Re: [avr-libc-dev] Problem accessing bytes in program space

[Paulo Marques]

Rich Teer wrote:
> Hi all,

Hi,

> I have some strings that I'm storing in program space like this:
>
> const char PROGMEM sms_keys_0[] = "abcABC0  ";
> const char PROGMEM sms_keys_1[] = "-_1      ";
> [...]
> PGM_P sms_keys[] PROGMEM = {
>     sms_keys_0,
>     sms_keys_1,
> [...]
> };
>
> I want to access characters of each string one at a time.  Something like
>
>     uart_puts (strcpy_P (buf, (PGM_P)pgm_read_word (&sms_keys[num])))
>
> will correctly print the numth string, but trying to print the string starting
> from elsewhere leads to data being read from God knows where:
>
>     uart_puts (strcpy_P (buf, (PGM_P)pgm_read_word (&sms_keys[num][2])))

This is not what you want to do. What you want is more like (untested):

uart_puts (strcpy_P (buf, (PGM_P)(pgm_read_word(&sms_keys[num]) + 2)));

or more simply (to send just one char):

uart_send_char(pgm_read_byte(pgm_read_word(&sms_keys[num]) + 2));

The casts are left as an exercise for the reader ;)

> I'd expect this to print the numth string starting at the 3 character
> (it works as expected in non-embedded normal C). 

No it doesn't...

> Am I doing something
> cluelessly wrong, or am I tickling a bug in the compiler? 

At best, you're fetching the address of the second character of string 
"num", reading a pointer from that address, and then using that pointer 
to copy a string from.

--
Paulo Marques
Software Development Department - Grupo PIE, S.A.
Phone: +351 252 290600, Fax: +351 252 290601
Web: www.grupopie.com

"One of my most productive days was throwing away 1000 lines of code."
Ken Thompson