432 lines
15 KiB
Markdown
432 lines
15 KiB
Markdown
---
|
|
title: C17
|
|
TARGET DECK: Obsidian::STEM
|
|
FILE TAGS: c17
|
|
tags:
|
|
- c17
|
|
---
|
|
|
|
## Overview
|
|
|
|
> A C program can be seen as a sort of machine that manipulates values: the particular values that variables of the program have at a given time, and also intermediate values that are the result of computed expressions.
|
|
|
|
This quote describes C's **abstract state machine**. Whatever instructions a C program compiles down to is "unimportant" provided that all **observable states** are correctly reproduced. This is the essence of optimization.
|
|
|
|
%%ANKI
|
|
Basic
|
|
What feature of C's abstract state machine makes C performant?
|
|
Back: The ability to optimize.
|
|
Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
|
|
<!--ID: 1723856661330-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
C can compile into any sequence of instructions provided what holds?
|
|
Back: All observable states are correctly reproduced.
|
|
Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
|
|
<!--ID: 1723856661337-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
Why is C's abstract state machine called the way it is?
|
|
Back: Compilers are free to realize the state machine however they see fit.
|
|
Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
|
|
<!--ID: 1723856661340-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
What three components make up C's abstract state machine?
|
|
Back: Values, types, and binary representations.
|
|
Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
|
|
<!--ID: 1723856661343-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Cloze
|
|
In C's abstract state machine, {binary representations} describe {types} which describe {values}.
|
|
Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
|
|
<!--ID: 1723856668033-->
|
|
END%%
|
|
|
|
## Values
|
|
|
|
An **object** is a region of data storage in the execution environment, the contents of which can represent **values**. An **lvalue** is an expression (with non-`void` object type) that potentially designates an object. An **rvalue** is the "value of the expression."
|
|
|
|
The notion of a value in C is an abstract entity. It exists beyond the program or the representation of the value in the program. For example, the value `0` (no matter how its represented) added to variable `x` should always yield result `x` regardless of platform.
|
|
|
|
%%ANKI
|
|
Basic
|
|
What does an object refer to?
|
|
Back: A region of data storage in the execution environment.
|
|
Reference: “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
|
|
<!--ID: 1723510994830-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
What does a value refer to?
|
|
Back: The contents of an object when interpreted as having a specific type.
|
|
Reference: “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
|
|
<!--ID: 1723510994835-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
Why does Gustedt refer to values as abstract entities?
|
|
Back: A value exists beyond a program or any particular representation.
|
|
Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
|
|
<!--ID: 1723856661349-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
How does Gustedt distinguish the data of a program execution from values?
|
|
Back: The data is the set of values of all objects at a given moment.
|
|
Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
|
|
<!--ID: 1723856661358-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Cloze
|
|
A {value} refers to the contents of an {object} when interpreted as having a specific type.
|
|
Reference: “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
|
|
<!--ID: 1723510994839-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
What is an lvalue?
|
|
Back: An expression (with non-`void` object type) that potentially designates an object.
|
|
Reference: “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
|
|
<!--ID: 1723510994874-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
Why are lvalues named the way they are?
|
|
Back: The name is an acronym for **l**ocator **value** (or **l**eft **value**).
|
|
Reference: “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
|
|
<!--ID: 1723510994878-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
What is an rvalue?
|
|
Back: The value of an expression.
|
|
Reference: “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
|
|
<!--ID: 1723510994881-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
Why are rvalues named the way they are?
|
|
Back: The name is an acronym for **r**ight **value**.
|
|
Reference: “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
|
|
<!--ID: 1723510994886-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
Can an lvalue designate an object?
|
|
Back: Yes.
|
|
Reference: “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
|
|
<!--ID: 1723510994900-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
Can an lvalue designate a function?
|
|
Back: No.
|
|
Reference: “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
|
|
<!--ID: 1723510994907-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
Is `x` an lvalue or rvalue in the following snippet?
|
|
```c
|
|
int x = 10;
|
|
```
|
|
Back: An lvalue.
|
|
Reference: “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
|
|
<!--ID: 1723510994914-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
Is `int` an lvalue or rvalue in the following snippet?
|
|
```c
|
|
int x = 10;
|
|
```
|
|
Back: Neither.
|
|
Reference: “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
|
|
<!--ID: 1723510994920-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
Is `10` an lvalue or rvalue in the following snippet?
|
|
```c
|
|
int x = 10;
|
|
```
|
|
Back: An rvalue.
|
|
Reference: “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
|
|
<!--ID: 1723510994925-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
*Why* is `x` an lvalue in the following?
|
|
```c
|
|
int x = 10;
|
|
```
|
|
Back: Because `x` refers to a memory location.
|
|
Reference: “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
|
|
<!--ID: 1723510994932-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
Is `x` an lvalue or rvalue in the following snippet?
|
|
```c
|
|
void x;
|
|
```
|
|
Back: Neither.
|
|
Reference: “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
|
|
<!--ID: 1723510994939-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
Is `x` an lvalue or rvalue in the following snippet?
|
|
```c
|
|
void *x;
|
|
```
|
|
Back: An lvalue.
