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: The ability to optimize. 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 - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.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 - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.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 - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.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 - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic Why are lvalues named the way they are? Back: The name is an acronym for locator value (or left value). Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

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.

END%%

%%ANKI Basic Why are rvalues named the way they are? Back: The name is an acronym for right value. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

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.

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.

END%%

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

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.

END%%

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

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.

END%%

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

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.

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 - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

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

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.

END%%

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

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.

END%%

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

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.

END%%

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

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.

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 - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.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 - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

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

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.

END%%

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

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.

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 - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

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.

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.

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.

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.

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.

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).

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).

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%%

%%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).

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).

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).

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).

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).

END%%

Compilation

Most compilation systems provide a compiler driver that invokes a preprocessor, compiler, assembler, and linkers/static:

1. The C preprocessor (e.g. cpp) translates C source files (.h and .c) into ASCII intermediate files (.i).
2. The C compiler (e.g. cc1) translates intermediate files into ASCII assembly-language files (.s).
3. The assembler translates assembly-language files into relocatable object files (.o).
4. The linker bundles all relocatable object files and produces an executable object file.

%%ANKI Basic The compiler driver usually consists of what four programs? Back: The preprocessor, compiler, assembler, and linker. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic The term "C compiler" usually refers to what more accurate term? Back: A compiler driver. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic On Unix systems, what kind of file typically has a .h extension? Back: C header files. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic On Unix systems, what kind of file typically has a .c extension? Back: C translation units. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic On Unix systems, what kind of file typically has a .i extension? Back: ASCII intermediate files. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic On Unix systems, what kind of file typically has a .s extension? Back: ASCII assembly-language files. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic On Unix systems, what kind of file typically has a .o extension? Back: Relocatable object files. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic On Unix systems, a C preprocessor usually inputs files with what extension(s)? Back: .h and .c. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic On Unix systems, a C preprocessor usually outputs files with what extension(s)? Back: .i Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic On Unix systems, a C compiler (not compiler driver) usually inputs files with what extension(s)? Back: .i Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic On Unix systems, a C compiler (not compiler driver) usually outputs files with what extension(s)? Back: .s Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic On Unix systems, an assembler usually inputs files with what extension(s)? Back: .s Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic On Unix systems, an assembler usually outputs files with what extension(s)? Back: .o Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic On Unix systems, a linker usually inputs files with what extension(s)? Back: .o Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What ambiguity exists when discussing the C compiler? Back: We could refer to the compiler (e.g. cc1) or the compiler driver. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic On Unix systems, what kind of file does a linker usually input? Back: Relocatable object files. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic On Unix systems, what kind of file does a linker usually output? Back: Executable object files. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic On Unix systems, what kind of file does an assembler usually input? Back: ASCII assembly-language files. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic On Unix systems, what kind of file does an assembler usually output? Back: Relocatable object files. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic On Unix systems, what kind of file does a C preprocessor usually input? Back: C source files. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic On Unix systems, what kind of file does a C preprocessor usually output? Back: ASCII intermediate files. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic On Unix systems, what kind of file does a C compiler (not compiler driver) usually input? Back: ASCII intermediate files. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic On Unix systems, what kind of file does a C compiler (not compiler driver) usually output? Back: ASCII assembly-language files. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic Which component of a compiler driver produces ASCII intermediate files? Back: The preprocessor. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic Which component of a compiler driver produces an executable object file? Back: The linker. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic Which component of a compiler driver produces ASCII assembly-language files? Back: The compiler. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic Which component of a compiler driver produces ASCII relocatable object files? Back: The assembler. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

The operating system provides a function called the loader that copies the code and data of the executable object file into memory, and then transfers control to the beginning of the program.

%%ANKI Basic What function does the OS provider to run an executable object file? Back: The loader. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What kind of file does a loader usually input? Back: An executable object file. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Cloze The loader copies {code and data} from an {executable object file} into {memory}. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic The loader copies code and data from what kind of file? Back: An executable object file. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic The loader copies code and data into what? Back: Memory. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Cloze The loader transfers {control} to the {beginning of a loaded program}. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic At a high-level, the loader is responsible for performing what two actions? Back: Copying code/data and beginning program execution. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

Bibliography

• Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.
• “ISO: Programming Languages - C17,” April 2017, 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).