474 lines
15 KiB
Markdown
474 lines
15 KiB
Markdown
---
|
|
title: Alignment
|
|
TARGET DECK: Obsidian::STEM
|
|
FILE TAGS: c17::alignment x86-64
|
|
tags:
|
|
- c17
|
|
- x86-64
|
|
---
|
|
|
|
## Overview
|
|
|
|
For a large class of modern ISAs, storage for basic C datatypes respect **self-alignmnet**. This means `char`s can start on any byte address, `short`s on any even address, 4-byte `int`s and `float`s must start on an address divisible by 4, and `double`s must start on an address divisible by 8. Likewise pointers are also self-aligned.
|
|
|
|
Wasted space introduced solely for alignment purposes is referred to as **slop**.
|
|
|
|
## Structures
|
|
|
|
A `struct`'s **stride address** refers to the first address following the `struct` data that has the same alignment as the `struct`. In general the compiler adds various constraints to how a `struct` is layed out:
|
|
|
|
1. The `struct`'s alignment follows that of its widest scalar member.
|
|
* This guarantees each member satisfies its own self-alignment requirement.
|
|
2. The `struct` introduces trailing padding up to its stride address.
|
|
1. This ensures each element in an array satsifies its self-alignment requirement.
|
|
|
|
The `sizeof` operator on a `struct` returns the total space used between its leading address and its stride address.
|
|
|
|
%%ANKI
|
|
Basic
|
|
What does self-alignment refer to?
|
|
Back: The placement of C datatypes on an address divisible by the size of the datatype.
|
|
Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. [http://www.catb.org/esr/structure-packing/](http://www.catb.org/esr/structure-packing/).
|
|
<!--ID: 1730831516929-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
What addresses can a `char` be stored at?
|
|
Back: Any address.
|
|
Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. [http://www.catb.org/esr/structure-packing/](http://www.catb.org/esr/structure-packing/).
|
|
<!--ID: 1730831516932-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
What addresses can a `int` be stored at?
|
|
Back: Any address divisible by 4.
|
|
Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. [http://www.catb.org/esr/structure-packing/](http://www.catb.org/esr/structure-packing/).
|
|
<!--ID: 1730831516934-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
What addresses can a `short` be stored at?
|
|
Back: Any address divisible by 2.
|
|
Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. [http://www.catb.org/esr/structure-packing/](http://www.catb.org/esr/structure-packing/).
|
|
<!--ID: 1730831516936-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
What addresses can a `double` be stored at (outside of a `struct`)?
|
|
Back: Any address divisible by 8.
|
|
Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. [http://www.catb.org/esr/structure-packing/](http://www.catb.org/esr/structure-packing/).
|
|
<!--ID: 1730831516938-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
What addresses can a `double` be stored at (inside of a `struct`)?
|
|
Back: Depending on compiler, addresses divisible by 4 or 8.
|
|
Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. [http://www.catb.org/esr/structure-packing/](http://www.catb.org/esr/structure-packing/).
|
|
<!--ID: 1730988416659-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
What addresses can a pointer be stored at?
|
|
Back: Any address divisible by 8.
|
|
Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. [http://www.catb.org/esr/structure-packing/](http://www.catb.org/esr/structure-packing/).
|
|
<!--ID: 1730831516941-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
How does self-alignment make access faster?
|
|
Back: It enables single instruction fetches and puts.
|
|
Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. [http://www.catb.org/esr/structure-packing/](http://www.catb.org/esr/structure-packing/).
|
|
<!--ID: 1730831516943-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Cloze
|
|
With respect to memory alignment, {slop} is {waste space for padding datatypes to their alignment}.
|
|
Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. [http://www.catb.org/esr/structure-packing/](http://www.catb.org/esr/structure-packing/).
|
|
<!--ID: 1730831516944-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
What value is slop initialized to?
|
|
Back: Undefined.
|
|
Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. [http://www.catb.org/esr/structure-packing/](http://www.catb.org/esr/structure-packing/).
|
|
<!--ID: 1730831516946-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
*Why* isn't equality for `struct`s well-defined?
|
|
Back: The value of slop is undefined.
|
|
Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. [http://www.catb.org/esr/structure-packing/](http://www.catb.org/esr/structure-packing/).
|
|
<!--ID: 1730831516947-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
*Why* isn't inequality for `struct`s well-defined?
|
|
Back: The value of slop is undefined.
|
|
Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. [http://www.catb.org/esr/structure-packing/](http://www.catb.org/esr/structure-packing/).
|
|
<!--ID: 1730831516948-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
Assume allocated order matches source order. How are the following variables aligned?
