avr-gcc在PROGMEM中未看到“ const”修饰符

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.

Answer 1

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

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

Answer 2

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.