[chbot] AVR linker issue (?)

[Mark Atherton]

Thanks everyone.

font.h already includes the extern ext, see below.

Col, thanks for taking the trouble, it is almost certainly a compiler 
problem.

I have been poking around in the .O files using avr-objdump and font.o 
appears to be missing ext_font whereas it is in one of the .text. 
sections associated with main.c.

I also updated the sample to use a volatile index to fetch data from 
each array (just in case the optimiser got clever and replaced each 
array with a single byte read); no change.

I moved both arrays into PROGMEM, and attempted to read using 
pgm_read_byte(). No change in reported error, and still no sign of an 
array in font.o.

Loaded both arrays with non-zero values just in case the compiler was 
trying to get clever, and return 0 for any indexed read.

Finally changed the arrays to have different content, just in case the 
compiler attempted to share identical arrays.

That was a couple of hours fun, and I don't want to be right, I just 
want to be happy (who said that ???).

Anyway, taken the B plan and dropped the font file directly into the LCD 
driver module, added a simple character interface and all done.

Now know a lot more than I would have liked about this particular 
toolchain (which includes the ATmega328PB) and the SSD1306 based OLED 
I2C LCD displays (only need text, and refuse to use the Arduino IDE).

Regards,

-Mark



On 18/07/2021 3:32 PM, Bry Ashman wrote:
> This is because C++ has internal linkage by default for const.
> 
> When building font.c with the c++ compiler it never only sees "const 
> char ..." which only has internal linkage, so it is not made available 
> outside of its translation unit. If that is changed to "extern const 
> char ext_font[] = {..};" the compiler will make it available for 
> external linkage and the linker will be able to resolve the symbol. (Or 
> if you include font.h into font.c so it sees the extern declaration)
> 
> The extern keyword in C++ has a few more modes than it does in C... and 
> translation unit visibility is a bit different.
> 
> Bry
> 
> 
> 
> On Sun, Jul 18, 2021 at 9:02 AM Mark Atherton <markaren1 at xtra.co.nz 
> <mailto:markaren1 at xtra.co.nz>> wrote:
> 
>     I need another pair of eyes to look at this issue as am obviously doing
>     something daft. This is part of a larger problem; minimal code to show
>     issue is below.
> 
>     All I am trying to do is move a table into a separate module, but the
>     linker is giving me a hard time.
> 
>     Remove 'const' in the external module and everything works fine.
> 
>     Thoughts please !
> 
>     -Mark
> 
> 
>     -------------
> 
>     mkdir -p .build
>     avr-g++ -c -g -Os -Wall -w -ffunction-sections -fdata-sections
>     -mmcu=atmega328p -DF_CPU=16000000L font.c -o .build/font.o
>     avr-g++ -c -g -Os -Wall -w -ffunction-sections -fdata-sections
>     -mmcu=atmega328p -DF_CPU=16000000L main.c -o .build/main.o
>     avr-gcc .build/font.o .build/main.o -o .build/hw.elf -Os
>     -Wl,--gc-sections -mmcu=atmega328p -lm
> 
>     .build/main.o: In function `main':
>     C:\Temp\linker_test/main.c:18: undefined reference to `ext_font'
>     collect2.exe: error: ld returned 1 exit status
>     make: *** [.build/hw.elf] Error 1
> 
>     ------------- font.c
> 
>     #include <avr\pgmspace.h>
> 
>     const unsigned char ext_font[] =        // links OK with const removed
>     {
>         0x00, 0x00, 0x00, 0x00, 0x00
>     };
> 
>     ------------- font.h
> 
>     #ifndef _FONT_H_
>     #define _FONT_H_
> 
>     extern const unsigned char ext_font[5]; // links OK with const removed
> 
>     #endif
> 
>     ------------- main.c
> 
>     #include <avr\io.h>
>     #include <avr\pgmspace.h>
>     #include <ctype.h>
>     #include "font.h"
> 
>     const unsigned char local_font[] = // const has no effect on linker !!
>     {
>         0x00, 0x00, 0x00, 0x00, 0x00
>     };
> 
>     int main(void)
>     {
>         volatile unsigned char val;
>         val = local_font[3];         // links OK
>         val = ext_font[3];           // link Fails
> 
>     }
> 
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