C Programming/Language Reference

Table of keywords

ANSI (American National Standards Institute) C (C89)/ISO C (C90)

Very old compilers may not recognize some or all of the C89 keywords const, enum, signed, void, volatile, as well as any later standards' keywords.

auto break case char const continue default do

double else enum extern float for goto if

int long register return short signed sizeof static

struct switch typedef union unsigned void volatile while

ISO C (C99)

These are supported in most new compilers.

_Bool _Complex

_Imaginary inline

restrict

ISO C (C11)

These are supported only in some newer compilers

alignof _Alignas _Atomic

_Generic _Noreturn _Static_assert

_Thread_local

Although not technically a keyword, C99-capable preprocessors/compilers additionally recognize the special preprocessor operator _Pragma, which acts as an alternate form of the #pragma directive that can be used from within macro expansions. For example, the following code will cause some compilers (incl. GCC, Clang) to emit a diagnostic message:

    #define EMIT_MESSAGE(str)    EMIT_PRAGMA(message(str))
    #define EMIT_PRAGMA(content) _Pragma(#content)
    EMIT_MESSAGE("Hello, world!")

Some compilers use a slight variant syntax; in particular, MSVC supports __pragma instead of _Pragma.

Specific compilers may also—in a non-standards-compliant mode, or with additional syntactic markers like __extension__—treat some other words as keywords, including asm, cdecl, far, fortran, huge, interrupt, near, pascal, or typeof. However, they typically allow these keywords to be overridden by declarations when operating in standards-compliant modes (e.g., by defining a variable named typeof), in order to avoid introducing incompatibilities with existing programs. In order to ensure the compiler can maintain access to extension features, these compilers usually have an additional set of proper keywords beginning with two underscores (__). For example, GCC treats asm, __asm, and __asm__ somewhat identically, but the latter two are always guaranteed to have the expected meaning since they can't be overridden.

Many of the newly introduced keywords—namely, those beginning with an underscore and capital letter like _Noreturn or _Imaginary—are intended to be used only indirectly in most situations. Instead, the programmer should prefer the use of standard headers such as <stdbool.h> or <stdalign.h>, which typically use the preprocessor to establish an all-lower-case variant of the keyword (e.g., complex or noreturn). These headers serve the purpose of enabling C and C++ code, as well as code targeting different compilers or language versions, to interoperate more cleanly. For example, by including <stdbool.h>, the tokens bool, true, and false can be used identically in either C99 or C++ without having to explicitly use _Bool in C99 or bool in C++.

See also the list of reserved identifiers .

Table of operators

Operators in the same row of this table have the same precedence and the order of evaluation is decided by the associativity (left-to-right or right-to-left). Operators closer to the top of this table have higher precedence than those in a subsequent group.

Operators	Description	Example Usage	Associativity
Postfix operators			Left to right
()	function call operator	swap (x, y)
[]	array index operator	arr [i]
.	member access operator for an object of struct/union type or a reference to it	obj.member
->	member access operator for a pointer to an object of struct/union type	ptr->member
Unary Operators			Right to left
!	logical not operator	!eof_reached
~	bitwise not operator	~mask
+ -	unary plus/minus operators	-num
++ --	post-increment/decrement operators	num++
++ --	pre-increment/decrement operators	++num
&	address-of operator	&data
*	indirection operator	*ptr
sizeof	sizeof operator for expressions	sizeof 123
sizeof()	sizeof operator for types	sizeof (int)
(type)	cast operator	(float)i
Multiplicative Operators			Left to right
* / %	multiplication, division and modulus operators	celsius_diff * 9.0 / 5.0
Additive Operators			Left to right
+ -	addition and subtraction operators	end - start + 1
Bitwise Shift Operators			Left to right
<<	left shift operator	bits << shift_len
>>	right shift operator	bits >> shift_len
Relational Inequality Operators			Left to right
< > <= >=	less-than, greater-than, less-than or equal-to, greater-than or equal-to operators	i < num_elements
Relational Equality Operators			Left to right
== !=	equal-to, not-equal-to	choice != 'n'
Bitwise And Operator			Left to right
&		bits & clear_mask_complement
Bitwise Xor Operator			Left to right
^		bits ^ invert_mask
Bitwise Or Operator			Left to right
|		bits | set_mask
Logical And Operator			Left to right
&&		arr != 0 && arr->len != 0
Logical Or Operator			Left to right
||		arr == 0 || arr->len == 0
Conditional Operator			Right to left
?:		size != 0 ? size : 0
Assignment Operators			Right to left
=	assignment operator	i = 0
+= -= *= /= %= &= |= ^= <<= >>=	shorthand assignment operators (foo op= bar represents foo = foo op bar)	num /= 10
Comma Operator			Left to right
,		i = 0, j = i + 1, k = 0

