notebook/notes/c17/declarations.md

title	TARGET DECK	FILE TAGS	tags
Declarations	Obsidian::STEM	c17	c17

Overview

C declarations were designed so that the declaration of an object looks like the use of the object. This isn't quite true - keywords like volatile and const only exist in declarations - but for the most part, this philosophy can be leveraged to read C declarations.

Declarators

A declarator in C is roughly an identifier along with pointers, function brackets, or array indications. Pointers will look like one of:

• *
• * const
• * volatile
• * const volatile
• * volatile const

whereas direct declarators will look like one of:

• identifier
• identifier[size]
• identifier(args)
• (declarator)

%%ANKI Basic What two qualifiers can be used in a pointer declaration? Back: const and volatile. Reference: Van der Linden, Peter. Expert C Programming: Deep C Secrets. Programming Languages / C. Mountain View, Cal.: SunSoft Pr, 1994.

END%%

Declarations

A declaration consists of at least one type-specifier (e.g. signed short), storage class (e.g. static), and/or type qualifier (e.g. const) as well as one or more declarators.

%%ANKI Basic How many declarators does a declaration have? Back: At least one. Reference: Van der Linden, Peter. Expert C Programming: Deep C Secrets. Programming Languages / C. Mountain View, Cal.: SunSoft Pr, 1994.

END%%

%%ANKI Basic How many declarations does a declarator have? Back: N/A. Reference: Van der Linden, Peter. Expert C Programming: Deep C Secrets. Programming Languages / C. Mountain View, Cal.: SunSoft Pr, 1994.

END%%

%%ANKI Basic Which part of the following are declarators?

int* a, b;

Back: * a and b. Reference: Van der Linden, Peter. Expert C Programming: Deep C Secrets. Programming Languages / C. Mountain View, Cal.: SunSoft Pr, 1994.

END%%

%%ANKI Basic Which part of the following are declarations?

int* a, b;

Back: The entire line is a single declaration. Reference: Van der Linden, Peter. Expert C Programming: Deep C Secrets. Programming Languages / C. Mountain View, Cal.: SunSoft Pr, 1994.

END%%

%%ANKI Basic Which part of the following declaration is the declarator?

const int *const x;

Back: *const x Reference: Van der Linden, Peter. Expert C Programming: Deep C Secrets. Programming Languages / C. Mountain View, Cal.: SunSoft Pr, 1994.

END%%

%%ANKI Basic Which part of the following declaration is the type specifier?

const int *const x;

Back: int Reference: Van der Linden, Peter. Expert C Programming: Deep C Secrets. Programming Languages / C. Mountain View, Cal.: SunSoft Pr, 1994.

END%%

%%ANKI Basic Which part of the following declaration is the type qualifier?

const int *const x;

Back: The first const. Reference: Van der Linden, Peter. Expert C Programming: Deep C Secrets. Programming Languages / C. Mountain View, Cal.: SunSoft Pr, 1994.

END%%

%%ANKI Basic What is the type of x in the following?

const int *const x, y;

Back: const int* const Reference: Van der Linden, Peter. Expert C Programming: Deep C Secrets. Programming Languages / C. Mountain View, Cal.: SunSoft Pr, 1994.

END%%

%%ANKI Basic What is the type of y in the following?

const int *const x, y;

Back: const int Reference: Van der Linden, Peter. Expert C Programming: Deep C Secrets. Programming Languages / C. Mountain View, Cal.: SunSoft Pr, 1994.

END%%

Declarations can be read by complying with the precedence rules outlined below:

1. Find the name of the declaration.
2. Obey the following precedence rules:
   1. Parentheses grouping together parts of a declaration
   2. Postfix operators () and []
   3. Prefix operator: the asterisk * denoting "pointer to"
3. If const and/or volatile keyword is next to a type specifier, it applies to the type specifier. Otherwise it applies to the pointer asterisk on its immediate left.

