![]() Within all the Serial.print() methods, there is a method Serial.print(const char* str) that will take in a string literal and print it to serial monitor, but it is not PROGMEM-aware, if you pass in a PROGMEM string literal Serial.print(PSTR("string literal")), the same method will accept what you pass in because it meet the type checking (remember PSTR is actually const char*), but it will not print out the correct string for PSTR("string literal") because it simply expecting a normal string. Function overload is a feature of C++ where multiple methods could have the same function name but each with different parameters. This has to do with C++'s function overload. If you further looks at the _FlashStringHelper, it is actually an empty class without any construct or method, it is literally empty, so why is this useful? and why can't just pass the string literal around by just using PSTR(). ![]() To understand the F() macro and the difference between F() and PSTR() take a little bit explanation.Īs you can see from the definition of F(), it casts a string literal into a const char* variable using PSTR(), and then re-cast it into a class _FlashStrngHelper. #define F(string_literal) (reinterpret_cast(PSTR(string_literal))) this handle the normal string array functionį() is also a macro but it is not part of the avr/progmem.h, it is actually part of the String class defined in WString.h together with a definition of a class called _FlashStringHelper: class _FlashStringHelper ![]() Flash memory in the case of Arduino), but allows the program to convert it into a const char *s PROGMEM variable so that it can be pass around into some PROGMEM-aware functions as a parameter. It looks a little bit intimidated but it actually quite simple, it not only tell the avr-gcc that the string literal s should be kept in program memory (i.e. PSTR came into Arduino from part of avr-libs(a C library) defined in avr/pgmspace.h, it is actually a macro, not a function as many think it was, and is defined as: #define PSTR(s) ((const PROGMEM char *)(s)) Serial.println((int)&top_of_stack - (int)_brkval) Įxplain in detail what exactly does each of the keywords F(), (PGM_P)F, PSTR, const PROGMEM. Serial.println(((int)&top_of_stack - (int)&_heap_start)) A simple corrected version for AVR that I used is here: extern unsigned int _heap_start Return _brkval ? &top - _brkval : &top - _malloc_heap_start įirst, the freeMemory() function that you used from Adafruit was originated from a github, I don't know about the _arm_ implementation, but for the avr, it is incomplete for handling the corner case when the program does not use the malloc() function, if your program never use malloc(), the formula used in freeMemory() will produce the wrong result. #elif defined(CORE_TEENSY) || (ARDUINO > 103 & ARDUINO != 151) should use uinstd.h to define sbrk but Due causes a conflict Now all different methods report a free ram of 2299, including the simple Serial.print(). For example, at the last example (PROGMEM, strcpy_P) when I included another Serial.print(freeMemory()) before the call of the printing functions, then both freeMemory() calls report 2299!Īnother interesting problem is that the freeMemory() reports 2299 as freeRam, while the SRAM of my arduino UNO is only 2kbytes.įollowing the suggestion in comments, I changed "test" to "testtesttesttesttesttesttesttesttesttesttesttesttesttesttesttest". Also it would be really useful if you could explain in detail what exactly does each of the keywords F(), (PGM_P)F, PSTR, const PROGMEM.ĭepending on where and how I use the call Serial.print(freeMemory()) the results change. The basic point of this question is to gather all the possible Flash memory printing techniques. I created a program that prints the word "test", with various methods: ![]() At the same time, I have been using the freeMemory() function from Adafruit. I have been testing on an Arduino UNO various techniques for serial printing from Flash Memory instead of RAM. ![]()
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