|
|
Reference: “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
|
|
<!--ID: 1723510994945-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
Is `y` an lvalue or rvalue in the following snippet?
|
|
```c
|
|
int y = x + 10;
|
|
```
|
|
Back: An lvalue.
|
|
Reference: “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
|
|
<!--ID: 1723510994952-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
Is `x` an lvalue or rvalue in the following snippet?
|
|
```c
|
|
int y = x + 10;
|
|
```
|
|
Back: An rvalue.
|
|
Reference: “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
|
|
<!--ID: 1723510994958-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
In the second line, is `ptr` an lvalue or rvalue?
|
|
```c
|
|
int *ptr = &x;
|
|
*ptr = 10;
|
|
```
|
|
Back: An lvalue.
|
|
Reference: “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
|
|
<!--ID: 1723510994964-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
In the second line, is `*ptr` an lvalue or rvalue?
|
|
```c
|
|
int *ptr = &x;
|
|
*ptr = 10;
|
|
```
|
|
Back: An lvalue.
|
|
Reference: “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
|
|
<!--ID: 1723510994970-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
Is `getValue` an lvalue or rvalue in the following snippet?
|
|
```c
|
|
int getValue () {
|
|
return 42;
|
|
}
|
|
```
|
|
Back: Neither.
|
|
Reference: “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
|
|
<!--ID: 1723510994975-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
Is `getValue()` an lvalue or rvalue in the following snippet?
|
|
```c
|
|
int d = getValue();
|
|
```
|
|
Back: An rvalue.
|
|
Reference: “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
|
|
<!--ID: 1723510994980-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
*Why* is `getValue` in the following snippet neither an lvalue nor an rvalue?
|
|
```c
|
|
int getValue() {
|
|
return 42;
|
|
}
|
|
```
|
|
Back: The function name is just syntax. That is, it isn't an expression.
|
|
Reference: “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
|
|
<!--ID: 1723510994984-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
Are variables typically lvalues or rvalues?
|
|
Back: lvalues.
|
|
Reference: “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
|
|
<!--ID: 1723510994988-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
Are constants typically lvalues or rvalues?
|
|
Back: rvalues.
|
|
Reference: “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
|
|
<!--ID: 1723510994992-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
Are array accesses typically lvalues or rvalues?
|
|
Back: lvalues.
|
|
Reference: “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
|
|
<!--ID: 1723510994997-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
Are dereferenced pointers typically lvalues or rvalues?
|
|
Back: lvalues.
|
|
Reference: “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
|
|
<!--ID: 1723510995001-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
Are temporary values typically lvalues or rvalues?
|
|
Back: rvalues.
|
|
Reference: “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
|
|
<!--ID: 1723510995006-->
|
|
END%%
|
|
|
|
## Types
|
|
|
|
Types are additional properties that C associates with values. All values have a type that is statically determined and all possible operations on a value are determined by its type.
|
|
|
|
%%ANKI
|
|
Cloze
|
|
Possible operations on a {value} are determined by its {type}.
|
|
Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
|
|
<!--ID: 1723856661364-->
|
|
END%%
|
|
|
|
## Representations
|
|
|
|
The **binary representation** of a type is the model used to represent values of said type on a given platform. The **object representation** of a type determines how values are stored in memory, disk, etc.
|
|
|
|
%%ANKI
|
|
Basic
|
|
What is the binary representation of a type?
|
|
Back: The model used to represent values of the type on a given platform.
|
|
Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
|
|
<!--ID: 1723856661371-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
What is the object representation of a type?
|
|
Back: How a value of a given type is actually stored in memory, disk, etc.
|
|
Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
|
|
<!--ID: 1723856661386-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Cloze
|
|
A {type}'s {binary representation} determines the results of all operations.
|
|
Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
|
|
<!--ID: 1723856661393-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
Why does Gustedt refer to binary representations as abstract entities?
|
|
Back: Binary representations don't completely determine how values are stored in memory.
|
|
Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
|
|
<!--ID: 1723856661399-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Cloze
|
|
A {binary} representation is abstract whereas an {object} representation is concrete.
|
|
Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
|
|
<!--ID: 1723856661405-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
Why might the same value have different binary representations?
|
|
Back: Because the binary representation corresponds to the type of the value.
|
|
Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
|
|
<!--ID: 1727432711873-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
The `sizeof` operator returns values in units of what?
|
|
Back: Bytes.
|
|
Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
|
|
<!--ID: 1732622657260-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
Consider object `A`. What is the "type" of its object representation?
|
|
Back: `unsigned char[sizeof A]`
|
|
Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
|
|
<!--ID: 1732622657263-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
Every object can be viewed as an array of what type?
|
|
Back: `unsigned char`
|
|
Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
|
|
<!--ID: 1732622657266-->
|
|
END%%
|
|
|
|
%%ANKI
|
|
Basic
|
|
Which type is considered the "atom" of all object types?
|
|
Back: `unsigned char`
|
|
Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
|
|
<!--ID: 1732622657269-->
|
|
END%%
|
|
|
|
## Bibliography
|
|
|
|
* “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
|
|
* Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020). |