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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#if defined(FOO)

byte mac[] = { 0x2A, 0xA0, 0xD8, 0xFC, 0x8B, 0xEE };

#endif

 

voidsetup()

{

  Serial.begin(9600);

  while(!Serial);

  Serial.println("Test...");

}

 

voidloop()

{

}

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

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#define FOO

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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voidsetup()

{

  Serial.begin(9600);

  while(!Serial);

  Serial.println("Test...");

}

 

voidloop()

{

}

 

#ifdef FOO

byte fileReadln(File myFile, char*buffer, byte count)

{

}

#endif

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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// BOF preprocessor bug prevent - insert me on top of your arduino-code

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

#if 1

__asmvolatile("nop");

#endif

…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…