Arduino IDE的#ifdef bug

From：http://subethasoftware.com/2013/04/09/arduino-compiler-problem-with-ifdefs-solved/

Arduino compiler problem with #ifdefs solved.

In C, “#ifdef” or “#if defined()” are used to hide or include portions of code only if certain conditions are met. For example, my recent *ALL RAM* BBS experiment contains code for using the SD card library as well as the Ethernet library. I used #ifdef around
specific blocks of code so I could compile versions with or without either of those libraries. But all is not well in Arduino land. Consider this following, simple example:

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This is supposed to only include the “byte mac[] =” line if “FOO” is defined, such as with:

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However, on the current Arduino IDE (1.0.4), this simple code will fail with:

ifdef.ino: In function ‘void setup()’:

ifdef:18: error: ‘Serial’ was not declared in this scope

What? Suddenly “Serial.println()” won’t work? Moving the byte declaration outside of the #if def make it work. Very weird.

I also found a similar example, where I tried to comment out a function that used SD library variable types:

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In this example, I did not want the fileReadln() function to be included unless I had defined FOO. But, compiling this produces:

ifdef:15: error: ‘File’ was not declared in this scope

ifdef:15: error: expected primary-expression before ‘char’

ifdef:15: error: expected primary-expression before ‘count’

ifdef:15: error: initializer expression list treated as compound expression

Unhelpful. And after wasting some time on this, I started a topic
in the Arduino.cc forums to ask if others were experiencing the same thing. And they were. A helpful post from parajew pointed
me to this site which helped explain the problem, and offered a workaround:

http://www.a-control.de/arduino-fehler/?lang=en

The pre-processor does some stuff behind the scenes, creating prototypes and including header files where needed, and it just does it wrong. The A-Control site figured out a simple workaround, which I trimmed a bit to just adding this at the top of my scripts:

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…and now either of my examples will compile as intended. Thank you, parajew and A-Control! I can now move on to my next problem…