F() Flash Macro No Longer Recognized

[woolseyj]

The F() flash macro used for storing strings in flash memory no longer appears to be working as of the 1.8.5 version. The following error is reported during compile for each use.

error: cannot convert 'const char*' to 'const __FlashStringHelper*' in return
#define F(str) (str)

[facchinm]

F() is useless in megaAVR architecture (it only increases flash occupation). Which code are you trying to compile?

[matthijskooijman]

F() is useless in megaAVR architecture (it only increases flash occupation). Which code are you trying to compile?

But for compatibility with existing sketches, all other cores have a working F() macro as well, even when they do not actually need.

[facchinm]

Also this core has it, this is why I'm asking for the code 🙂

[woolseyj]

The code is the Adafruit_IO_Arduino library.

[facchinm]

I just compiled one random example from that library and I didn't get any error. Could you provide more info?

[woolseyj]

Here is an example sketch that uses the library.

[facchinm]

Got it. This patch makes the project compile again

diff --git a/cores/arduino/Arduino.h b/cores/arduino/Arduino.h
index 1f02a58..3891105 100644
--- a/cores/arduino/Arduino.h
+++ b/cores/arduino/Arduino.h
@@ -28,6 +28,7 @@
 
 #undef F
 #define F(str) (str)
+#define __FlashStringHelper char
 
 #ifdef __cplusplus
 extern "C"{

But I'm not 100% sure it is the right approach.
@MCUdude do you think it could work as is?

[woolseyj]

Confirmed.

[MCUdude]

@facchinm even though it would fix this exact issue, it's not a very elegant solution. Will this mess up other libraries that use __FlashStringHelper in some way?

[matthijskooijman]

I think it will, since libraries might overload a method for both char* and __FlashStringHelper*. It might be better to make this a separate type and heave F() cast to __FlashStringHelper* as well.

How does SAM and SAMD solve this? I suspect like above?

[woolseyj]

@facchinm, could you please explain a little further why the F() macro is useless for the megaAVR architecture? I did not understand what you meant by "only increases flash occupation". Both the Uno R3 and the WiFi R2 have flash memory; are they used differently?

[facchinm]

Classic AVR architecture is referred as modified Harward, because data and memory are in different address spaces (so address 0 could refer to ram or flash, and there are different functions to access them).
New AVR (rebranded megaAVR) architecture is instead more conventional as flash and ram share the same address space (like ARM) so there's no need for special treatment if a variable is only in flash (adding const modifier is enough).

[woolseyj]

Great, thanks!

[facchinm]

Anyway, __FlashStringHelper is clearly not intended to be used as-is but as an helper to print/compare strings in Flash with strings in RAM on a very peculiar architecture via the F() macro. Clearly it's too late to hide it but I have no better idea than defining it as const char or char.