|
|
```c
|
|
char *p;
|
|
char c;
|
|
short x;
|
|
```
|
|
Back:
|
|
```c
|
|
char *p; // 8 bytes
|
|
char c; // 1 byte
|
|
char pad[1]; // 1 byte
|
|
short x; // 2 bytes
|
|
```
|
|
Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. [http://www.catb.org/esr/structure-packing/](http://www.catb.org/esr/structure-packing/).
|
|
<!--ID: 1730902219762-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
Assume allocated order matches source order. How are the following variables aligned?
|
|
```c
|
|
long x;
|
|
short c;
|
|
float f;
|
|
```
|
|
Back:
|
|
```c
|
|
long x; // 8 bytes
|
|
short c; // 2 bytes
|
|
char pad[2]; // 2 bytes
|
|
float f; // 4 bytes
|
|
```
|
|
Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. [http://www.catb.org/esr/structure-packing/](http://www.catb.org/esr/structure-packing/).
|
|
<!--ID: 1730902219769-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
Assume allocated order matches source order. What is the value of `N`?
|
|
```c
|
|
char c;
|
|
char pad1[M];
|
|
char *p;
|
|
char pad2[N];
|
|
int x;
|
|
```
|
|
Back: $0$
|
|
Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. [http://www.catb.org/esr/structure-packing/](http://www.catb.org/esr/structure-packing/).
|
|
<!--ID: 1730902219773-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
Assume allocated order matches source order. What is the value of `M`?
|
|
```c
|
|
char c;
|
|
char pad1[M];
|
|
char *p;
|
|
char pad2[N];
|
|
int x;
|
|
```
|
|
Back: Between $0$ and $7$ inclusive.
|
|
Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. [http://www.catb.org/esr/structure-packing/](http://www.catb.org/esr/structure-packing/).
|
|
<!--ID: 1730902219776-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
Consider `short A[N]`. What is the internal padding of this array?
|
|
Back: $0$, i.e. `A` has no internal padding.
|
|
Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. [http://www.catb.org/esr/structure-packing/](http://www.catb.org/esr/structure-packing/).
|
|
<!--ID: 1730902219780-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
Consider `char* A[N]`. What is the internal padding of this array?
|
|
Back: $0$, i.e. `A` has no internal padding.
|
|
Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. [http://www.catb.org/esr/structure-packing/](http://www.catb.org/esr/structure-packing/).
|
|
<!--ID: 1730902219784-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
What is the alignment of a `struct` instance?
|
|
Back: That of its widest scalar member.
|
|
Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. [http://www.catb.org/esr/structure-packing/](http://www.catb.org/esr/structure-packing/).
|
|
<!--ID: 1730902219788-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
*Why* is the alignment of a `struct` instance that of its widest scalar member?
|
|
Back: It is an easy way to ensure all members are self-aligned.
|
|
Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. [http://www.catb.org/esr/structure-packing/](http://www.catb.org/esr/structure-packing/).
|
|
<!--ID: 1730902219792-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
*Why* does a `struct` sometimes introduce trailing padding?
|
|
Back: To ensure each element of an array of these `struct`s is self-aligned.
|
|
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
|
|
<!--ID: 1731165663191-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Cloze
|
|
The address of a `struct` is the same as its {first member}.
|
|
Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. [http://www.catb.org/esr/structure-packing/](http://www.catb.org/esr/structure-packing/).
|
|
<!--ID: 1730902219797-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
How much leading padding does a `struct` instance have?
|
|
Back: $0$
|
|
Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. [http://www.catb.org/esr/structure-packing/](http://www.catb.org/esr/structure-packing/).
|
|
<!--ID: 1730902219802-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
How are members of the following `struct` aligned?
|
|
```c
|
|
struct foo {
|
|
char *p;
|
|
char c;
|
|
long x;
|
|
};
|
|
```
|
|
Back:
|
|
```c
|
|
struct foo {
|
|
char *p; // 8 bytes
|
|
char c; // 1 byte
|
|
char pad[7]; // 7 bytes
|
|
long x; // 8 bytes
|
|
};
|
|
```
|
|
Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. [http://www.catb.org/esr/structure-packing/](http://www.catb.org/esr/structure-packing/).
|
|
<!--ID: 1730902219809-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
How are members of the following `struct` aligned?
|
|
```c
|
|
struct foo {
|
|
char c;
|
|
char *p;
|
|
long x;
|
|
};
|
|
```
|
|
Back:
|
|
```c
|
|
struct foo {
|
|
char c; // 1 byte
|
|
char pad[7]; // 7 bytes
|
|
char *p; // 8 bytes
|
|
long x; // 8 bytes
|
|
};
|
|
```
|
|
Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. [http://www.catb.org/esr/structure-packing/](http://www.catb.org/esr/structure-packing/).
