[drt1245]

Additionally, he keeps talking about dereferencing the pointer, which I don't think is right. The pointer never gets deferenced in the code shown.

I'm not an x86 guru, but I think that "movq ptr(%rip), %rsi" is different because ptr needs to be moved from relative to the instruction pointer (because it is on the stack, as a non-const variable).

[bonzini]

It's a global variable, it's not on the stack. It's in the data section.

$arr is copying the address of "arr", so it must use an immediate move (possibly 64-bit). It could also use RIP-relative with the "lea" (load effective address) instruction.

$ptr(%rip) is accessing memory, so it uses RIP-relative addressing with the "mov" instruction.

[magnetic]

> It's a global variable, it's not on the stack. It's in the data section.

And because it's a const, it's in the .rodata section... (read-only) Playing too liberally with the data often leads to a SEGV (as it should).

[AbacusAvenger]

The code is more interesting if you -do- actually dereference the pointer, too. I've changed the prototype of "bogus" to take a single char as the first argument, and dereference the two pointers inside the do_arr/do_ptr functions:

https://goo.gl/jZYx0f

So the "do_arr" version "knows" what value is at * arr because "arr" is immutable, so the compiler can just choose to load a constant. The "do_ptr" version has to load from memory instead.

But what if we tell the compiler that the value of "ptr" won't change (i.e. "char * const" instead of "const char * ")? The code becomes the same:

https://goo.gl/iNaUHe

So basically "const char []" and "char * const" are more logically equivalent here.

[novia]

This is a little off topic, but does anyone have any good resources to help me wrap my head around pointers in C? Right now they are very confusing to me.

[Zelizz]

Imagine all memory is a big array. A pointer is just an index into that array. A pointer dereference is like accessing something at an array index. A pointer to a pointer is array index to a location where you'll find another array index.

[novia]

I understand pointers conceptually. It's the syntax (in C specifically) that is giving me trouble. I am fine with pointers in ASM.

[iainmerrick]

Hmm, it's been a long time since I read it, but this sounds like something that good old K&R probably does well. It's famously short and well-written.

It's old-fashioned but I assume (maybe others can correct me) the latest edition is up to date enough that it won't teach you any outright bad habits.

[novia]

Thank you very much! I'll give it a look.

[kccqzy]

You are right. This is RIP-relative addressing.