Re: Symbols defined via linker script: local or global?

[Sebastian Huber <sebastian dot huber at embedded-brains dot de>]

Hello,

sorry for the delay. I have now a test case. It seems to be related to 
the --gc-sections option.

I used "ld --verbose > linkcmds" to get the default linker command file 
on my Linux machine and removed the header and trailer.

Test program:

extern char SA[];
extern char SC[];
extern char SE[];

__attribute__((section(".my"))) int i;

static const char *const t[] = { SA, SC, SE };

int main()
{
    return (int)(long)t;
}

Test linker command file:

SA = 4;
SB = 8;

SECTIONS {
    .my : {
        SC = .;
        SD = .;
        KEEP (*(.my*))
    }

    SE = SC;
    SF = SD;
}

INCLUDE linkcmds

We get:

gcc -T linkcmds.my test.c
objdump -t a.out | grep '\<S[A-F]\>'
0000000000000000 g       .my    0000000000000000              SD
0000000000000008 g       *ABS*  0000000000000000              SB
0000000000000000 g       .my    0000000000000000              SF
0000000000000000 g       .my    0000000000000000              SC
0000000000000004 g       *ABS*  0000000000000000              SA
0000000000000000 g       .my    0000000000000000              SE

gcc -T linkcmds.my test.c -Wl,--gc-sections
objdump -t a.out | grep '\<S[A-F]\>'
0000000000000000 l       .my    0000000000000000              SD
0000000000000008 l       *ABS*  0000000000000000              SB
0000000000000000 g       .my    0000000000000000              SF
0000000000000000 g       .my    0000000000000000              SC
0000000000000004 g       *ABS*  0000000000000000              SA
0000000000000000 g       .my    0000000000000000              SE

So, the --gc-sections turns SB and SD into local symbols in contrast to 
SA and SE which are referenced by t.

On 12/05/16 14:43, Sebastian Huber wrote:
> On 12/05/16 14:42, Alan Modra wrote:
> > On Thu, May 12, 2016 at 02:17:41PM +0200, Sebastian Huber wrote:
> > > >
> > > >
> > > >On 12/05/16 14:15, Alan Modra wrote:
> > > > > >On Thu, May 12, 2016 at 01:04:52PM +0200, Sebastian Huber wrote:
> > > > > > >>Hello,
> > > > > > >>
> > > > > > >>I define a symbol in a custom linker script, e.g.
> > > > > > >>
> > > > > > >>foo = 0x123;
> > > > > > >>
> > > > > > >>In case I reference this symbol in my executable (e.g. 
> > > > > printf("%p\n", foo)),
> > > > > > >>then the symbol is global in the ELF file. In case it is not 
> > > > > referenced by
> > > > > > >>the executable, then it is local.
> > > > > >What target, and what options were you passing to ld?
> > > > > >
> > > >
> > > >Target is ARM EABI, command line generated via GCC:
> > > >
> > > >arm-rtems4.11-ld -dc -dp -N -o b-atsamv/app.exe
> > > >/opt/rtems-4.12/arm-rtems4.12/atsamv/lib/start.o 
> > > /opt/rtems-4.12/lib64/gcc/arm-rtems4.12/6.1.1/thumb/cortex-m7/fpv5-d16/hard/crti.o 
> > > /opt/rtems-4.12/lib64/gcc/arm-rtems4.12/6.1.1/thumb/cortex-m7/fpv5-d16/hard/crtbegin.o 
> > >
> > > >-e _start
> > > >-L/opt/rtems-4.12/lib64/gcc/arm-rtems4.12/6.1.1/thumb/cortex-m7/fpv5-d16/hard 
> > > -L/opt/rtems-4.12/lib64/gcc/arm-rtems4.12/6.1.1/../../../../arm-rtems4.12/lib/thumb/cortex-m7/fpv5-d16/hard 
> > >
> > > >-L/opt/rtems-4.12/arm-rtems4.12/atsamv/lib
> > > >-L/opt/rtems-4.12/lib64/gcc/arm-rtems4.12/6.1.1
> > > >-L/opt/rtems-4.12/lib64/gcc/arm-rtems4.12/6.1.1/../../../../arm-rtems4.12/lib 
> > >
> > > >--gc-sections -Map b-atsamv/app.map b-atsamv/init.o
> > > >b-atsamv/dev/cfi/cfi_core.o b-atsamv/atsam_flash.o
> > > >b-atsamv/test-file-system.o -ljffs2 --start-group -lgcc --start-group
> > > >-lrtemsbsp -lrtemscpu -latomic -lc -lgcc --end-group --end-group 
> > > /opt/rtems-4.12/lib64/gcc/arm-rtems4.12/6.1.1/thumb/cortex-m7/fpv5-d16/hard/crtend.o 
> > > /opt/rtems-4.12/lib64/gcc/arm-rtems4.12/6.1.1/thumb/cortex-m7/fpv5-d16/hard/crtn.o 
> > >
> > > >-T linkcmds.sdram
> > I don't see anything odd there, but I can't reproduce the global to
> > local change with a simple testcase.  All of my linker script symbols
> > are global.
>
> Ok, I try to create a simple test case.

--
Sebastian Huber, embedded brains GmbH

Address : Dornierstr. 4, D-82178 Puchheim, Germany
Phone   : +49 89 189 47 41-16
Fax     : +49 89 189 47 41-09
E-Mail  : sebastian.huber@embedded-brains.de
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