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.
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/).
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%%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/).
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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/).
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%%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/).
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/).
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/).
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/).
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%%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/).
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%%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/).
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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/).
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%%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/).
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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/).
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/).
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%%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/).
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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/).
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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/).
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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/).
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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/).
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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/).
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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/).
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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.
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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/).
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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/).
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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/).
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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.
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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.
* 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/).
