On 30-Sep-2016, at 10:10 pm, Román Martínez <rgmf@riseup.net> wrote:
Hi,
I compile main.s and it generates main.o. With objdump I can see:
0000000000000000 <_start>: 0: b8 01 00 00 00 mov $0x1,%eax 5: bb 00 00 00 00 mov $0x0,%ebx a: cd 80 int $0x80
After link main.o it generates main. With objdump I now can see:
0000000000400078 <_start>: 400078: b8 01 00 00 00 mov $0x1,%eax 40007d: bb 00 00 00 00 mov $0x0,%ebx 400082: cd 80 int $0x80
So, linker generates virtual address, doesn't it? But why it starts at 400078 and not in other any location? Is there any logic here? A virtual address can start at 0?
The linker script would normally leave out some area and _not_ start from 0. (although this is not an absolute necessity ). It will pick up and valid virtual address and assign start of .text,.bss and .stack. This is possible because we now have virtual memory. And every program thinks that it has _all_ the memory for itself. So, 400078 is perfectly valid virtual address space and linker has chosen this for this executable. Also note that if the address space is getting shifted by little , this is because of the KASLR (layout randomisation) - If this is disabled then the linker would generate the same virtual address for all the executable every time. <snip>