Re: [avr-gcc-list] Link question

[Kári Davíðsson]

Got confused with the behavior of __attribute__((constructor)) and
__attribute__((destructor)) for shared libraries. I thought I could do
something similar through the .init8 and .fini8 sections. But as I
discovered the behavior of the linker between static and dynamic
libraries is quite different.

I read a little up on the topic since your answer did limit my search
space on google. The solution seems to be to use -whole-archive for the
linker. To prevent clash with the standard library the trick is to use
-whole-arcive in pairs with -no-whole-archive enclosing the libraries
to be linked, e.g

$ avr-g++ -g -Os -DF_CPU=16000000UL -mmcu=atmega328p myapp.o \
-L. -Wl,-whole-archive -lmylib -Wl,-no-whole-archive -o myapp

More in man ld.

Thanks for your answer which definitely pushed me in the right
direction.

brg
kd

PS: I am not to keen on your "-u symbol" solution although I am sure it
works since it breaks "information hiding" principle and I did not
have global symbols in all of the .o files. Each .o file would have to
have its own symbol to prevent symbol clashing in the linker. Also
starting to introduce global symbols to the .o files just to have to
maintain them also in the Makefile(s) seem counter productive.

On 3/23/12 8:07 AM, Joerg Wunsch wrote:
> (Please suscribe to the list to not miss replies.)
>
> Kári Davíðsson<address@hidden>  wrote:
>
> > I can compile my application and link in this library without errors,
> > but then .init8 and .fini8 sections are stripped out of the final
> > executable, e.g.
>
> No, they are not "stripped", they simply never made it there, because
> the linker had no reason to include those object modules in the first
> place.
>
> > If I on the other hand link the application by specifying all of the
> > library object files on the command line, every thing works just fine.
>
> Sure, object files specified on the command line are always linked.
>
> Object modules within a library are only pulled in to satisfy global
> references which are undefined at the point where the library is being
> processed (that's why the position of a library on the command line is
> important).
>
> Presumably, your init/fini object modules don't define any symbol that
> is being needed by the linker, so they are not considered.
>
> What you could do is putting one global symbol into each of your
> modules (perhaps you've already got some?), and then explicitly tell
> the linker that just this symbol is global undefined, using the -u
> option.  This will make the linker pull in that entire object module
> from the library.
>
> avr-g++ -g -Os -DF_CPU=16000000UL -mmcu=atmega328p myapp.o \
> -Wl,-u,symbol1 -Wl,-u,symbol2 \
> -L. -lmylib -o myapp
>
> (The -Wl option is used to prefix a linker-only option on the compiler
> command line.)