Accessing Makefile variables in code?

问题

UPDATED PROGRESS

I am sorry I forgot to specify this question as an Arduino question. I just assumed that it's a preprocessor problem which is kind of independent of what platform this is being executed on.

I am using Arduino-Make and I am trying to pass in USERNAME and PASSWORD

BOARD_TAG    = mega2560
CPPFLAGS     = -DUSERNAME="$(USERNAME)" -DPASSWORD="$(PASSWORD)"
include $(ARDMK_DIR)/Arduino.mk

Command line:

make USERNAME="HELLO" PASSWORD="WORLD"

Code:

void setup() {
  Serial.begin(9600);

  String auth_raw2(USERNAME : PASSWORD);
  Serial.println(auth_raw2);
}


void loop() {}

I am getting this error:

macro.ino:10:29: error: found ‘:’ in nested-name-specifier, expected ‘::’
macro.ino:10:20: error: ‘HELLO’ has not been declared

回答1:

What you want is

String auth_raw( USERNAME ":" PASSWORD );

That will do the proper literal string concatenation you're looking for. The compiler will run adjacent string literals together into a single string. So if you have

char a[] = "The " "quick" " brown " "fox";

it treats it the same as if you wrote

char a[] = "The quick brown fox";

I'm not sure about putting " around the values provided on the command line to make.

回答2:

According to the literature at Arduino's Site:

Concatenating in a constructor "gives unpredictable results because 'auth_raw' never got an initial value before you started concatenating different data types. For best results, initialize your Strings before you concatenate them."

---

As far as the variables you are passing to your makefile are concerned, the syntax is correct, and the commandline -D to define those variables is as well valid. However, as the documentation points out, you should avoid mixing integers and strings when passing to a constructor, though it would be perfectly valid to construct the String first, and then concatenate the values.

来源：https://stackoverflow.com/questions/24963654/accessing-makefile-variables-in-code