What does this code mean?

Hi,
right now I am at a project and I wanted to know what this means.

#if defined(__SAM3X8E__)
    #undef __FlashStringHelper::F(string_literal)
    #define F(string_literal) string_literal
#endif

I'm pretty sure that's for defining FLASH memory strings. FLASH memory (PROGMEM) is non volatile so it stays when you restart/lose power, and is where the sketch file is stored.

but that's about all I know of it. There should be some information on it in the reference area of this site (under resources)

it's common that a header file will define a string (e.g. SAM3X8E) when it is include to prevent the header from being processes multiple times because it could also be included in other header files

if the header defining SAM3X8E is included, it's redefining the "F()" macro, but it must first undefine any pre-existing definition of the "F()" macro to avoid an error/warning.

SAM3X8E is another Atmel Microcontroler (based on ARM Cortex-M3). So it seems likely SAM3X8E is defined as an indicator of the platform type.

So all this is doing is saying...
if this is being complied for a SAM3X8E based board...
Remove the traditional definition of the F macro
Replace it with one that has no effect (it just inserts the string literal)

I am assuming, with ARM being a Von Neumann architecture as opposed to Harvard, const strings are placed in FLASH anyway and no need to jump through hoops with F macro and the like to access them.

gcjr:
it's common that a header file will define a string (e.g. SAM3X8E) when it is include to prevent the header from being processes multiple times because it could also be included in other header files

if the header defining SAM3X8E is included, it's redefining the "F()" macro, but it must first undefine any pre-existing definition of the "F()" macro to avoid an error/warning.

That's about 10% correct. The code is defining the behavior of the F() macro based on the processor type. "SAM3X8E" is a flavor of micro-controller from Microchip: https://www.microchip.com/wwwproducts/en/ATsam3x8e.

The 'SAM3X8E' macro is likely defined via a command line option when the Arduino IDE invokes the compiler. Ultimately, it's determined by what you set in the IDE's "Board" field.

thanks.
so SAM3X8E is a processor type, like AVR_ATmega328P defined by the compiler