Linking files

[Frans]

Hello,

Yesterday, I restarted working on an OS. I thought it would be easier if I did'nt write a bootloader, and let grub do that work for me. And maybe start later with my own bootloader.

I found Bare bones and C PlusPlus bare bones

They were really helpful, and it's now working, but I don't understand how the linker script works:

Code: Select all

ENTRY (_loader)

SECTIONS{
    . = 0x00100000;

    .text :{
        *(.text)
    }

    .rodata ALIGN (0x1000) : {
        *(.rodata)
    }

    .data ALIGN (0x1000) : {
        *(.data)
    }

    .bss : {
        _sbss = .;
        *(COMMON)
        *(.bss)
        _ebss = .;
    }

    .data ALIGN (0x1000) : {
        start_ctors = .;
        *(.ctor*)
        end_ctors = .;
        start_dtors = .;
        *(.dtor*)
        end_dtors = .;
        *(.data)
    }
}

The first line is the only one I understand (It tells the linker what function starts the program). Can someone explain the other lines?

[AJ]

Every program can be sub-divided in to a number of sections. These are normally something like:

• .text - contains your executable code
  .data - contains initialised variables etc..
  .rodata - contains constant (read only) data
  .bss - contains zeroed data
  .ctor - contains c++ global / static class constructor pointer list
  .dtor - same for destructors

What you are saying is that, in memory, the binary starts at 0x100000 (1MB). Immediately following this, is your code (.text). You then have .rodata, aligned on a 4kb memory boundary (0x1000 bytes). This is followed by your other sections which are also 4kb aligned.

Wherever you see x = .; , x is a variable which is accessible from your code (by declaring an extern variable). In this case, start_ctors and end_ctors allows your code to find pointers to the global / static class constructors and call them when necessary (probably after heap initialisation).

This is pretty basic and shows my limited understanding of linker scripts, but HTH.
Adam