Keywords

These keywords define the type of data a variable holds.

Keyword	Std	Description
`void`	C89	Represents an empty set of values (e.g., function returning nothing) or a generic pointer (`void*`).
`char`	C89	Smallest addressable unit (usually 1 byte); stores characters.
`short`	C89	Short signed integer type.
`int`	C89	Basic signed integer type.
`long`	C89	Long signed integer type (at least 32 bits).
`float`	C89	Single-precision floating-point.
`double`	C89	Double-precision floating-point.
`signed`	C89	Explicitly specifies a signed integer.
`unsigned`	C89	Specifies a non-negative integer.
`_Bool`	C99	Boolean type (0 or 1). Note: `bool` is a macro in `<stdbool.h>` prior to C23.
`_Complex`	C99	Complex number type.
`_Imaginary`	C99	Imaginary number type (optional support in implementations).
`struct`	C89	Defines a complex data structure grouping variables under one name.
`union`	C89	Defines a data structure where all members share the same memory location.
`enum`	C89	Defines a set of named integer constants.

These keywords determine the lifetime, visibility (scope), and linkage of variables.

Keyword	Std	Description
`auto`	C89	Default storage class for local variables (automatic duration). Rarely used explicitly.
`extern`	C89	Declares a variable/function is defined in another translation unit (file).
`register`	C89	Suggests the variable be stored in a CPU register for speed (hint only).
`static`	C89	Internal linkage (global) or Static duration (local). Preserves value between function calls.
`typedef`	C89	Creates an alias name for an existing type.
`_Thread_local`	C11	Specifies a variable has thread storage duration (unique per thread).

These keywords modify how the memory associated with a type is accessed.

Keyword	Std	Description
`const`	C89	Read-only. The program cannot modify the variable after initialization.
`volatile`	C89	Prevents compiler optimization. Indicates value may change unexpectedly (e.g., hardware register).
`restrict`	C99	Optimization hint. Assumes this pointer is the only way to access the underlying memory in that scope.
`_Atomic`	C11	Specifies that access to the variable is atomic (thread-safe without locks).

Keywords that control the execution path of the program.

Keyword	Std	Description
`if`	C89	Conditional branching statement.
`else`	C89	The alternative branch of an `if` statement.
`switch`	C89	Multi-way branch based on an integer value.
`case`	C89	A specific label within a `switch` block.
`default`	C89	The fallback label within a `switch` block.
`while`	C89	Loop that checks condition before execution.
`do`	C89	Loop that checks condition after execution (guarantees one run).
`for`	C89	Loop with initialization, condition, and increment expressions.
`goto`	C89	Unconditional jump to a labeled statement.
`continue`	C89	Skips the rest of the current loop iteration.
`break`	C89	Exits the current loop or `switch` statement immediately.
`return`	C89	Terminates a function and optionally returns a value.

Keywords that act like operators or compiler directives.

Keyword	Std	Description
`sizeof`	C89	Compile-time operator returning the size of a type or variable in bytes.
`inline`	C99	Suggests the compiler substitute the function code directly (inlining) to reduce call overhead.
`_Alignas`	C11	Specifies the memory alignment requirement for a variable/type.
`_Alignof`	C11	Returns the alignment requirement of a type.
`_Generic`	C11	Enables compile-time generic selection (C-style polymorphism).
`_Noreturn`	C11	Specifies that a function never returns to the caller (e.g., `exit()`).
`_Static_assert`	C11	Performs a compile-time assertion check.