C Reference - Data Types Keywords

Basic Types

Type Group	Type Name	Size [bits]	Hex Range
	void	0	-
Characters	signed char char	8	0x80..0x7F
	unsigned char	8	0x00..0xFF
Integers	signed short int short int short	16	0x8000..0x7FFF
	unsigned short int unsigned short	16	0x0000..0xFFFF
	signed int int	32*	0x80000000.. 0x7FFFFFFF
	enum	32*	0x80000000.. 0x7FFFFFFF
	unsigned int unsigned	32*	0x00000000.. 0xFFFFFFFF
	signed long int long int long	32	0x80000000.. 0x7FFFFFFF
	unsigned long int unsigned long	32	0x00000000.. 0xFFFFFFFF
	long long int long long	64	0x8000000000000000.. 0x7FFFFFFFFFFFFFFF
	unsigned long long int unsigned long long	64	0x0000000000000000.. 0xFFFFFFFFFFFFFFFF
Floating Point	float	32*	1.17549e-38.. 3.40282e+38
	double	64*	2.2250e-308.. 1.7976e+308
	long double	64*	2.2250e-308.. 1.7976e+308

* machine dependent

Derived Types

Name	Declaration Syntax	Description
Pointer	type *name;	Contains address of another variable
Array	type name[size];	Set of variables of the same type
Structure	struct tag {...} name;	Set of variables of different types
Union	union tag {...} name;	Set of variables of different types that are allocated on the same memory

 

Type Specifiers and Qualifiers

Name	Syntax	Description
typedef	typedef old_type new_type;	Type definition
extern	extern type name;	External variable declaration
static	static type name;	Static variable has a lifetime until the end of program and a local scope
const	const type name=value;	Constant variable - can't be changed
auto	auto type name;	Declare local variable inside function. Can be omitted from the code.
register	register type name;	Declare local variable inside function. Suggests compiler to keep variable in CPU register. Used for optimization.
volatile	volatile type name;	Avoid compiler optimization, used when can be changed externally. i.e: I/O addressed variable.

 

 

---

void

Syntax:

void

Size:

0 bits 0 bytes

Description:

Void type

Example1:

void *p; p = (void *)(&x);

Example2:

void my_func(int x) {    :      /* no return value */ }

 

---

char

Syntax:

char variable name;

Size:

8 bits 1 bytes

Range:

Dec:   -128..127 Hex:   0x80..0x7F Const: CHAR_MIN..CHAR_MAX

Description:

Signed character

Examples:

char c; char c=32; char c='a'; char *msg="Hello there"; char msg[]="Hello there";

 

---

signed char

Syntax:

signed char variable name;

Size:

8 bits 1 bytes

Range:

Dec:   -128..127 Hex:   0x80..0x7F Const: SCHAR_MIN..SCHAR_MAX

Description:

Signed character. Same as char.

Examples:

signed char c; signed char c=-100; signed char c='a'; signed char *msg="Hello there"; signed char msg[]="Hello there";

 

---

unsigned char

Syntax:

unsigned char variable name;

Size:

8 bits 1 bytes

Range:

Dec:   0..255 Hex:   0..0xFF Const: 0..UCHAR_MAX

Description:

Unsigned character.

Examples:

unsigned char c; unsigned char c=200; unsigned char c='a';

 

---

short

Syntax:

short variable name;

Size:

16 bits 2 bytes

Range:

Dec:   -32768..32767 Hex:   0x8000..0x7FFF Const: SHRT_MIN..SHRT_MAX

Description:

Signed short integer

Examples:

short c; short c=-20000; short c=20000;

 

---

unsigned short

Syntax:

unsigned short variable name;

Size:

16 bits 2 bytes

Range:

Dec:   0..65535 Hex:   0..0xFFFF Const: 0..USHRT_MAX

Description:

Unsigned short integer.

Examples:

unsigned short c; unsigned short c=40000;

 

---

int

Syntax:

int variable name;

Size:

32 bits (machine dependent) 4 bytes

Range:

Dec:   -2147483648..2147483647 Hex:   80000000..0x7FFFFFFF Const: INT_MIN..INT_MAX

Description:

Signed integer.

Examples:

int c; int c=-200000; int c=200000;

 

---

enum

Syntax:

enum name {    name1 [= number1],    name2 [= number2],    name3 [= number3],    : };

Size:

32 bits 4 bytes

Range:

Dec:   -2147483648..2147483647 Hex:   80000000..0x7FFFFFFF Const: INT_MIN..INT_MAX

Description:

Enumeration constant list of signed integers.

Examples:

enum subject {    MATH        = 1,    PHYSICS     = 2,    CHEMISTRY   = 3,    PROGRAMMING = 4,    ELECTRONICS = 9 };

 

---

unsigned int

Syntax:

unsigned int variable name;

Size:

32 bits (machine dependent) 4 bytes

Range:

Dec:   0..4294967295 Hex:   0..0xFFFFFFFF Const: 0..UINT_MAX

Description:

Unsigned integer.

Examples:

unsigned int c; unsigned int c=200000U;

 

---

long

Syntax:

long variable name;

Size:

32 bits 4 bytes

Range:

Dec:   -2147483648..2147483647 Hex:   80000000..0x7FFFFFFF Const: LONG_MIN..LONG_MAX

Description:

Signed long integer.

Examples:

long c; long c=-200000L; long c=200000L;

 

---

unsigned long

Syntax:

unsigned long variable name;

Size:

32 bits 4 bytes

Range:

Dec:   0..4294967295 Hex:   0..0xFFFFFFFF Const: 0..ULONG_MAX

Description:

Unsigned long integer.

