--- title: Functions TARGET DECK: Obsidian::STEM FILE TAGS: c17::function tags: - 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? ```c 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? ```c (&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? ```c (*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? ```c (*&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? ```c (&*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: ```c 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|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: ```c // 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](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](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](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: ```c 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](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: ```c 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](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? ```c 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](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? ```c 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](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? ```c 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](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? ```c 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](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? ```c 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](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? ```c 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](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? ```c 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](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? ```c 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](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? ```c 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](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? ```c 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](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? ```c 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](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? ```c 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](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? ```c 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](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? ```c 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](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? ```c 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](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? ```c 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](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? ```c 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](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? ```c 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](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: ```c 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: ```c 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: ```c 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: `` Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020). END%% %%ANKI Basic Which library defines `EXIT_FAILURE`? Back: `` 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? ```c noreturn void exit(int) ``` Back: `` 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: `` 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]`? ```c 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]`? ```c 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]`? ```c 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`? ```c 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`? ```c int main(int argc, char* argv[argc + 1]); ``` Back: `2` Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020). 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).