Reading bytes from progmem

Question

I'm trying to write a simple program (as a pre-cursor to a more complicated one) that stores an array of bytes to progmem, and then reads and prints the array. I've looked through a million blog/forums posts online and think I'm doing everything fine, but I'm still getting utter gibberish as output.

Here is my code, any help would be much appreciated!

void setup() {

  byte hello[10] PROGMEM = {1,2,3,4,5,6,7,8,9,10};

  byte buffer[10];

  Serial.begin(9600);

  memcpy_P(buffer, (char*)pgm_read_byte(&hello), 10);

  for(int i=0;i<10;i++){
   //buffer[i] = pgm_read_byte(&(hello[i])); //output is wrong even if i use this
   Serial.println(buffer[i]);
  }
}

void loop() {
}

If I use memcpy, I get the output:

148
93
0
12
148
93
0
12
148
93

And if I use the buffer = .... statement in the for loop (instead of memcpy):

49
5
9
240
108
192
138
173
155
173

Answer 1

You're thinking about two magnitudes too complicated.

memcpy_P wants a source pointer, a destination pointer and a byte count. And the PROGMEM pointer is simply the array. So, your memcpy_P line should like like

memcpy_P (buffer, hello, 10);

that's it.

memcpy (without the "P") will not be able to reach program memory and copy stuff from data RAM instead. That is not what you want.