Problems trying to use print F

[dave-in-nj]

using example digitalreadserial
ESP8266 board
1.8.2 IDE

/*
  DigitalReadSerial

  Reads a digital input on pin 2, prints the result to the Serial Monitor

  This example code is in the public domain.

  http://www.arduino.cc/en/Tutorial/DigitalReadSerial
*/

// digital pin 2 has a pushbutton attached to it. Give it a name:
int pushButton = D2;

// the setup routine runs once when you press reset:
void setup() {
  // initialize serial communication at 9600 bits per second:
  Serial.begin(9600);
  // make the pushbutton's pin an input:
  pinMode(pushButton, INPUT);
}

// the loop routine runs over and over again forever:
void loop() {
  // read the input pin:
  int buttonState = digitalRead(pushButton);
  // print out the state of the button:
  Serial.println((buttonState));
  delay(1);        // delay in between reads for stability
}

compiles properly.

Sketch uses 263316 bytes (25%) of program storage space. Maximum is 1044464 bytes.
Global variables use 26792 bytes (32%) of dynamic memory, leaving 55128 bytes for local variables. Maximum is 81920 bytes.

===
only change is to add (F in the print line

/*
  DigitalReadSerial

  Reads a digital input on pin 2, prints the result to the Serial Monitor

  This example code is in the public domain.

  http://www.arduino.cc/en/Tutorial/DigitalReadSerial
*/

// digital pin 2 has a pushbutton attached to it. Give it a name:
int pushButton = D2;

// the setup routine runs once when you press reset:
void setup() {
  // initialize serial communication at 9600 bits per second:
  Serial.begin(9600);
  // make the pushbutton's pin an input:
  pinMode(pushButton, INPUT);
}

// the loop routine runs over and over again forever:
void loop() {
  // read the input pin:
  int buttonState = digitalRead(pushButton);
  // print out the state of the button:
  Serial.println(F(buttonState));
  delay(1);        // delay in between reads for stability
}

will not compile.
error :

In file included from /home/dave/.arduino15/packages/esp8266/hardware/esp8266/2.5.0/cores/esp8266/Arduino.h:261:0,
from sketch/DigitalReadSerial.ino.cpp:1:
/tmp/arduino_modified_sketch_452053/DigitalReadSerial.ino: In function 'void loop()':
/home/dave/.arduino15/packages/esp8266/hardware/esp8266/2.5.0/tools/sdk/libc/xtensa-lx106-elf/include/sys/pgmspace.h:37:114: error: initializer fails to determine size of '__c'
#define PSTR(s) (extension({static const char __c[] attribute((aligned(4))) PROGMEM = (s); &__c[0];}))
^
/home/dave/.arduino15/packages/esp8266/hardware/esp8266/2.5.0/cores/esp8266/WString.h:38:76: note: in definition of macro 'FPSTR'
#define FPSTR(pstr_pointer) (reinterpret_cast<const __FlashStringHelper *>(pstr_pointer))
^
/home/dave/.arduino15/packages/esp8266/hardware/esp8266/2.5.0/cores/esp8266/WString.h:39:34: note: in expansion of macro 'PSTR'
#define F(string_literal) (FPSTR(PSTR(string_literal)))
^
/tmp/arduino_modified_sketch_452053/DigitalReadSerial.ino:27:18: note: in expansion of macro 'F'
Serial.println(F(buttonState));
^
/home/dave/.arduino15/packages/esp8266/hardware/esp8266/2.5.0/tools/sdk/libc/xtensa-lx106-elf/include/sys/pgmspace.h:37:114: error: array must be initialized with a brace-enclosed initializer
#define PSTR(s) (extension({static const char __c[] attribute((aligned(4))) PROGMEM = (s); &__c[0];}))
^
/home/dave/.arduino15/packages/esp8266/hardware/esp8266/2.5.0/cores/esp8266/WString.h:38:76: note: in definition of macro 'FPSTR'
#define FPSTR(pstr_pointer) (reinterpret_cast<const __FlashStringHelper *>(pstr_pointer))
^
/home/dave/.arduino15/packages/esp8266/hardware/esp8266/2.5.0/cores/esp8266/WString.h:39:34: note: in expansion of macro 'PSTR'
#define F(string_literal) (FPSTR(PSTR(string_literal)))
^
/tmp/arduino_modified_sketch_452053/DigitalReadSerial.ino:27:18: note: in expansion of macro 'F'
Serial.println(F(buttonState));
^
exit status 1
Error compiling for board LOLIN(WEMOS) D1 R2 & mini.

[WattsThat]

buttonState isn’t a string.

The F macro expects a literal string, not an integer.

[gfvalvo]

Serial.println(F(buttonState));

What did you expect this to do?
What benefit did you expect it to provide?

[6v6gt]

Are you thinking of Serial.printf( ) ?

[dave-in-nj]

As i understand the F() function
Puts the information in PROGMEM and not RAM.

i thought Serial.printf()
Is used with character strings. But not sure.

[WattsThat]

Serious question: Why would you put a run time variable in flash? Unless of course it’s a constant...

As someone previously asked, what is the benefit?

The whole point of (F) is to leave constant strings in flash rather than pointlessly copying them to ram.

[6v6gt]

Ok. It was an attempt to write to flash memory at runtime. Well, on the ESP you can do it.
For example, the eeprom read/write is emulated in flash. On AVR (uno etc.) The flash can be written to in the context of the bootloader only.

[Koepel]

Not just like that.

The ESP8266 can write to Flash with a very special function: ESP.flashWrite().
It can also select part of the Flash as a file system.
And it can do OTA (Over The Air upload of a sketch).
Those are all very special things.

By the way, there is a special bootloader for the Uno that can write to Flash during runtime: https://hackaday.com/2015/07/03/arduinos-and-other-avrs-write-to-own-flash/.
The Uno can not execute code from ram and can not read data from Flash. The usage of PROGMEM requires again a very special function.

[dave-in-nj]

I think I got [ answers for ] what I need and I need to do some more studying.

one of the reasons is that over time. memory gets corrupted or there is a higher use just due to time.

WhattsThat mentioned putting a constant in flash, not variables.
so...

Serial.print(F("buttonState = "));
Serial.println(buttonState);

did compile , and lets the variable go where it belongs.