Examples:

unsigned long c; unsigned long c=200000UL;

---

long long

Syntax:

long long variable name;

Size:

64 bits 8 bytes

Range:

Dec:   9223372036854775808..        9223372036854775807 Hex:   0x8000000000000000..        0x7FFFFFFFFFFFFFFF Const: LLONG_MIN..LLONG_MAX

Description:

long long integer.

Examples:

long long c; long long c=200000LL;

---

unsigned long long

Syntax:

unsigned long long variable name;

Size:

64 bits 8 bytes

Range:

Dec:   0..18446744073709551615 Hex:   0..0xFFFFFFFFFFFFFFFF Const: 0..ULLONG_MAX

Description:

Unsigned long long integer.

Examples:

unsigned long long c; unsigned long long c=200000ULL;

 

---

float

Syntax:

float variable name;

Size:

32 bits 4 bytes

Range:

Dec:   1.17549e-38..        3.40282e+38 Const: FLT_MIN..FLT_MAX

Description:

Single precision floating point.

Examples:

float c; float c=200.8F; float c=-2e-8F;

 

---

double

Syntax:

double variable name;

Size:

64 bits 8 bytes

Range:

Dec:   2.2250e-308..        1.7976e+308 Const: DBL_MIN..DBL_MAX

Description:

Double precision floating point.

Examples:

unsigned char c; unsigned char c=200.8; unsigned char c=-2e-8;

 

---

long double

Syntax:

long double variable name;

Size:

64 bits 8 bytes

Range:

Dec:   2.2250e-308..        1.7976e+308 Const: LDBL_MIN..LDBL_MAX

Description:

Long double precision floating point.

Examples:

unsigned char c; unsigned char c=200.8L; unsigned char c=-2e-8L;

 

---

Pointer

Syntax:

type *name; type (*func_ptr)(type1, type2,..);

Description:

Contains the memory address of another variable

Example1:

int x=10; int *px;   /* px points to x -    px contains the address of x */ px = (int *)(&x);   /* x now equal to 20 */ *px = 20;

Example2:

int square(int x) {    return x*x; }   int main(void) {    /* function pointer declaration */    int (*pfunc)(int) = square;      /* call function -> y=100 */    int y = (*pfunc)(10); }

 

---

Array

Syntax:

type name[size]; type name[size1][size2];

Description:

Contains a set of variables of the same type

Example1:

int x[10]; int y[10][20]; int z[5]={10,20,30,40,50}; char s[]="Hello";   int i,j;   /* reset x array */ for(i=0; i<10; i++) {    x[i] = 0;    for(j=0; j<10; j++) {       y[i][j] = 0;    } }   /*  */ *px = 20;

Example2:

int square(int x) {    return x*x; }   int main(void) {    /* function pointer declaration */    int (*pfunc)(int) = square;      /* call function -> y=100 */    int y = (*pfunc)(10); }

 

---

Structure

Syntax:

struct tag {    type1 member1;    type2 member2;    ... } name;

Description:

Contains a set of variables of different types

Example1:

struct time {    int h;    int m; };   struct time *pt; struct time dt; struct time t1 = {8,0}; struct time t2 = {16,0};   dt.h = t2.h - t1.h; dt.m = t2.m - t1.m; if(dt.m < 0) {    dt.m += 60;    dt.h -= 1; }

Example2 - bit fields:

struct reg {    unsigned int in       : 3;    unsigned int out      : 4;    unsigned int reserved : 9; }; int input; int output;   struct reg r=0; // sets all bits to 0 input = r.in; output = 3; r.out = output;

 

---

Union

Syntax:

union tag {    type1 member1;    type2 member2;    ... } name;

Description:

Contains a set of variables, that are allocated on the same memory

Example:

union data {    unsigned long num;    char name[4]; } union data d1.name="abc"; union data d2.num =0x00434241; /* now d1 is equal to d2 */

 

---

typedef

Syntax:

typedef old_type new_type;

Description:

New type definition

Example:

typedef unsigned char Byte typedef unsigned int Word typedef struct time {    char h;    char m; } Time;   Byte c=255; Word x; Time t;

 

---

extern

Syntax:

extern type name;

Description:

External variable  declaration, used in other files to broaden the variable scope to other files. The external declaration is usually written in header file.

Example:

See Program Structure

 

---

static

Syntax:

static type name; static type function(type arg1,...)

Description:

Static variables are freed only when program ends, even when defined inside a function. Static variables have local file scope for global variables, and local function scope for local variables. Static functions have local file scope and are not visible outside the file.

Example:

// external static variable static int a;   void myfunc() {    // local static variable    static int x=10;      x = x+1; /* =10,11,12,...         */             /* for each function call*/ }

 

---

const

Syntax:

const type name=value;

Description:

Constant variable - can't be changed

Example:

const int linesnum=100; linenum = 0; /* error! */

 

---

auto

Syntax:

auto type name;

Description:

Declare local variable inside function. Can be omitted from the code.

Example:

void myfunc() {    auto int x;    ... }

 

---

register

Syntax:

register type name;

Description:

Declare local variable inside function. Suggests compiler to keep variable in CPU register. Used for optimization.

Example:

void myfunc() {    register int x;    ... }

 

---

volatile

Syntax:

volatile type name;

Description:

Avoid compiler optimization, used when can be changed externally (i.e: I/O addressed variable).

Example:

volatile unsigned short data;