[C language] data types, storage classes

ref

• https://www.runoob.com/cprogramming/c-data-types.html
  

---

type of data

Integer type

type	size	scope
char	1byte	-128~127
unsigned char	1byte	0~255
int	2/4byte	-32768~32767 / $-2^{31}$ ~$2^{31}-1$
unsigned int	2/4byte	0~65535 / $0$ ~$2^{32}-1$
short	2byte	-32768~32767
unsigned short	2byte	0~65535
long	4/8byte	$-2^{31}$ ~$2^{31}-1$ /$-2^{63}$ ~$2^{63}-1$
unsigned long	4/8byte	$0$ ~$2^{32}-1$ /$0$ ~$2^{64}-1$
long long	8byte	$-2^{63}$ ~$2^{63}-1$

• 16-bit operating system: int occupies 2 bytes, long occupies 4 bytes
• 32-bit operating system: int occupies 4 bytes, long occupies 4 bytes
• 64-bit operating system: int occupies 4 bytes, long occupies 8 bytes

floating point type

type	size	scope	precision
float	4byte	1.2E-38~3.4E+38	6 significant bits
double	8byte	2.3E-308~1.7E+308	15 significant bits
long double	16byte	3.4E-4932~1.1E+4932	19 significant bits

Embedded common replacement

typedef char int8;
typedef volatile char vint8;
typedef unsigned char uint8;
typedef volatile unsigned char vuint8;
typedef int int16;
typedef unsigned short uint16;
typedef long int32;
typedef unsigned long uint32;

---

storage class

• auto: default storage class, can only be used in functions, can only modify local variables

  int year;
  auto int month;   // 两个相同存储类得变量
  

• register: local variables stored in registers instead of RAM, maximum size = register size, cannot use &operators (no memory location)

  register int day;
  

  • Only used for variables (counters) that need fast access; may be in registers (depending on hardware)
• static: keep the existence of local variables during the life cycle of the program, if in the file, the scope is limited to the file

  static int hour;
  

• extern: provide a reference, the variable name points to a previously defined storage location

  extern int minute;
  

---

operator

• ^: or, dissimilar=1
• &: return variable address
• *: points to a variable
• ?:: conditional expression

  (a==10)?20:30
  

priority

---

function

• Call by value: The actual value is assigned to the formal parameter of the function, and the modification of the formal parameter does not affect the actual parameter
• Call by reference: pass a pointer to a formal parameter, the operation is equivalent to the operation of the actual parameter itself

---

scope

• The global variable has the same name as the local variable, and the local variable is used first in the function
  • Formal parameters are also treated as local variables of the function