arduino flash memory array

Question

I have a number of arrays all of different sizes that I have stored in flash memory. I can access single array entries with

byte j = pgm_read_byte(&(array[x]));

What I want to do is to pass the array from the flash memory as an argument to a function. I have tried giving a pointer to the array, as an argument but this gives a compilation error:

void callPGM2(byte arr_size, byte *arr) {
..
..
}

ptr2 = &pgm_read_byte(&(array_1[0]));
callPGM2(5, &ptr2);

Can full arrays be passed from flash memory as function arguments?

Whatever the function pgm_read_byte returns, it can't be used as a pointer. And if it returns an int (as shown in the first code snippet) then why would you pass it as an array of bytes to the callPGM2 function? — Some programmer dude
– Some programmer dude, Commented Sep 25, 2016 at 10:13
Also, in C (as well as in C++) arrays naturally decays to pointers to their first element. If a byte * is expected, and you have an array of byte, then passing the array as is will work just as fine as doing e.g. &array[0]. — Some programmer dude
– Some programmer dude, Commented Sep 25, 2016 at 10:15
pgm_read_byte is a function in the avr/pgmspace lib. The code snippet above I have made a mistake, i think as it returns byte. — Daniel Hall
– Daniel Hall, Commented Sep 25, 2016 at 10:21
The reason to want to pass the array to the function is so the function can perform operation and write values defined in the array to secondary device through i2c — Daniel Hall
– Daniel Hall, Commented Sep 25, 2016 at 10:23
I think you may use pointer to PROGMEM as function argument, but don't mix it with non PROGMEM pointer. Would you mind to test this in real hardware, since I don't have it in my hand right now. If this is not what you mean, please ignore my comment. — putu
– putu, Commented Sep 25, 2016 at 11:19

tofro · Accepted Answer · 2016-09-25 10:23:13Z

2

There's no way to directly pass a pointer to PROGMEM variables, because of the AVR's Harvard architecture with 2 address spaces that C has no way to straightforwardly express - You need to temporarily copy the memory to RAM using memcpy_P, for example.

And you want to learn about the functions provided in the pgmspace library. It holds equivalents to a number of C functions like strcmp, that allow you to work with a constant argument in program space.

answered Sep 25, 2016 at 10:23

tofro

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4 Comments

Daniel Hall Over a year ago

Thanks, i did come across another post referring to the Harvard architecture , but no one confirmed if a pointer to flash memory could be used as an argument in a function.

putu Over a year ago

I'm little bit confused with the explanation. Isn't function like memcpy_P take the pointer to PROGMEM as one of its argument? So technically, we can define similar function which accept pointer to PROGMEM? Of course, inside the function, we must used PROGMEM functions to access the data.

tofro Over a year ago

Sure - A progmem pointer is just a value - can be handed over to PROGMEM functions to have it dereferenced into program space. Working with "normal" and PROGMEM pointers in parallel can, however, be extremely confusing (you can dereference one, but not the other), so it is recommended to copy the memory to RAM temporarily and work on it with "normal" pointers.

Daniel Hall Over a year ago

@tofro can I ask you for a little more information?

Daniel Hall · Accepted Answer · 2016-09-25 13:28:06Z

0

To copy a string from flash memory to RAM

#include<avr/pgmspace.h>
const byte Update_1[5]  PROGMEM = {0x01, 0xB2, 0x02, 0xFF, 0xFF};
byte buffer2[5];
setup {
  memcpy_P (buffer2, &(Update_1),5);
}

answered Sep 25, 2016 at 13:28

Daniel Hall

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