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

From: Paulo Marques
Subject: Re: [avr-libc-dev] Problem accessing bytes in program space
Date: Mon, 28 Jan 2008 20:37:36 +0000
User-agent: Thunderbird (X11/20070509)

Rich Teer wrote:
Hi all,


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 = {

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

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