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Posted: **Mon Jul 30, 2007 11:37 am**

hi
suppose i have a a couple object files and want to link them together with ld. also suppose that this binary program is not going to be loaded at address 0, how do i tell the linker to add the appropriate offset to each memory reference in the code?
the ld man page has got the following to say:

--image-base value
Use value as the base address of your program or dll. This is the
lowest memory location that will be used when your program or dll
is loaded. To reduce the need to relocate and improve performance
of your dlls, each should have a unique base address and not over-
lap any other dlls. The default is 0x400000 for executables, and
0x10000000 for dlls. [This option is specific to the i386 PE tar-
geted port of the linker]

but i do:

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sancho@Kiste:~/assembly/boot$ ld boot.o -o boot.bin -e main --oformat binary -N --image-base 0x123

and get

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ld: unrecognized option '--image-base'
ld: use the --help option for usage information

Am I choosing the wrong option?

Posted: **Mon Jul 30, 2007 12:05 pm**

-Ttext 0x123

Regards,
John.

Posted: **Mon Jul 30, 2007 3:06 pm**

jnc100 wrote:-Ttext 0x123

Regards,
John.

I had tried that before but it doesnt do what I want. In fact, "-Ttext 0x123" pads the executable file with 0x123 bytes of zero at the start before placing any executable code. I simply want that the offset 0x123 be added to all memory references within the code...

Posted: **Mon Jul 30, 2007 6:16 pm**

Have you tried a linker script?

Posted: **Tue Jul 31, 2007 4:43 am**

ive had a look at the manual, but all i an find is this -Tsection option, even for scripts..the problem is as follows:

suppose you have a bunch of .o files and want to link them together to a single binary..the linker would for example place the text section first and then append the data section
if varible x is the very first variable in the data section and the text section is 0x10 bytes long, then

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mov x, eax

would be linked to

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mov [0x10], eax

but of course, suppose this binary file instead of being loaded at address 0x0, is loaded at address 0x10000. then of course i would want the linker to write

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mov [0x100010], eax

instead of

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mov [0x10], eax

How would I achieve that? How is this "normally done" (relocation sure is a fairly common thing, isnt it?)

Posted: **Tue Jul 31, 2007 5:01 am**

Looking at the ".bin" extension on your linker output file I am assuming you are outputting in plain binary format. As this format has no section headers it does not support relocation of any segment - that's why when relocating the .text segment ld actually just pads the file.

You have 2 options:

(1) - Use ELF. ELF has headers and thus you can change the address of any section without using padding.

(2) - Use position independent code - compile with -fPIC.

JamesM

Posted: **Tue Jul 31, 2007 9:22 am**

It is possible to force a binary to think that it is going to be running at a certain address. That's what I do in my OS. However it is pretty impossible to relocate a flat binary. So what I do is link every one of my programs at 0x40000000 and load them there using virtual memory. Here is the linker script that I do that with:

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ENTRY( app_entry )

OUTPUT_FORMAT( binary )

SECTIONS
{
        .text 0x40000000 : AT( 0x40000000 )
        {
                *(.text)
                *(.text.*)
                *(.gnu.linkonce.t*)
                *(.const*)
                *(.ro*)
                *(.gnu.linkonce.r*)
        }
        .data :
        {
                *(.data)
                *(.gnu.linkonce.d*)
        }
        
        /* The uninitialized data section */
        .bss :
        {
                bss_start = ABSOLUTE( . );

                *(.bss .bss.* .gnu.linkonce.b.*)

                *(COMMON)

                bss_end = ABSOLUTE( . );

                bss_length = bss_end - bss_start;
                
        }
        }

You can just change 0x40000000 to whatever address you load the binary to and it should work.

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