%%ANKI Basic In the precedence rules for C declarations, what available postfix operators are there? Back: () and []. Reference: Van der Linden, Peter. Expert C Programming: Deep C Secrets. Programming Languages / C. Mountain View, Cal.: SunSoft Pr, 1994.

END%%

%%ANKI Basic In the precedence rules for C declarations, what available prefix operators are there? Back: Just *. Reference: Van der Linden, Peter. Expert C Programming: Deep C Secrets. Programming Languages / C. Mountain View, Cal.: SunSoft Pr, 1994.

END%%

%%ANKI Basic In the precedence rules for C declarations, what available type qualifiers are there? Back: const and volatile. Reference: Van der Linden, Peter. Expert C Programming: Deep C Secrets. Programming Languages / C. Mountain View, Cal.: SunSoft Pr, 1994.

END%%

%%ANKI Basic What is the type of foo in the the following declaration?

char *const *(*foo)();

Back: A pointer to a function returning a pointer to a const pointer-to-char. Reference: Van der Linden, Peter. Expert C Programming: Deep C Secrets. Programming Languages / C. Mountain View, Cal.: SunSoft Pr, 1994.

END%%

%%ANKI Basic What is the type of foo in the the following declaration?

char *const *foo();

Back: A function returning a pointer to a const pointer-to-char. Reference: Van der Linden, Peter. Expert C Programming: Deep C Secrets. Programming Languages / C. Mountain View, Cal.: SunSoft Pr, 1994.

END%%

%%ANKI Basic What is the type of foo in the the following declaration?

int (*(*foo)(void))[3]

Back: A pointer to a function (accepting void) returning a pointer to an array (size 3) of ints. Reference: Van der Linden, Peter. Expert C Programming: Deep C Secrets. Programming Languages / C. Mountain View, Cal.: SunSoft Pr, 1994.

END%%

%%ANKI Basic What is the type of foo in the the following declaration?

const int (* volatile foo)[64]

Back: A volatile pointer to an array (size 64) of const ints. Reference: Van der Linden, Peter. Expert C Programming: Deep C Secrets. Programming Languages / C. Mountain View, Cal.: SunSoft Pr, 1994.

END%%

%%ANKI Basic What is the type of foo in the the following declaration?

const int * const foo;

Back: A const pointer to a const int. Reference: Van der Linden, Peter. Expert C Programming: Deep C Secrets. Programming Languages / C. Mountain View, Cal.: SunSoft Pr, 1994.

END%%

%%ANKI Basic What is the type of foo in the the following declaration?

const int * foo;

Back: A pointer to a const int. Reference: Van der Linden, Peter. Expert C Programming: Deep C Secrets. Programming Languages / C. Mountain View, Cal.: SunSoft Pr, 1994.

END%%

%%ANKI Basic What is the type of foo in the the following declaration?

int const * foo;

Back: A pointer to a const int. Reference: Van der Linden, Peter. Expert C Programming: Deep C Secrets. Programming Languages / C. Mountain View, Cal.: SunSoft Pr, 1994.

END%%

%%ANKI Basic What is the type of foo in the the following declaration?

int * const foo;

Back: A const pointer-to-int. Reference: Van der Linden, Peter. Expert C Programming: Deep C Secrets. Programming Languages / C. Mountain View, Cal.: SunSoft Pr, 1994.

END%%

%%ANKI Basic What is the type of foo in the the following declaration?

char *(*foo[10])(int **);

Back: An array (size 10) of pointers to functions (accepting int **) returning pointer-to-char. Reference: Van der Linden, Peter. Expert C Programming: Deep C Secrets. Programming Languages / C. Mountain View, Cal.: SunSoft Pr, 1994.

END%%

%%ANKI Basic What is the type of signal in the the following declaration?

void (*signal(int sig, void (*func)(int)))(int);

Back: A function (accepting an int and void (*)(int)) returning a pointer to a function (accepting an int) returning void. Reference: Van der Linden, Peter. Expert C Programming: Deep C Secrets. Programming Languages / C. Mountain View, Cal.: SunSoft Pr, 1994.

