HW1
released: 01/26, 22:00
due: 02/02, 23:59

Answer the following questions.
Submit your answers to Canvas assignments. There is an entry for this homework.

1. C pointer basics.

Below is a piece of valid C code. Read it and answer questions:

#include<stdio.h>

void foo(int p) {
    p = p + 1;
}

void bar(int *p) {
    int w = *p;
    w = w + 1;
}

void baz(int *p) {
    *p = *p + 1;
}

int main() {
    int x=0;
    foo(x);
    printf("foo: %d\n", x);
    bar(&x);
    printf("bar: %d\n", x);
    baz(&x);
    printf("baz: %d\n", x);
}

  1.a What the output of this program?
  (you can copy-paste the code to a file, compile it, and then run)

    [answer:

        foo: 0
        bar: 0
        baz: 1
    ]

  1.b What does "&" in "bar(&x)" mean? Explain in one sentence.

    [answer:
      "&" means to get the memory address of the variable "x"
      (which is on stack btw).
    ]


  1.c What does "*" in "int w = *p;" mean? Explain in one sentence.

    [answer:
      "*" means to dereference the pointer "p"; in other words, to get
      the value from the memory pointed by "p".
    ]



2. Stack frame.

Study handout.week2a (https://naizhengtan.github.io/22spring/notes/handout_w02a.pdf)
Answer the questions below.

Consider the state of the stack when arriving at line 21 (which is empty)
of as.txt (the right panel). Below is the stack.
  Notes:
  -- each slot is 8bytes (indicated by "[8B]")
  -- %rbp and %rsp point to the beginning of the slot.
  -- (if you find the above bullet useless/confusing, you can ignore it.)

high
    |       ... [8B]     |
    |   previous %rbp    | <- %rbp (base pointer)
    |   x (in main) [=?] |
    |       arg     [=?] | <- %rsp
    |              [=??] |
    |                    |
low

  2.a What are the values of "x" and "arg" (indicated by "[=?]")?

    [answer: x=0, arg=8]


  2.b If the program continue to run, what will appear in the next slot of the
  stack (indicated by "[=??]" in the stack above)?
  [hint: your answer should be the address of one line of code.
   Why? Revisit the assembly instruction "call".]


    [answer: address of line 24 in the assembly code (as.txt).]


  2.c Now consider what happens when the call and the function prologue is
  executed (line 42 in as.txt).
    [update 1/29: the stack below is when %rip points to line 42;
     meaning line 42 hasn't been executed.]

  high
       |       ...          |
   A   |   previous %rbp    |
   B   |   x (in main)      |
   C   |       arg          |
   D   |              [=??] |
   E   |  address A ("pr")  | <- %rbp
   F   |      x (in f)      |    [%rbp-8]
       |                    |    [%rbp-16]
       |  address ??        |    [%rbp-24]
       |                    | <- %rsp
  low

      "pr" = "previous rbp" (which was actually address A)


  What does the stack element "%rbp-24" contain (indicated by "address ??")?
  Again, it contains a pointer, pointing to somewhere on stack.
  Choose from address A--F (labels on the left of the stack).
  Explain why in 1-2 sentences.


    [answer:

      C

      "ptr" is a local variable defined in function "main" which is passed to "f"
      through the first argument (%rdi). Later, the value of the argument (%rdi)
      is copied to the current position in line 33 of as.txt.

      See also line 23 in the C code (example.c).]