--- title: Declarations TARGET DECK: Obsidian::STEM FILE TAGS: c17 tags: - 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? ```c 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? ```c 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? ```c 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? ```c 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? ```c 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? ```c 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? ```c 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%% %%ANKI Basic What is the type of `x` in the following? ```c #define int_ptr int * int_ptr x, y; ``` 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 What is the type of `y` in the following? ```c #define int_ptr int * int_ptr x, y; ``` 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 What is the type of `x` in the following? ```c typedef int_ptr int * int_ptr x, y; ``` 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 What is the type of `y` in the following? ```c typedef int_ptr int * int_ptr x, y; ``` Back: `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? ```c 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? ```c 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? ```c int (*(*foo)(void))[3] ``` Back: A pointer to a function (accepting `void`) returning a pointer to an array of `int`s. 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? ```c const int (* volatile foo)[64] ``` Back: A `volatile` pointer to an array of `const int`s. 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? ```c 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? ```c 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? ```c 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? ```c 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? ```c char *(*foo[10])(int **); ``` Back: An array 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? ```c 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%% ### Type Specifiers Signed | Unsigned | 32-bit | 64-bit ----------- | ------------------- | ------ | ------ signed char | unsigned char | 1 | 1 short | unsigned short | 2 | 2 int | unsigned | 4 | 4 long | unsigned long | 4 | 8 long long | unsigned long long | 8 | 8 char * | - | 4 | 8 float | - | 4 | 4 double | - | 8 | 8 Fixed width data integral types (e.g. `int32_t`) can be found by including ``. %%ANKI Cloze The {``} header file contains {fixed width data integral types}. 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 header file contains `INT32_MAX`? Back: `` 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 does the "width" of an integer type refer to? Back: The number of bits used to represent its value. Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI What two variants does a C integral type declaration have? Back: Signed and unsigned. Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI What does it mean for an integer to be "signed"? Back: It can represent negative, zero, and positive values. Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% %%ANKI What does it mean for an integer to be "unsigned"? Back: It can only represent nonnegative values. 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 How large is a word? Back: This is a machine-dependent value. 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 word sizes are typical nowadays? Back: 32- and 64-bit word sizes. 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 `char` *typically* represents {1} byte(s) on a 64-bit platform. 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 `short` *typically* represents {2} byte(s) on a 64-bit platform. 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 `int` *typically* represents {4} bytes(s) on a 64-bit platform. 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 `unsigned` *typically* represents {4} bytes(s) on a 64-bit platform. 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 `long` *typically* represents {8} bytes(s) on a 64-bit platform. 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 `long long` *typically* represents {8} bytes(s) on a 64-bit platform. 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 distinguishes `long` from `long long`? Back: `long long`s are guaranteed to be at least 64-bit wide. 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 *Why* is there both a `long` and `long long`? Back: `long long`s are at least 64-bit wide, even on 32-bit platforms. 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 `char *` *typically* represents {8} bytes(s) on a 64-bit platform. 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 `float` *typically* represents {4} bytes(s) on a 64-bit platform. 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 `double` *typically* represents {8} bytes(s) on a 64-bit platform. 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 Is declaration `int` signed or unsigned? Back: Signed. 