Table of data types

Type	Size in Bits	Comments	Alternative Names
Primitive Types in ANSI C (C89)/ISO C (C90)
char	≥ 8	sizeof gives the size in units of chars. These "C bytes" need not be 8-bit bytes (though commonly they are); the number of bits is given by the CHAR_BIT macro in the limits.h header. Signedness is implementation-defined. Any encoding of 8 bits or less (e.g. ASCII) can be used to store characters. Integer operations can be performed portably only for the range 0 ~ 127. All bits contribute to the value of the char, i.e. there are no "holes" or "padding" bits.	—
signed char	same as char	Characters stored like for type char. Can store integers in the range -127 ~ 127 portably.	—
unsigned char	same as char	Characters stored like for type char. Can store integers in the range 0 ~ 255 portably.	—
short	≥ 16, ≥ size of char	Can store integers in the range -32767 ~ 32767 portably. Used to reduce memory usage (although the resulting executable may be larger and probably slower as compared to using int.	short int, signed short, signed short int
unsigned short	same as short	Can store integers in the range 0 ~ 65535 portably. Used to reduce memory usage (although the resulting executable may be larger and probably slower as compared to using int.	unsigned short int
int	≥ 16, ≥ size of short	Represents the "normal" size of data the processor deals with (the word-size); this is the integral data-type used normally. Can store integers in the range -32767 ~ 32767 portably.	signed, signed int
unsigned int	same as int	Can store integers in the range 0 ~ 65535 portably.	unsigned
long	≥ 32, ≥ size of int	Can store integers in the range -2147483647 ~ 2147483647 portably.	long int, signed long, signed long int
unsigned long	same as long	Can store integers in the range 0 ~ 4294967295 portably.	unsigned long int
float	≥ size of char	Used to reduce memory usage when the values used do not vary widely. The floating-point format used is implementation defined and need not be the IEEE single-precision format. unsigned cannot be specified.	—
double	≥ size of float	Represents the "normal" size of data the processor deals with; this is the floating-point data-type used normally. The floating-point format used is implementation defined and need not be the IEEE double-precision format. unsigned cannot be specified.	—
long double	≥ size of double	unsigned cannot be specified.	—
Primitive Types added to ISO C (C99)
long long	≥ 64, ≥ size of long	Can store integers in the range -9223372036854775807 ~ 9223372036854775807 portably.	long long int, signed long long, signed long long int
unsigned long long	same as long long	Can store integers in the range 0 ~ 18446744073709551615 portably.	unsigned long long int
intmax_t	the maximum width supported by the platform	Can store integers in the range -(1 << n-1)+1 ~ (1 << n-1)-1, with 'n' the width of intmax_t. Used by the "j" length modifier in the C Programming/File IO#Formatted output functions: the printf family of functions.	—
uintmax_t	same as intmax_t	Can store integers in the range 0 ~ (1 << n)-1, with 'n' the width of uintmax_t.	—
User Defined Types
struct	≥ sum of size of each member	Said to be an aggregate type.	—
union	≥ size of the largest member	Said to be an aggregate type.	—
enum	≥ size of char	Enumerations are a separate type from ints, though they are mutually convertible.	—
typedef	same as the type being given a name	typedef has syntax similar to a storage class like static, register or extern.	—
Derived Types
type* (pointer)	≥ size of char	0 always represents the null pointer (an address where no data can be placed), irrespective of what bit sequence represents the value of a null pointer. Pointers to different types may have different representations, which means they could also be of different sizes. So they are not convertible to one another. Even in an implementation which guarantess all data pointers to be of the same size, function pointers and data pointers are in general incompatible with each other. For functions taking variable number of arguments, the arguments passed must be of appropriate type, so even 0 must be cast to the appropriate type in such function-calls.	—
type [integer] (array)	≥ integer × size of type	The brackets ([]) follow the identifier name in a declaration. In a declaration which also initializes the array (including a function parameter declaration), the size of the array (the integer) can be omitted. type [] is not the same as type*. Only under some circumstances one can be converted to the other.	—
type (comma-delimited list of types/declarations) (function)	—	Functions declared without any storage class are extern. The parentheses (()) follow the identifier name in a declaration, e.g. a 2-arg function pointer: int (* fptr) (int arg1, int arg2).	—

Character sets

Programs written in C can read and write any character set, provided the libraries that support them are included/used.

The source code for C programs, however, is usually limited to the ASCII character set.

In a file containing source code, the end of a line is sometimes, depending on the operating system it was created on not a newline character but compilers treat the end of each line as if it were a single newline character.

Virtually all compilers allow the $, @, and ` characters in string constants. Many compilers also allow literal multibyte Unicode characters, but they are not portable.

Certain characters must be escaped with a backslash to represent themselves in a string or character constant. These are:

\\ Literal backslash \" Literal double quote \' Literal single quote

\n Newline \t Horizontal tab \f Form feed

\v Vertical tab

Additionally, some compilers allow these characters:

• \r Carriage return
• \a Alert (audible bell)
• \b Backspace

\xhh, where the 'h' characters are hexadecimal digits, is used to represent arbitrary bytes (including \x00, the zero byte).

\uhhhh or \Uhhhhhhhh, where the 'h' characters are hexadecimal digits, is used to portably represent Unicode characters.