I'm new to the Arduino and do not quite understand the F() macro yet.

In my code, I have a String, and as I want to transfer it via WebServer, I need to write it to the answer.

Somehow I need to convert the String, to fit it into the fastrprintln() function.

String weather_json = getWeatherData();
char weather[2048];

Do you have any idea for me?

  • What do you mean by "I need to write it to the answer."? Is that a typo? Commented Jan 8, 2016 at 10:28
  • The F() macro is for literals (eg. F("foo") ) not variables.
    – Nick Gammon
    Commented Jan 8, 2016 at 11:12
  • I need to convert the String, to fit it into the fastrprintln() function.” No, you don't: client.println(getWeatherData()); Commented Jan 8, 2016 at 13:19

1 Answer 1


The Arduino core macro F() takes a string literal and forces the compiler to put it in program memory. This reduces the amount of SRAM needed as the string is not copied to a data memory buffer. Special functions are required to access the data stored in program memory. The Arduino core hides a lot of the typical usages such as:

Serial.println(F("Hello world"));

Further details of the low level access program memory access may be found in the AVR GCC libc documentation. The F() macro is defined as:

class __FlashStringHelper;
#define F(string_literal) (reinterpret_cast<const __FlashStringHelper*>(PSTR(string_literal)))

The FlashStringHelper class is used to help the compiler recognize this type of string literal when passed to Arduino functions. Below are some examples:

// String constructor with program memory string literal
String::String(const __FlashStringHelper *str);

// Print (Serial, etc) of program memory string literal
size_t Print::print(const __FlashStringHelper *);
size_t Print::println(const __FlashStringHelper *);

You should not use the Arduino core F() macro with anything other than a string literal.

  • I was having an issue with my program locking up so after reading your answer I added the F() macro to my string literals and that was literally all it took to resolve this issue. I must be lower on memory than I had previously imagined.. Anyway, thanks for introducing me to this lock-up saving macro.
    – Jacksonkr
    Commented Sep 4, 2016 at 13:43

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