The meaning of a value stored in an object is determined solely by the type of the expression used to access it - for example the type used in the variable declaration. There are two categories of types:

• object types - types used to describe objects
• function types - types used to describe functions

Additionally an object may have a complete or incomplete type. An incomplete type is a type that describes an object but lacks information needed to determine its size. For example: an array of unknown size or struct or union types whose members were not specified. A complete type is a type that describes an object with sufficient information needed to determine its size.

Basic Types

Basic types are the fundamental building blocks of all C programs. All basic types are complete types. Basic types are divided into four sub-categories: char type, signed integer types, unsigned integer types, and floating types.

Character Types

There are three distinct character types: char, signed char, and unsigned char. Depending on the implementation used, a char might be signed or unsigned.

Type	Explanation	Min Macro	At least Min Value	Max Macro	At least Max Value
char	Either a "signed char" or a "unsigned char".	CHAR_MIN		CHAR_MAX
signed char	Same as char, guaranteed to be signed.	SCHAR_MIN	-127	SCHAR_MAX	+127
unsigned char	Same as char, guaranteed to be unsigned.		0	UCHAR_MAX	+255

Signed Integer Types

The standard defines five signed integer types:

• signed char
• short int
• int
• long int
• long long int^C99

In this list, each type provides at least as much storage as those preceding it.

Type	Explanation	Min Macro	At least Min Value	Max Macro	At least Max Value	since
short short int signed short signed short int	Short signed integer type. At least 16 bits in size.	SHRT_MIN	-32767	SHRT_MAX	+32767	C89
int signed int	An integer type. At least 16 bits in size.	INT_MIN	-32767	INT_MAX	+32767
long long int signed long signed long int	A long integer type. At least 32 bits in size.	LONG_MIN	-2147483647	LONG_MAX	+2147483647
long long long long int signed long long signed long long int	A long long integer type. At least 64 bits in size.	LLONG_MIN	-9223372036854775807	LLONG_MAX	+9223372036854775807	C99

The macros in the table can be found in the <limits.h> header.

Unsigned Integer Types

For each of the signed integer types defined, there is a corresponding unsigned type. Unsigned types are designated with the keyword unsigned. The unsigned types occupy the same amount of space as their signed counterparts. The range of nonnegative values of a signed integer type is a subrange of the corresponding unsigned type.

Note that unsigned integer types computations can never overflow. Results that cannot be represented by the specified type are reduced modulo the number that is one greater than the largest value that can be represented by that type.

Type	Explanation	Max Macro	At least Max Value	since
unsigned short unsigned short int	Short unsigned integer type. At least 16 bits in size.	USHRT_MIN	+65535	C89
unsigned unsigned int	An unsigned integer type. At least 16 bits in size.	UINT_MAX	+65535
unsigned long unsigned long int	An unsigned long integer type. At least 32 bits in size.	ULONG_MIN	+4294967295
unsigned long long unsigned long long int	An unsigned long long integer type. At least 64 bits in size.	ULLONG_MIN	18446744073709551615	C99

The macros in the table can be found in the <limits.h> header.

Extended Unsigned/Signed Integer Types

Extended signed integer types and extended unsigned integer types are implementation-specific integer types that are provided as an extension to the ones defined by the standard. Although the C standard can't define them or describe their underlying types, it does setup a set of rule by which implementations much conform to in order to ensure well-defined behavior.

Floating Types

Standard C has two types of floating point types: real and complex

Real Floating Types

Standard C provides three real floating types.

• float
• double
• long double^C99

Each type is a subset of the smaller type.

Complex Types^C99

Standard C provides three complex types. Complex types are the same as the real floating types with the addition of the _Complex keyboard. The types are:

Void Type

The void type comprises an empty set of values. The void type is an incomplete type which cannot be completed.

• float _Complex
• double _Complex
• long double _Complex

Boolean Type^C99

Starting with C99, the C Standard introduced the _Bool type - an object large enough to store the values 0 and 1.

The standard also introduced a new header: <stdbool.h>. It is recommended that the header be used wherever the use of bool is needed instead of using _Bool directly. The header defines 4 macros:

• bool - expands to _Bool
• true - expands to 1
• false - expands to 0
• __bool_true_false_are_ - expands to 1

For example:

#include <stdio.h>
#include <stdbool.h>
int main(void)
{
    bool b = true;
    if (b)
        puts("It's true!");
    return 0;
}

Enumerated Type

An enumerated type is a named integer constant value. An enumeration comprises a set of enumerated types. For example:

enum enemies
{
    ghosts = 1,
    goblins,
    octopi,
    monsters
};

Array Type

An array type describes a contiguously allocated nonempty set of objects. Each individual element member object is called an element type. The array type is derived from its element type. (For example, if the element type is 'int', the array is said to be an 'array of int'.)

Structure Type

A structure type describes a set of nonempty member objects with an optional name and type.

struct student
{
    unsigned int student_id;
    char *first_name;
    char *last_name;
    float gpa;
};

Union Type

A union type describes a nonempty set of overlapping member objects.

Function Type

A function type describes a function with a specified return type. The number and types of its parameters is used to characterize the function type.

Pointer Type

A pointer type describes an object whose value provides a reference to an object of a stated type. A pointer type can be derived from both function and object types.

Atomic Type ^C11

An atomic type describes the type designated by the atomic type specifier. Array types, function types, and qualified types cannot be atomic types.