Data Types

Basic Data Types

Type	Base Type	Min^c	Max^c	Constants^d
char^a	integer	0	255	32
signed char	integer	-128	127	32
unsigned char	integer	0	255	32
short^b	integer	-32768	32767	32
unsigned short	integer	0	65536	32
int^a,b	integer	-2³¹	2³¹-1	32
unsigned int	integer	0	2³²-1	32U
long^b	integer	-2³¹	2³¹-1	32L
unsigned long	integer	0	2³²-1	32UL
float^a	floating point	6/-38	6/38	3.7F
double	floating point	15/-308	15/308	3.7
long double^a	floating point	15/-308	15/308	3.7L

^aThe most common definition is given, but might differ for other compilers.

^bThe preceeding keyword signed (and the succeding keyword int for short and long types) is optional

^cFor floating point numbers the two numbers are the number of digits (precission) and the exponent (of powers in 10). The minimum is here the smallest possible number, which are non-zero. Smaller numbers will raise the underflow exception.

^dLiteral constants, when used in the source text. The minimal integer constant is of type signed integer, when not overridden by the suffix 'L' or 'U'.

Derived Data Types

Type	Comment
bool	bool is equivalent to the char type, which predefined compiler definitions of true as 1 and false as 0.
Pointers	Each data type can have a pointer associated with. Pointers don't have a keyword, but are marked by a preceeding asterix in the variable and function declaration/definition. Most compilers supplies the predefined constant NULL, which is equivalent to 0.
void	Only allowed in function declaration/definition to indicate that the function has no return type or in case of a returning pointer that the pointer is not associated to a specific data type (e.g. malloc() is of type void *)
Aliases	Aliases to existing data types can be defined with the typedef command outside a function block. (Example: typedef int error_number;)
Structs	Combination of existing data type to a combined data type. Typically definition of structs are combined with typedef instructions to avoid the struct keyword in varable declaration.
Enumeration	A list of names associated with an ascending series of integers. The definition is identical to structs (e.g. enum day {sun,mon,tue,wed,thue,fri,sat};), except that the curly brackets contains only the name associated with an ascending list, starting from zero.

Data Type Prefixes

Prefix	Comment
const	Indicates that a variable is constant and cannot be changed. Allows for better optimization
register	Stores variable in internal memory of CPU chip for faster access. Not recommended because compilers are doing a better job with optimizations.
volatile	Exclude certain optimzation if the content and memory location is sensitive for the execution (e.g. when the program is relying on low level input from external programs or hardware)
extern	Indicates global variables in include files that they are declared but that the initial value is given somewhere else in the source text (e.g. a different source file).
static	Preserves the content of variables of function to the next call of that function.
struct	Indicates that the following data type is a struct. Is often avoided by making an alias to the struct definition with the typedef command.
enum	Indicates that the following data type is an enumeration. Is often avoided by making an alias to the enumeration definition with the typedef command.