|
|
<!--ID: 1730902219814-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Cloze
|
|
A `struct`'s {stride address} is {the first address following the `struct`'s data with the same alignment as the `struct`}.
|
|
Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. [http://www.catb.org/esr/structure-packing/](http://www.catb.org/esr/structure-packing/).
|
|
<!--ID: 1730902219820-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
What is the result of `sizeof(struct foo)`?
|
|
```c
|
|
struct foo {
|
|
char *p;
|
|
char c;
|
|
};
|
|
```
|
|
Back: `16`
|
|
Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. [http://www.catb.org/esr/structure-packing/](http://www.catb.org/esr/structure-packing/).
|
|
<!--ID: 1730902219825-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
What is the result of `sizeof(struct foo)`?
|
|
```c
|
|
struct foo {
|
|
short s;
|
|
char c;
|
|
};
|
|
```
|
|
Back: `4`
|
|
Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. [http://www.catb.org/esr/structure-packing/](http://www.catb.org/esr/structure-packing/).
|
|
<!--ID: 1730902219830-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
What "hidden" space is returned by `sizeof` on `struct` types?
|
|
Back: Slop.
|
|
Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. [http://www.catb.org/esr/structure-packing/](http://www.catb.org/esr/structure-packing/).
|
|
<!--ID: 1730902219834-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
How are members of the following `struct` aligned?
|
|
```c
|
|
struct foo {
|
|
char c;
|
|
struct bar {
|
|
char *p;
|
|
short x;
|
|
} bar;
|
|
};
|
|
```
|
|
Back:
|
|
```c
|
|
struct foo {
|
|
char c; // 1 byte
|
|
char pad[7]; // 7 bytes
|
|
struct bar {
|
|
char *p; // 8 bytes
|
|
short x; // 2 bytes
|
|
char pad2[6]; // 6 bytes
|
|
} bar;
|
|
};
|
|
```
|
|
Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. [http://www.catb.org/esr/structure-packing/](http://www.catb.org/esr/structure-packing/).
|
|
<!--ID: 1730902219840-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
How are members of the following `struct` aligned?
|
|
```c
|
|
struct foo {
|
|
char c;
|
|
struct bar {
|
|
short x;
|
|
} bar;
|
|
};
|
|
```
|
|
Back:
|
|
```c
|
|
struct foo {
|
|
char c; // 1 byte
|
|
char pad[1]; // 1 byte
|
|
struct bar {
|
|
short x; // 2 bytes
|
|
} bar;
|
|
};
|
|
```
|
|
Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. [http://www.catb.org/esr/structure-packing/](http://www.catb.org/esr/structure-packing/).
|
|
<!--ID: 1730902219846-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
What is the alignment of a nested `struct`?
|
|
Back: That of its widest scalar member.
|
|
Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. [http://www.catb.org/esr/structure-packing/](http://www.catb.org/esr/structure-packing/).
|
|
<!--ID: 1730902219851-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
The `sizeof` operator on `struct`s returns the space used between what two addresses?
|
|
Back: The `struct`'s leading address and stride address.
|
|
Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. [http://www.catb.org/esr/structure-packing/](http://www.catb.org/esr/structure-packing/).
|
|
<!--ID: 1731165663196-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
What is the offset of field `u` in the following?
|
|
```c
|
|
struct foo {
|
|
char u;
|
|
int v;
|
|
};
|
|
```
|
|
Back: `0`
|
|
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
|
|
<!--ID: 1731165663205-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
What is the offset of field `v` in the following?
|
|
```c
|
|
struct foo {
|
|
char u;
|
|
int v;
|
|
};
|
|
```
|
|
Back: `4`
|
|
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
|
|
<!--ID: 1731165663209-->
|
|
END%%
|
|
|
|
## Unions
|
|
|
|
The `sizeof` operator on a `union` returns that of its widest member.
|
|
|
|
%%ANKI
|
|
Basic
|
|
The `sizeof` operator on `union`s returns what?
|
|
Back: The `sizeof` that of its widest member.
|
|
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
|
|
<!--ID: 1731165663214-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
What is the offset of field `u` in the following?
|
|
```c
|
|
union foo {
|
|
char u;
|
|
int v;
|
|
};
|
|
```
|
|
Back: `0`
|
|
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
|
|
<!--ID: 1731165663237-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
What is the offset of field `v` in the following?
|
|
```c
|
|
union foo {
|
|
char u;
|
|
int v;
|
|
};
|
|
```
|
|
Back: `0`
|
|
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
|
|
<!--ID: 1731165663242-->
|
|
END%%
|
|
|
|
## Bibliography
|
|
|
|
* Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
|
|
* Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. [http://www.catb.org/esr/structure-packing/](http://www.catb.org/esr/structure-packing/). |