Store floats in and reading them from flash

Question

How do I store floats in flash and how do I read them from there? (I need a look-up table for pairs of float values.)

The storing part doesn't seem to be so difficult as this compiles:

PROGMEM float value1 = 1.23456;

But how do I read them? Do I need to copy them byte by byte to a float variable, or is there a better way? (I am using an Arduino ATmega 328p with 4-byte floats.)

Answer 1

There is a helper macro in avr-libc that is designed just for this purpose:

#define pgm_read_float_near(address_short) __LPM_float((uint16_t)(address_short))
Read a float from the program space with a 16-bit (near) address.
Note
 The address is a byte address. The address is in the program space. 

Use it like:

PROGMEM float pi=3.141592653; // constant value in flash
float diameter = 24.332154; // variable user value
float circumference = diameter * pgm_read_float_near(&pi);

Answer 2

I realize that the OP asked about a single value, but I thought I'd add info on retrieving from an array.

const float pmdata[] PROGMEM = {1.0, 2.0, 3.0};

then retrieve thusly:

float val = pgm_read_float(&pmdata[idx]);

• Muhammad Ikhwan Perwira: what "&" means on &pmdata