Strange additional data in text section of arm-coff object files

Strange additional data in text section of arm-coff object files

[David Lin]

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Hi, everybody
 
There is something strange about the COFF file 
generated by arm-coff tools
I built an environment for building COFF files on 
target machine with ARM processor
As the manual of ld descriped, executable object 
codes could be generated
and relocatable object files could also be created 
with additional flag '-r'
 
for example:

 
arm-coff-gcc -c init.c
arm-coff-gcc -c hello.c
arm-coff-ld -e _hello -o hello_cof.o hello.o 
init.o
arm-coff-ld -r -e _hello -o hello_rel.o hello.o 
init.o
 
 
By observing the executable COFF file 
hello_cof.o and relocatable file hello_rel.o, 
there is surely something different
 
Concerning the text section only, 
I found size of text in  hello_cof.o is 
16-byte more than size in hello_rel.o
content of these 16 bytes of data is "FFFFFFFF 
00000000 FFFFFFFF 00000000"
 
On the experience of prevous work, it is similiar to 
the m68k case, the cross-platform tools for Motorola 68k processors
size of text section in executable object files is 
also 16 byte more than in relocatable ones
content of these 16 bytes of data, however, is 
"00000000 00000000 00000000 00000000"
 
does this circumstance imply something?
I am interested in it :)
 
Here is my experimental environment
 
CPU: Intel Pentium 755

OS: MS Windows 98
Environment: cygwin 1.3.3-1
version of binutils: 2.11.90 (with BFD 
2.11.90)
version of gcc: 2.95.3-4 (cygwin 
special)
version of newlib: 1.9.0
target: arm-coff
 
 

Re: Strange additional data in text section of arm-coff object files

[Anders Norlander]

David Lin wrote:

> Hi, everybody
> 
> There is something strange about the COFF file generated by arm-coff tools
> 
> I built an environment for building COFF files on target machine with
> ARM processor
> 
> As the manual of ld descriped, executable object codes could be generated
> 
> and relocatable object files could also be created with additional
> flag '-r'
> 
> for example:
> 
> arm-coff-gcc -c init.c
> 
> arm-coff-gcc -c hello.c
> 
> arm-coff-ld -e _hello -o hello_cof.o hello.o init.o
> 
> arm-coff-ld -r -e _hello -o hello_rel.o hello.o init.o
> 
> By observing the executable COFF file hello_cof.o and relocatable file
> hello_rel.o,
> 
> there is surely something different
> 
> Concerning the text section only,
> 
> I found size of text in hello_cof.o is 16-byte more than size in
> hello_rel.o
> 
> content of these 16 bytes of data is "FFFFFFFF 00000000 FFFFFFFF 00000000"
> 
> On the experience of prevous work, it is similiar to the m68k case,
> the cross-platform tools for Motorola 68k processors
> 
> size of text section in executable object files is also 16 byte more
> than in relocatable ones
> 
> content of these 16 bytes of data, however, is "00000000 00000000
> 00000000 00000000"
> 
> does this circumstance imply something?
> 
> I am interested in it :)
> 
It's the constructor/destructor lists. For arm-coff they start
with -1 and end with 0, so both lists are empty. For the m68k the
tables start with the number of entries and are terminated by 0.

Relocateable links do not generate constructor tables, but you
can force it with the -Ur linker option.

Regards,
Anders

Re: Strange additional data in text section of arm-coff object files

[Nick Clifton]

Hi David,

> Concerning the text section only, 
> I found size of text in  hello_cof.o is 16-byte more than size in
> hello_rel.o content of these 16 bytes of data is "FFFFFFFF 00000000
> FFFFFFFF 00000000"

The answer becomes obvious once you use disassemble the two text
sections.  In the fully linked executable you will find this at the
end of the .text section:

  00008028 <__CTOR_LIST__>:
    8028:	ffffffff 	swinv	0x00ffffff
    802c:	00000000 	andeq	r0, r0, r0

  00008030 <__DTOR_LIST__>:
    8030:	ffffffff 	swinv	0x00ffffff
    8034:	00000000 	andeq	r0, r0, r0

These words are inserted by the linker script.  (Run ld --verbose to
see the default script that the linker will use).  If this is a
problem for you, you can create your own linker script that does not
add these words.  (Always assuming that you Ir not using any C++ in
your program, since these words are used to build the static
constructor and destructor lists).

Cheers
        Nick