END%%

Prototypes

A function declaration/definition has two ways of using declarators: parameter type lists and identifier type lists. To make the distinction clear, consider the following two ways of defining an add function:

int f(int x, int y) { return x + y; }  // Paramter type list
int f(x, y) int x; int y; { return x + y }  // Identifier type list

A function prototype is a kind of function declaration that specifies the function signature. There are three important points to make note of:

• Empty identifier lists are interpreted as "the compiler has not been told what this function's arguments are."
• The standard prohibits declaring functions with a non-empty identifier list.
• Empty parameter lists are not allowed.

Therefore:

// Uses an empty identifer list. This declares a function `foo`
// that takes an unknown specification of arguments.
void foo();
// Uses a non-empty identifier list. Compiler error.
void foo(x, y);
// Uses a non-empty identifier list. Compiler error.
void foo(x, y) int x; int y;
// Uses a non-empty identifier list. Definitions allow this.
void foo(x, y) int x; int y; { }
// Uses a non-empty parameter list. This prototypes a function
// `foo` that takes no arguments.
void foo(void);
// Uses a non-empty parameter list. This prototypes and defines
// a function `foo` that takes no arguments.
void foo(void) {}

Together these points imply a function prototype must use a parameter type list.

%%ANKI Basic Which of prototypes or declarations are more general? Back: Declarations. Reference: “ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

%%ANKI Basic What two ways are parameters declared in function declarations and definitions? Back: Identifier type lists and parameter type lists. Reference: “ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

%%ANKI Basic Which of identifier type lists and/or parameter type lists considered obsolete? Back: Identifier type lists. Reference: “ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

%%ANKI Basic Define an addition function using identifier type lists. Back:

int add(x, y) int x; int y; { return x + y; }

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

END%%

%%ANKI Basic Define an addition function using parameter type lists. Back:

int add(int x, int y) { return x + y; }

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

END%%

%%ANKI Basic Is the following a prototype or a declaration?

void foo();

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

END%%

%%ANKI Basic What compilation error does the following raise?

void foo();

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

END%%

%%ANKI Basic What compilation error does the following raise?

void foo(x, y);

Back: A function declaration cannot have a non-empty identifier list. Reference: “ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

%%ANKI Basic What compilation error does the following raise?

void foo(x, y) int x; int y;

Back: A function declaration cannot have a non-empty identifier list. Reference: “ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

%%ANKI Basic What compilation error does the following raise?

void foo(x, y) int x; int y; {}

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

END%%

%%ANKI Basic What function prototype is declared in the following?

void foo(x, y) int x; int y; {}

Back: N/A. No prototype has been declared. Reference: “ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

%%ANKI Basic What compilation error does the following raise?

void foo();

int main(void) { foo(1); }

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

END%%

%%ANKI Basic Is the following a prototype or a declaration?

void f(void);

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

END%%

%%ANKI Basic What compilation error does the following raise?

void foo(void);

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

END%%

%%ANKI Basic What compilation error does the following raise?

void foo(void);

int main(void) { foo(1); }

Back: Too many arguments to function call foo. Reference: “ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

%%ANKI Basic What function prototype is declared in the following?

void foo(int x, int y);

Back: void foo(int, int) Reference: “ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

%%ANKI Basic What function prototype is declared in the following?

void foo(int x, int y) {}

Back: void foo(int, int) Reference: “ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

%%ANKI Basic How many arguments does the following declaration specify?

void foo();

Back: Some number unknown to the compiler. Reference: “ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

%%ANKI Basic How many arguments does the following declaration specify?

void foo(void);

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

END%%

%%ANKI Basic Why might the following snippet raise a compilation error?

int foo();
int foo(int a);

Back: N/A. It likely wouldn't. Reference: “ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

%%ANKI Basic Why might the following snippet raise a compilation error?

int foo();
int foo(float a);

Back: Conflicting types. The first foo declares any float passed to it default promotes to a double. Reference: “ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

%%ANKI Basic Why might the following snippet raise a compilation error?

int foo();
int foo(char a);

Back: Conflicting types. The first foo declares any char passed to it default promotes to an int. Reference: “ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

%%ANKI Basic Why might the following snippet raise a compilation error?

int foo();
int foo(double a);

Back: N/A. It likely wouldn't. Reference: “ISO: Programming Languages - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.

