Array and pointers... Confused

Projects Programming
1

I'm getting confused on pointer and arrays and hopefully someone may shine some light.
I have the following arrays:

const prog_char array1[] PROGMEM ={0x00,0x01};
const prog_char array2[] PROGMEM ={0x00,0x01};

My function uses the array:

void test(const prog_char *iptr)
{
//Do something with iptr
}

Everything is well until here. I can call test(array1); and everything works as expected.
Now, I'd like to have an array like this:

const prog_char *myarray[] PROGMEM= {array1,array2};

How do I need to call my test() function and pass myarray[0] as argument?

Thanks in advance!!!
RI

2

To call the function is no different.

As myarray is an array of pointers, passing 'myarray[ 0 ]' is the same as 'array1', or:

test( myarray[0] );

test( *myarray ); //This is fine and is equivalent to accessing the first element.
3

The pointer myarray[0] resides in PROGMEM, not in SRAM. So you need to read itpgm_read_byte(myarray). Then you can use the value you just read and pgm_read_byte to get the values.

4

Good catch, I didn't see the PROGMEM on 'myarray',

To modify the function so you can do the pgm_read_byte inside the function, change it to:

void test(const char **iptr)
{
  const char *arr1 = ( char* ) pgm_read_word( iptr );
  const char *arr2 = ( char* ) pgm_read_word( iptr + 1 );
}

EDIT: changed byte to word for pointer size.

You don't need to use prog_char when passing the type. PROGMEM is an attribute, not a modifier like 'static', or 'const' and isn't remembered.

5

Yes right! Reading address is word and reading the actual char array element is byte. XD

6

Awesome!!!
Got it working now :slight_smile:
Thanks a lot!!!

7

No problem. If your data is a few arrays but all with the same length, then it's easier to chain them into one array in PROGMEM so you don't have to use pgm_read_xxx twice.