avr-gcc not seeing "const" modifier with PROGMEM

Question

I have a file like this:

#include <avr/io.h>
#include <avr/pgmspace.h>

const PROGMEM char* str = "Hello UART!\r\n";

I'm trying to compile it with a Makefile, this is the final command:

avr-gcc -std=gnu99 -mmcu=atmega328p -DF_CPU=16000000UL -I. -Ilib/
    -funsigned-char -funsigned-bitfields -fpack-struct -fshort-enums
    -Wall -Wno-main -Wno-strict-prototypes -Wno-comment -g2 -Wextra
    -Wfatal-errors -Wno-unused-but-set-variable -ffunction-sections
    -fdata-sections -Wl,--gc-sections -Wl,--relax -Os main.c lib/uart.c
    --output main.elf

I am getting the following error:

main.c:9:21: error: variable 'str' must be const in order to be put into read-only section by means of '__attribute__((progmem))'
 const PROGMEM char* str = "Hello UART!\r\n";
                     ^
compilation terminated due to -Wfatal-errors.
Makefile:71: recipe for target 'main.elf' failed
make: *** [main.elf] Error 1

What's wrong with my code?

I tried moving the PROGMEM keyword in various places of the declaration, without any change.

Try: const char str[] PROGMEM = "Hello UART!\r\n";

Sir Jo Black
– Sir Jo Black

2015-04-26 06:38:44 +00:00
Commented Apr 26, 2015 at 6:38 — Sir Jo Black
– Sir Jo Black, Commented Apr 26, 2015 at 6:38

ndim · Accepted Answer · 2017-08-13 17:10:13Z

4

const char str[] PROGMEM = "Hello UART!\r\n";

should work. You're creating a non-const pointer to const data.

edited Aug 13, 2017 at 17:10

ndim

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answered Apr 23, 2015 at 18:53

Unimportant

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4 Comments

MightyPork Over a year ago

will this put the \0 at the end, too?

Unimportant Over a year ago

Yes it will 0 terminate the string.

MightyPork Over a year ago

I still don't really understand the difference, but this works. Maybe you could try to add some more info? I always thought array is the same as pointer in c.

Unimportant Over a year ago

const char str[] PROGMEM = "Hello UART!\r\n"; declares a const array of 14 bytes (including 0 termination) to be put in program memory. const PROGMEM char* str = "Hello UART!\r\n"; only declares a non-const pointer to be put in program memory pointing to const data not in program memory. The non-const pointer cannot be put in program memory and therefore gives you the error.

John Bollinger · Accepted Answer · 2015-04-23 18:52:05Z

0

Your code declares a non-const pointer to a(n array of) const char. That is, the compiler is exactly right that variable str is non-const, it is the thing to which it points that is declared const. If you want both pointer and pointed-to array to be const, then that would be this:

const PROGMEM char* const str = "Hello UART!\r\n";

or equivalently, this:

PROGMEM char const * const str = "Hello UART!\r\n";

Those two mean exactly the same thing as each other.

answered Apr 23, 2015 at 18:52

John Bollinger

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