END%%

main

main is a special function serving as the entrypoint to C programs. It can have several different prototypes, but the following two are always possible:

int main(void);
int main(int argc, char* argv[argc+1]);

The only two return values guaranteed to work on all platform is EXIT_SUCCESS and EXIT_FAILURE. Reaching the end of main is equivalent to a reutrn with value EXIT_SUCCESS.

%%ANKI Basic Which function serves as the entrypoint of C programs? Back: main Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic How many valid prototypes of main are available? Back: Indeterminate. Depends on the system. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic How many "official" prototypes of main are available? Back: Two. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic What are the "official" prototypes of main? Back:

int main(void);
int main(int argc, char* argv[argc + 1]);

Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic What are the "official" prototypes of main? Back:

int main(void);
int main(int argc, char* argv[argc + 1]);

Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020). END%%

%%ANKI Basic What are the only portable values that main can return? Back: EXIT_SUCCESS and EXIT_FAILURE. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic Which library defines EXIT_SUCCESS? Back: stdlib.h Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic Which library defines EXIT_FAILURE? Back: stdlib.h Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic What happens when main does not explicitly return a value? Back: EXIT_SUCCESS is implicitly returned. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Cloze Returning {1:s} in {1:main} is equivalent to invoking function {2:exit} with argument {2:s}. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic Which library declares the following prototype?

noreturn void exit(int)

Back: stdlib.h Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic What does keyword _Noreturn indicate? Back: The associated callee will never return control back to the caller. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic What kind of syntactical construct is _Noreturn? Back: A special keyword. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic What kind of syntactical construct is noreturn? Back: A macro. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Cloze {1:_Noreturn} is a {2:keyword} whereas {2:noreturn} is a {1:macro}. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic Which library is noreturn defined in? Back: stdnoreturn.h Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic Consider the following prototype. What is the value of argv[0]?

int main(int argc, char* argv[argc + 1]);

Back: The name of the program invocation. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic Consider the following prototype. What is the value of argv[argc]?

int main(int argc, char* argv[argc + 1]);

Back: 0 Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic Consider the following prototype. What is the value of argv[1]?

int main(int argc, char* argv[argc + 1]);

Back: 0 if argc == 1 else the first argument to the program. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic Consider the following prototype. What is the minimum value of argc?

int main(int argc, char* argv[argc + 1]);

Back: 1 Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic Consider the following prototype. What is the minimum length of argv?

int main(int argc, char* argv[argc + 1]);

Back: 2 Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

Initializers

An initializer is an expression that gives an object a value at time of declaration. Only variable-length arrays (VLAs) do not allow for an initializer. The default initializer looks like {0}.

%%ANKI Basic Which part of the following are initializers?

int a = 3;

Back: 3 Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic Which part of the following are initializers?

double a = { 7.0 };

Back: { 7.0 } Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic Which part of the following are initializers?

double a = { 7.0 };

Back: { 7.0 } Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020). END%%

%%ANKI Basic How is the following updated to include initializers 1 and 2 for a and b respectively?

int a, b;

Back:

int a = 1, b = 2;

Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic What object types allow initializers? Back: All but VLAs. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic What object types do not allow initializers? Back: Just variable-length arrays. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic What is the default initializer? Back: {0} Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic {0} is a valid initializer for what object types? Back: All but VLAs. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic {0} is not a valid initializer for what object types? Back: Just variable-length arrays. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

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 - C,” April 12, 2011, https://port70.net/~nsz/c/c11/n1570.pdf.
• Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).
• Van der Linden, Peter. Expert C Programming: Deep C Secrets. Programming Languages / C. Mountain View, Cal.: SunSoft Pr, 1994.