Does a linker generate absolute virtual addresses when linking

Question

Assume a simple hello world in C, compiled using gcc -c to an object file and disassembled using objdump will looks like this:

_main:
       0:   55  pushq   %rbp
       1:   48 89 e5    movq    %rsp, %rbp
       4:   c7 45 fc 00 00 00 00    movl    $0, -4(%rbp)
       b:   c7 45 f8 05 00 00 00    movl    $5, -8(%rbp)
      12:   8b 05 00 00 00 00   movl    (%rip), %eax

As you can see the memory addresses are 0, 1, 4, .. and so on. They are not actual addresses.

Linking the object file and disassembling it looks like this:

_main:
100000f90:  55  pushq   %rbp
100000f91:  48 89 e5    movq    %rsp, %rbp
100000f94:  c7 45 fc 00 00 00 00    movl    $0, -4(%rbp)
100000f9b:  c7 45 f8 05 00 00 00    movl    $5, -8(%rbp)
100000fa2:  8b 05 58 00 00 00   movl    88(%rip), %eax

My question is, is 100000f90 an actual address of a byte of virtual memory or is it an offset?

How can the linker give an actual address prior to execution? What if that memory address isn't available when executing? What if I execute it on another machine with much less memory (maybe paging kicks in here).

Is't it the job of the loader to assign actual addresses?

Is the linker generating actual addresses for he final executable file?

Answer 1

(The following answers assume that the linker is not creating a position-independent executable.)

My question is, is 100000f90 an actual address of a byte of virtual memory or is it an offset?

It's the actual virtual address. Strictly speaking, it is the offset from the base of the code segment, but since modern operating systems always set the base of the code segment to 0, it is effectively the actual virtual address.

How can the linker give an actual address prior to execution? What if that memory address isn't available when executing? What if I execute it on another machine with much less memory (maybe paging kicks in here).

Each process gets its own separate virtual address space. Because it is virtual memory, the amount of physical memory in the machine doesn't matter. Paging is the process by which virtual addresses get mapped to physical address.

Isn't it the job of the loader to assign actual addresses?

Yes, when creating a process, the operating system loader allocates physical page frames for the process and maps the pages into the process's virtual address space. But the virtual addresses are those assigned by the linker.