![]() Here are the three lines of code you’ll need: char buffer We can print out as many variables into our string as we want, and the amount of code required always stays at a manageable three lines. So for this helpfully informative burrito update, you’d be staring down the barrel of nine lines of code. ![]() “The 3 burritos are 147.7 degrees F, weigh 14oz, and were finished 3 minutes ago.”įor every variable you add to the output, you add two more serial prints in the code. What if you wanted to print a line with 4 variables inserted into a string like this: In fact, using Serial.print() to build a string gets even more clunky the more variables you add. But Great Auntie Gertrude’s Carbunkle, is it clunky. Now to be clear, there’s nothing inherently wrong with using Serial.print() to build a string. Using Serial.print() you’d typically write five lines of code to print out that single line of text. In this example, consider both the number of burritos and the temperature value as variables. How would you print a string using text and variables using good old Serial.print()? Let’s say you want to print this line of text to the Serial Monitor: So here’s an Arduino 101 question for you. The PSTR() macro it uses has to be inside a function to work.First, let’s revisit the clunky way to print a string Sadly, you can't use the F() macro to initialize a global pointer. See Print.cpp in the Arduino core sources for examples of how you would treat the argument differently. Then you can use the F() macro to keep those string literals in FLASH. If you write your own function and one of the arguments is often a fairly large string literal you might want to make a version of the function that accepts the '_FlashStringHelper *' type. println() methods can all accept the '_FlashStringHelper *' type and fetch the string from the FLASH address space. Serial, LCD, Ethernet, WiFi, etc are all objects that inherit behavior from the Print class so their. The methods then cast the _FlashStringHelper pointer BACk to a character pointer and fetch each character of the string from FLASH. The compiler chooses those methods when you pass the '_FlashStringHelper *' created by the F() macro. println() methods similar to the 'char *' methods that accept a '_FlashStringHelper *' instead. The ONLY safe operation is to cast the value BACK to what it was before. Unlike a regular cast, no conversion is done. The "reinterpret_cast" tells the compiler that you know that the value being cast is not compatible with the destination type. The class _FlashStringHelper has no body, just a type. The F() macro uses the macro PSTR() to tell the compiler to keep the string in FLASH and then changes the value type from 'char *' (character pointer) to a '_FlashStringHelper *'. #define F(string_literal) (reinterpret_cast(PSTR(string_literal))) There is a macro named "F" in the standard include file Wstring.h: class _FlashStringHelper there is a special trick used to make keeping them out of SRAM easier. Because they are often used for text output to Serial, LCD, Ethernet, WiFi, etc. They are treated like initialized read-only variables and their space in SRAM is initialized by copying the data from FLASH. String literals are a special case of initialized variables. It is up to you to add the special function calls to fetch data from FLASH whenever you want to fetch data from that variable. It won't remember that your variable is in FLASH and if you try to use it like you would any other variable it will use that FLASH address to fetch data from SRAM and get the wrong data. Unfortunately, the compiler will only keep track of the address of the variable, not which address space it is in. (Note: The special instructions for writing into FLASH can only be executed from the BOOTLOADER area of the FLASH memory. For larger initialized read-only variables, such as lookup tables, it is good to tell the compiler to keep the data in FLASH. ![]() This is great for initialized variables that will be modified by the sketch but for constant (read-only) variables it is a waste of precious SRAM space. Any initialized variables will be initialized by copying the data from FLASH to the variable's location in SRAM before the sketch starts. You have to use special machine instructions for reading (or writing) FLASH or reading or writing EEPROM. Instructions are fetched from FLASH (a.k.a. ![]() On the AVR processor the SRAM, FLASH, and EEPROM are in separate address spaces.
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