notebook/notes/c17/index.md

title	TARGET DECK	FILE TAGS	tags
C17	Obsidian::STEM	c17	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: It enables optimization. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

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).

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).

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).

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).

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 - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

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 - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

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).

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).

END%%

%%ANKI Cloze A {value} refers to the contents of an {object} when interpreted as having a specific type. Reference: “ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

%%ANKI Basic What is an lvalue? Back: An expression (with non-void object type) that potentially designates an object. Reference: “ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

%%ANKI Basic Why are lvalues named the way they are? Back: The name is an acronym for locator value. Reference: “ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

%%ANKI Basic What is an rvalue? Back: The value of an expression. Reference: “ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

%%ANKI Basic Why are rvalues named the way they are? Back: The name is an acronym for right value. Reference: “ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

%%ANKI Basic Can an lvalue designate an object? Back: Yes. Reference: “ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

%%ANKI Basic Can an lvalue designate a function? Back: No. Reference: “ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

%%ANKI Basic Is x an lvalue or rvalue in the following snippet?

int x = 10;

Back: An lvalue. Reference: “ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

%%ANKI Basic Is int an lvalue or rvalue in the following snippet?

int x = 10;

Back: Neither. Reference: “ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

%%ANKI Basic Is 10 an lvalue or rvalue in the following snippet?

int x = 10;

Back: An rvalue. Reference: “ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

%%ANKI Basic Why is x an lvalue in the following?

int x = 10;

Back: Because x refers to a memory location. Reference: “ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

%%ANKI Basic Is x an lvalue or rvalue in the following snippet?

void x;

Back: Neither. Reference: “ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

%%ANKI Basic Is x an lvalue or rvalue in the following snippet?

void *x;

Back: An lvalue. Reference: “ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

%%ANKI Basic Is y an lvalue or rvalue in the following snippet?

int y = x + 10;

Back: An lvalue. Reference: “ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

%%ANKI Basic Is x an lvalue or rvalue in the following snippet?

int y = x + 10;

Back: An rvalue. Reference: “ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

%%ANKI Basic In the second line, is ptr an lvalue or rvalue?

int *ptr = &x;
*ptr = 10;

Back: An lvalue. Reference: “ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

%%ANKI Basic In the second line, is *ptr an lvalue or rvalue?

int *ptr = &x;
*ptr = 10;

Back: An lvalue. Reference: “ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

%%ANKI Basic Is getValue an lvalue or rvalue in the following snippet?

int getValue () {
  return 42;
}

Back: Neither. Reference: “ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

%%ANKI Basic Is getValue() an lvalue or rvalue in the following snippet?

int d = getValue();

Back: An rvalue. Reference: “ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

%%ANKI Basic Why is getValue in the following snippet neither an lvalue nor an rvalue?

int getValue() {
  return 42;
}

Back: The function name is just syntax. That is, it isn't an expression. Reference: “ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

%%ANKI Basic Are variables typically lvalues or rvalues? Back: lvalues. Reference: ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

%%ANKI Basic Are constants typically lvalues or rvalues? Back: rvalues. Reference: ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

%%ANKI Basic Are array elements typically lvalues or rvalues? Back: lvalues. Reference: ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

%%ANKI Basic Are dereferenced pointers typically lvalues or rvalues? Back: lvalues. Reference: ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

%%ANKI Basic Are temporary values typically lvalues or rvalues? Back: rvalues. Reference: ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

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.

Types are categorized as function types and object types. An object type is complete if there is sufficient information to determine the size of objects of that type. Otherwise we say it is incomplete.

%%ANKI Cloze Possible operations on a {value} are determined by its {type}. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic Types are partitioned into what two categories? Back: Object types and function types. Reference: “ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

%%ANKI Basic What is an object type? Back: A type that describes objects. Reference: “ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

%%ANKI Basic What is a function type? Back: A type that describes functions. Reference: “ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

%%ANKI Basic What two parts characterize a function type? Back: The return type and the number/types of its parameters. Reference: “ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

%%ANKI Basic What does it mean for an object type to be complete? Back: There is sufficient information to determine the size of objects of that type. Reference: “ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

%%ANKI Basic What does it mean for an object type to be incomplete? Back: There is insufficient information to determine the size of objects of that type. Reference: “ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

%%ANKI Basic Does x have complete or incomplete object type in the following?

void x;

Back: Incomplete. Reference: ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

%%ANKI Basic Does x have complete or incomplete object type in the following?

int x;

Back: Complete. Reference: ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

%%ANKI Basic Does x have complete or incomplete object type in the following?

void *x;

Back: Complete. Reference: ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

%%ANKI Basic What object type can an lvalue not have? Back: void Reference: “ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

%%ANKI Basic What object type can an lvalue have? Back: Any object type other than void. Reference: “ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

Representation

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).

END%%

%%ANKI Basic What is the binary representation of a value? Back: N/A. Binary representations describe types not values. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

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).

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).

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).

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).

END%%

Bibliography

• “ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.
• Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).