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 How is declaration `unsigned` written more precisely? Back: `unsigned int`. 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 Is declaration `long` signed or unsigned? Back: Signed. 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 {1:`float`} has {2:4} byte precision whereas {2:`double`} has {1:8} byte precision. 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 C standard defines {lower} bounds on numeric ranges of data types. 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 C standard does *not* define {1:upper} bounds on numeric ranges of data types (except for {1:fixed-size} types). Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016. END%% ### Structs A `struct` is a grouping of data together. It has the following general form: ```c struct optional_tag { type_1 ident1; ... type_N identN; } optional_var1 ... optional_varM; ``` %%ANKI Basic What is the underlined portion of the following declaration called? ```c struct ___ { ... }; ``` Back: The tag. Reference: Van der Linden, Peter. _Expert C Programming: Deep C Secrets_. Programming Languages / C. Mountain View, Cal.: SunSoft Pr, 1994. END%% %%ANKI Basic What distinguishes the `X`'s from one another in the following? ```c struct X { ... } X; ``` Back: The first `X` is a tag whereas the second is a variable with type `struct X`. Reference: Van der Linden, Peter. _Expert C Programming: Deep C Secrets_. Programming Languages / C. Mountain View, Cal.: SunSoft Pr, 1994. END%% %%ANKI Basic What distinguishes the `X`'s from one another in the following? ```c typedef struct X { ... } X; ``` Back: The first `X` is a tag whereas the second is an alias for type `struct X`. Reference: Van der Linden, Peter. _Expert C Programming: Deep C Secrets_. Programming Languages / C. Mountain View, Cal.: SunSoft Pr, 1994. END%% ### Unions A `union` is a grouping of data together but with overlaid storage. It has the following general form: ```c union optional_tag { type1 ident1; ... typeN identN; } optional_var1 ... optional_varN; ``` %%ANKI Basic What is the underlined portion of the following declaration called? ```c union ___ { ... }; ``` Back: The tag. Reference: Van der Linden, Peter. _Expert C Programming: Deep C Secrets_. Programming Languages / C. Mountain View, Cal.: SunSoft Pr, 1994. END%% %%ANKI Basic What distinguishes the `X`'s from one another in the following? ```c union X { ... } X; ``` Back: The first `X` is a tag whereas the second is a variable with type `union X`. Reference: Van der Linden, Peter. _Expert C Programming: Deep C Secrets_. Programming Languages / C. Mountain View, Cal.: SunSoft Pr, 1994. END%% %%ANKI Basic What distinguishes the `X`'s from one another in the following? ```c typedef union X { ... } X; ``` Back: The first `X` is a tag whereas the second is an alias for type `union X`. Reference: Van der Linden, Peter. _Expert C Programming: Deep C Secrets_. Programming Languages / C. Mountain View, Cal.: SunSoft Pr, 1994. END%% ### Enums An `enum` is a mapping of identifiers with integer values. They have general form: ```c enum optional_tag { type1 ident1; ... typeN identN; } optional_var1 ... optional_varN; ``` %%ANKI Basic What is the underlined portion of the following declaration called? ```c enum ___ { ... }; ``` Back: The tag. Reference: Van der Linden, Peter. _Expert C Programming: Deep C Secrets_. Programming Languages / C. Mountain View, Cal.: SunSoft Pr, 1994. END%% %%ANKI Basic What distinguishes the `X`'s from one another in the following? ```c enum X { ... } X; ``` Back: The first `X` is a tag whereas the second is a variable with type `enum X`. Reference: Van der Linden, Peter. _Expert C Programming: Deep C Secrets_. Programming Languages / C. Mountain View, Cal.: SunSoft Pr, 1994. END%% %%ANKI Basic What distinguishes the `X`'s from one another in the following? ```c typedef enum X { ... } X; ``` Back: The first `X` is a tag whereas the second is an alias for type `enum X`. 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 value of `A` in the following? ```c enum X { A, B, C }; ``` Back: `0` 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 value of `B` in the following? ```c enum X { A, B, C }; ``` Back: `1` 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 value of `B` in the following? ```c enum X { A=2, B, C }; ``` Back: `3` Reference: Van der Linden, Peter. _Expert C Programming: Deep C Secrets_. Programming Languages / C. Mountain View, Cal.: SunSoft Pr, 1994. END%% %%ANKI Basic According to Linden, what is the "only advantage" of `enum`s over `#define`? Back: `enum`s can usually be traced in a debugger. Reference: Van der Linden, Peter. _Expert C Programming: Deep C Secrets_. Programming Languages / C. Mountain View, Cal.: SunSoft Pr, 1994. 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](https://port70.net/~nsz/c/c11/n1570.pdf). * Van der Linden, Peter. _Expert C Programming: Deep C Secrets_. Programming Languages / C. Mountain View, Cal.: SunSoft Pr, 1994.