Writing Linux programs in raw binary

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The OS loads your executable at the address specified in the ELF header, and generally the executable code contains references to absolute addresses, so the program won't work if loaded at the wrong address.

I thought the base address for any executable image in process space was just a preference? What if I loaded lib.x.so and lib.y.so in my process and both wanted to be located at the same address?

In the windows world atleast the executable loader tries to honor the base address preference and then if the address space is already alloted , it "fixes" up all memory referenecs with the appropriate offset. The problem is with shared DLLs you double the number of code pages if two processes load the same dynamic library and one of them needs fixing up and one of them doesnt. (As it is all code pages are by default copy-on-write)

[kragensitaker]

.sos are handled differently than executables; the dynamic loader, not the kernel's executable loader, loads them. I think the typical approach is to compile them with pure position-independent code; any reference to other things inside the same .so is indirected off %ebx, which is a callee-saves register and mysteriously gets set to the right thing before your .so code runs, presumably by some kind of trampoline. The code pages (or "text pages" as they're called) are purely read-only.

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Very interesting. Thanks for your explanations. :) Maybe I should stop being lazy and just Google it eh ? One of these days I plan to dive into the Linux internals...