Re: Archive (library) which overrides syscalls from toolchain's archive?

[Freddie Chopin <freddie_chopin at op dot pl>]

Hello Nick!

First of all thank you for your helpful message which got me further with the 
problem!

On Friday 25 of September 2015 11:44:16 Nick Clifton wrote:
> This sounds like a linker command line ordering problem.  [Aside: It
> would really help if you posted your linker command line ...]

Yes, that's a good idea, the linking command is:

arm-none-eabi-g++ -L./output/  -T./output/STM32F407VGT.ld -mcpu=cortex-m4 -
mthumb -mfloat-abi=hard -mfpu=fpv4-sp-d16 -g -Wl,-
Map=./output/distortos.map,--cref,--gc-sections <a lot of object files> -
ldistortos -o ./output/distortos.elf

> What is probably happening is that your archive is being placed on the
> linker command line *before* the object file that calls _sbrk_r.  So at
> the time that your archive is scanned, there are no undefined references
> to _sbrk_r.  So it is discarded.  Later on on the command line something
> else does call _sbrk_r, so an unresolved reference is created.  Then
> later on still libg.a appears on the command line, this is scanned and
> it provides a definition of _sbrk_r.  Hence the version in libg.a is
> pulled in.  Unfortunately this version wants to call _sbrk, but no
> further libraries left on the command line provide that function, so the
> linker generates the error message above.
> 
> > Is there any (possibly simple and generic) way to solve it?
> 
> Probably.  Reordering your linker command line ought to work.

The problem is that _sbrk_r() (let's stick to this function as the best 
example) from libg.a is pulled by malloc() from libg.a... That's why I don't 
think any reordering will help here. Reordering would probably be the simplest 
option here (but really fragile as a solution).
 
> > or forcing _sbrk_r() to be initially undefined with
> > EXTERN(_sbrk_r) in the linker script, but this didn't change much.
> 
> Did you try using --undefined=_sbrk_r on the linker command line ?

I've tried that now, and - surprisingly - it worked. Yesterday I had also some 
--start-group ... --end-groups in the linker command (basically wrapping all 
the objects and libdistortos.a). But today it worked - to completely fix the 
link I had to mark _sbrk_r() and _kill_r() as undefined. It also works with 
the EXTERN() in the linker script (; The only problem here is that I should 
mark all the syscalls (there are 18 of them) as undefined, so that all 
possible scenarios would actually work. This forces the right objects to be 
linked, but they are linked even if not needed, which is problematic for 
targets with small memories. Currently that's not an issue, as most of the 
syscalls are just stubs, with the only exception being _sbrk_r(), but in 
future syscalls like _open_r() or _read_r() would pull all the I/O code into a 
project that might not need them, which would be a real issue...

> > Basically I'm looking for a way to just force some functions to be taken
> > from designated archive.
> 
> Have you tried using the --wrap linker command line option ?  You could
> wrap calls to _sbrk_r and then provide the wrapped version in your own
> archive.

I didn't know that option (; I didn't try it yet, but it seems that it would 
solve the problem that I described above (the functions wouldn't be forced 
into the executable if they are not used). The problem I see here is that this 
cannot be done with a linker script (or maybe it can be done?), so the link 
command would get pretty long and complicated... I'll try that soon and find 
out.

> PS. There is also the --whole-archive linker command line option, which
> could be used to force all of your archive to be included in the link,
> but it is probably overkill.

Hmm - I've just tried it and it seems to be the easiest option. It seems to 
solve my issue, it doesn't force any functions into the executable if they are 
not needed and the commands are pretty short... Why do you think it is 
overkill? Does it have some downsides that I'm not aware of? I added "... -
Wl,--whole-archive -ldistortos -Wl,--no-whole-archive ..." to my linker 
command and it seems to work fine.

Thanks again!

Regards,
FCh

P.S. Previously I used the wrong e-mail address for my post, now I'm using the 
right one.