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title TARGET DECK FILE TAGS tags
Functions Obsidian::STEM c17::function
c17

Overview

A function f without a following opening ( is converted to a pointer to its start. This is called function decay.

%%ANKI Basic What is the effect of function decay? Back: Evaluation of a function f without a following opening ( is converted to a pointer to its start. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic What name is given to the implicit conversion of a function to a pointer? Back: Function decay. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic According to Gustedt, what C feature explains why are there no "function values"? Back: Function-to-pointer decay. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic Why can't functions directly be made arguments to functions? Back: Because function arguments decay to pointers. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Cloze {1:Function pointers} are to {2:(...)} whereas {2:pointers} are to {1:[...]}. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic In what order are decays, dereferences, address ofs, and calls performed in the following?

f(3);

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

END%%

%%ANKI Basic In what order are decays, dereferences, address ofs, and calls performed in the following?

(&f)(3);

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

END%%

%%ANKI Basic In what order are decays, dereferences, address ofs, and calls performed in the following?

(*f)(3);

Back: Decay, dereference, decay, call. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic In what order are decays, dereferences, address ofs, and calls performed in the following?

(*&f)(3);

Back: Address of, dereference, decay, call. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic In what order are decays, dereferences, address ofs, and calls performed in the following?

(&*f)(3);

Back: Decay, dereference, address of, call. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Cloze {1:Pointers} refer to {2:arrays} whereas {2:function pointers} refer to {1:functions}. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

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 c17/declarations 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 - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.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 - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic Which of identifier type lists and/or parameter type lists considered obsolete? Back: Identifier type lists. 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 Define an addition function using identifier type lists. Back:

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

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 Define an addition function using parameter type lists. Back:

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

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 the following a prototype or a declaration?

void foo();

Back: A declaration. 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 compilation error does the following raise?

void foo();

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

END%%

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

void f(void);

Back: A prototype. 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 compilation error does the following raise?

void foo(void);

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

END%%

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

void foo(void);

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

Variable-Length Arrays

Within a function prototype, a parameter can denote a derived#Variable-Length using [*] syntax. For example, the following prototypes are all (more or less) equivalent:

int sum2d(int  , int  , int a[*][*]);
int sum2d(int n, int  , int a[n][*]);
int sum2d(int  , int m, int a[*][m]);
int sum2d(int n, int m, int a[n][m]);
int sum2d(int  , int  , int a[][*]);
int sum2d(int  , int  , int (*a)[*]);
int sum2d(int  , int m, int (*a)[m]);

%%ANKI Basic In what context are VLAs declared with [*] actually useful? Back: Function prototypes. Reference: https://stackoverflow.com/a/17371914

END%%

%%ANKI Basic Maintaining array syntax, rewrite the following without parameter names.

int sum2d(int n, int m, int a[n][m]);

Back:

int sum2d(int, int, int a[*][*]);

Reference: https://stackoverflow.com/a/17371914

END%%

%%ANKI Basic Why is the first * considered redundant in the following?

int sum2d(int, int, int a[*][*]);

Back: Array decay. Reference: https://stackoverflow.com/a/17371914

END%%

%%ANKI Basic How can we rewrite the following with as few *s as possible?

int sum2d(int, int, int a[*][*]);

Back:

int sum2d(int, int, int a[][*]);

Reference: https://stackoverflow.com/a/17371914

END%%

Bibliography