Reference: Keywords

Every C keyword, grouped by what it's for.

4 minute read

Big idea

C has a small vocabulary. These keywords are reserved: you can't use them as names for variables or functions. This page lists each one, what it's for, and where it's taught.

Types

Keyword	What it does	Example
`char`	one byte; holds a character or a small integer	`char grade = 'A';`
`int`	the everyday whole number (usually 32 bits)	`int count = 0;`
`short`	a smaller int (usually 16 bits)	`short s = 300;`
`long`	a bigger int (at least 32 bits; `long long` at least 64)	`long long big = 9000000000LL;`
`float`	a decimal, about 7 digits of precision	`float f = 2.5f;`
`double`	a decimal, about 15 digits — the default choice	`double pi = 3.14159;`
`signed`	can hold negative values (the default for `int`)	`signed char c = -5;`
`unsigned`	no negatives; doubles the positive range	`unsigned int u = 4000000000u;`
`void`	"nothing": no return value, no parameters, or an untyped pointer	`void greet(void);`
`_Bool`	true/false; use it as `bool` from `<stdbool.h>`	`bool done = false;`

See Data Types and sizeof.

Building your own types

Keyword	What it does	Example
`struct`	groups members of different types	`struct Point { int x, y; };`
`union`	members share one piece of memory	`union { int i; float f; } u;`
`enum`	a set of named integer constants	`enum Dir { UP, DOWN };`
`typedef`	a new name for an existing type	`typedef struct Point Point;`

See Structures, Unions, Enums, typedef.

Control flow

Keyword	What it does	Example
`if` / `else`	run code only when a condition is true	`if (x > 0) … else …`
`switch` / `case` / `default`	jump to the branch matching a value	`switch (op) { case '+': … }`
`while`	repeat while a condition is true	`while (n > 0) n--;`
`do`	repeat, checking the condition after each pass	`do { … } while (again);`
`for`	a loop with start, condition, and step	`for (int i = 0; i < n; i++)`
`break`	leave the loop or switch now	`if (found) break;`
`continue`	skip to the loop's next pass	`if (x < 0) continue;`
`return`	leave the function, optionally with a value	`return total;`
`goto`	jump to a label in the same function — rarely a good idea	`goto cleanup;`

Storage and qualifiers

Keyword	What it does	Example
`const`	read-only	`const int MAX = 10;`
`static`	local: keeps its value between calls · global: private to its file	`static int calls = 0;`
`extern`	declares something defined in another file	`extern int counter;`
`auto`	"an ordinary local" — the default, so never written in C	`auto int x;`
`register`	old hint to keep a variable in a CPU register	`register int i;`
`volatile`	the value may change behind the compiler's back (hardware, signals)	`volatile int flag;`
`inline`	a hint to paste a small function's body at each call	`inline int sq(int x) { return x * x; }`
`restrict`	promise that a pointer is the only way to reach its data	`void memcpy(void restrict d, …)`

See Scope and Storage Classes and Const Correctness.

Operator keyword

Keyword	What it does	Example
`sizeof`	the size of a type or variable, in bytes	`sizeof(int)`, `sizeof arr`

Newer additions

C11 and later added keywords starting with an underscore and a capital — _Alignas, _Alignof, _Atomic, _Generic, _Noreturn, _Static_assert, _Thread_local, _Complex, _Imaginary. You'll rarely need them while learning. C23 also makes bool, true, false, nullptr, constexpr, and typeof keywords.

Not keywords (but just as reserved in practice)

main, printf, NULL, true, false (before C23), and bool (before C23) are not keywords — they're names from the library or headers. You could reuse some of them, but don't.

Which of these can't be used as a variable name?

Key terms

Keyword — A word reserved by the language for its own meaning.
Identifier — A name you choose: letters, digits, and _, not starting with a digit, not a keyword.
Qualifier — A keyword that adjusts a type: const, volatile, restrict.

Check yourself

Which of these is NOT a C keyword?

Which keyword makes a global private to its file?