUTPUT);

You only need to care about that bit - nothing else is of any concern unless you're using a "RH_PLATFORM_GENERIC_AVR8" (whatever that is).

• Neel bedarkar: Thanks for the prompt reply, I understand that to rewrite this I don't need to care about the #ifdef block but how does this file compile in the first place without actually defining the macros? I added this library as a .zip file using the library manager of the Arduino IDE, the entire folder does not mention them.
• Majenko: The top two thirds of the block only gets compiled if they have been defined. Since they haven't been defined it gets completely ignored. C compiling is done in multiple phases. The first is the "preprocessor" that looks at the #ifdef etc bits and expands them where it can. The C compiler doesn't see the top part at all because the preprocessor deletes it.