[chbot] AVR linker issue (?)

[Richard Jones]

Did you try using the 'C' compiler instead of C++?

Richard

On Sun, Jul 18, 2021 at 5:03 PM Mark Atherton <markaren1 at xtra.co.nz> wrote:

> 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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