In a previous question (Append Text to Variable Names: Please Help Me Shorten This Code) I learned that in order to shorten this code:

fanSpeed = readEEPROM(AT24C32_ADDRESS, 1, fanSpeedPos);

I could define a macro at the top of the script like this:

#define readEE(VAR) VAR = readEEPROM(AT24C32_ADDRESS, 1, VAR ## Pos)

And then just use the following shorter code:


Which would be expanded out to:

foo = readEEPROM(AT24C32_ADDRESS, 1, fooPos);

My current question is: How can I write a macro that accepts 2 arguments instead of just 1? I couldn't find much information about this concerning Arduinos, so I studied about how to do it in C++ (like from here: http://en.cppreference.com/w/cpp/preprocessor/replace) but it was too complicated for me to understand.

In particular, I would like to learn if it's possible to shorten the following code:

void writeToEE(byte location, bool value, bool &var) {
  writeEE(loc, value); var = value;

(Sidenote: the writeEE function looks like this: void writeEE(uint8_t entry, uint8_t data) { writeEEPROM(AT24C32_ADDRESS, 1, entry, data); }. Since the first 2 arguments are almost always constants, that's why I shortened it with another function).

Since whenever I'm using writeToEE, the "entry" variable name is always the "data" variable name with "Pos" appended, I would ideally like to do this (pseudo-code):

#define wrEE(VAR, VAL) VAR = writeToEE(VAR ## Pos, VAL, VAR)

so that when I write this:

wrEE(fanSpeed, 3);

it will be expanded out to:

writeToEE(fanspeedPos, 3, fanspeed);

I just don't know how to pass the 2nd argument ("VAL") into the macro, which is the value being stored in the "fanSpeed" variable, in the "fanSpeedPos" location in the EEPROM. When I try to compile my pseudo code example (was hoping to get lucky!), I get the following error message: "void value not ignored as it ought to be"

  • ... Did you mean to put "VAR = " in front of it? Commented May 10, 2016 at 14:39
  • I really don't know what I'm doing, and I took the "VAR = " part from the first link in the first sentence of the post.
    – Jerry
    Commented May 10, 2016 at 15:00
  • 1
    But the return type is void. Which means it doesn't return anything. Commented May 10, 2016 at 15:01
  • Thanks for the guidance! Now I understand that in the first example (from the other thread), the readEEPROM was returning a variable, which is why VAR = was needed. I removed it so that the macro then became: #define wrEE(VAR, VAL) writeToEE(VAR ## Pos, VAL, VAR), and it seems to work! It seems like it's storing my variable correctly. Is it really that simple? Does it look like that's the correct way to do it?
    – Jerry
    Commented May 10, 2016 at 15:13
  • Small side question: since the writeToEE function is calling the writeEE function, would the code run faster if I skipped the writeToEE function and just had the macro expand out to the writeEE function? (Sorry for the confusing similar names). Or does it all end up to be the same after it goes through the compiler?
    – Jerry
    Commented May 10, 2016 at 15:19

1 Answer 1


In the parameter list of the #define for the macro, separate the second argument from the first with only a comma. For example:

#define wrEE(speed,var) writeToEE((etc))

The "void value not ignored as it ought to be" error message is a consequence of trying to store the void result of writeToEE. The solution is to not try to store it.

In C, variable, parameter, and constant names are case-sensitive.

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