store and retrieve array of chars in flash memory

Projects Programming
18

sterretje:
"The smart solution would be to implement a function which could manipulate the flash variables on a byte-by-byte basis without buffering

Which is not possible on AVR based Arduinos without modifying the bootloader or other tricks.

He means reading a byte of FLASH and deciding whether it should be passed on to its final destination or not, or with modification. (I see Danois90 has clarified.) For example:

// strings I want to save in flash
const char STR_0[] PROGMEM = "string0_";
const char STR_1[] PROGMEM = "string1_";
const char STR_2[] PROGMEM = "string2_";
const char STR_3[] PROGMEM = "string3_";
const char STR_4[] PROGMEM = "string4_";
const char STR_5[] PROGMEM = "string5_";

const char* const STR[] PROGMEM =
 {
   STR_0,
   STR_1,
   STR_2,
   STR_3,
   STR_4,
   STR_5
 };

const size_t MAX_STRINGS = sizeof(STR) / sizeof(STR[0]);

// Handy macro for casting the "const char *" to the correct type for printing
#define CF(s) ((const __FlashStringHelper *)s)

void function()
{
 uint8_t i=0;

 while (i<MAX_STRINGS) {
   printUppercaseFLASHstr( STR[i] );
   i++;

   if (i<MAX_STRINGS)
     printUppercaseFLASHstr( STR[i] );
   i++;

   Serial.println();
 }

 Serial.println();
}

void printUppercaseFLASHstr( const char *flashPtr )
{
 for (;;) {
   char c = pgm_read_byte( flashPtr++ );
   if (c == '\0')
     break;

   if (('a' <= c) and (c <= 'z'))
     c -= 'a' - 'A'; // offset is 32, so subtracting 32 from 'd' (80) is a 'D' (68)
   Serial.write( c );
 }
}


void setup() {
 Serial.begin(9600);
 function();
}


void loop() {
}

This prints the uppercase versions of all those strings without ever copying them to a RAM buffer. The output:

STRING0_STRING1_
STRING2_STRING3_
STRING4_STRING5_

Actually, I suspect the sketch performs everything in registers, so it may not use any RAM.