C data types

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In the C programming language, data types refers to an extensive system for declaring variables of different types. The language itself provides basic arithmetic types and syntax to build array and compound types. Several headers in the standard library contain definitions of support types, that have additional properties, such as exact size, guaranteed.[1][2]

Basic types[edit]

The C language provides many basic types. Most of them are formed from one of the four basic arithmetic type specifiers in C (char, int, float and double), and optional specifiers (signed, unsigned, short, long). All available basic arithmetic types are listed below:

Type Explanation
char smallest addressable unit of the machine that can contain basic character set. It is an integer type. Actual type can be either signed or unsigned depending on the implementation.
signed char same size as char, but guaranteed to be signed.
unsigned char same size as char, but guaranteed to be unsigned.
short
short int
signed short
signed short int
short signed integer type. At least 16 bits in size.
unsigned short
unsigned short int
same as short, but unsigned.
int
signed int
basic signed integer type. At least 16 bits in size.
unsigned
unsigned int
same as int, but unsigned.
long
long int
signed long
signed long int
long signed integer type. At least 32 bits in size.
unsigned long
unsigned long int
same as long, but unsigned.
long long
long long int
signed long long
signed long long int
long long signed integer type. At least 64 bits in size. Specified since the C99 version of the standard.
unsigned long long
unsigned long long int
same as long long, but unsigned. Specified since the C99 version of the standard.
float single precision floating-point type. Actual properties unspecified (except minimum limits), however on most systems this is the IEEE 754 single-precision binary floating-point format. This format is required by the optional Annex F "IEC 60559 floating-point arithmetic".
double double precision floating-point type. Actual properties unspecified (except minimum limits), however on most systems this is the IEEE 754 double-precision binary floating-point format. This format is required by the optional Annex F "IEC 60559 floating-point arithmetic".
long double extended precision floating-point type. Actual properties unspecified. Unlike types float and double, it can be either 80-bit floating point format, the non-IEEE "double-double" or IEEE 754 quadruple-precision floating-point format if a higher precision format is provided, otherwise it is the same as double. See the article on long double for details.

The actual size of integer types varies by implementation. The standard only requires size relations between the data types and minimum sizes for each data type:

The relation requirements are that the long long is not smaller than long, which is not smaller than int, which is not smaller than short. As char's size is always the minimum supported data type, all other data types can't be smaller.

The minimum size for char is 8 bit, the minimum size for short and int is 16 bit, for long it is 32 bit and long long must contain at least 64 bit.

The type int should be the integer type that the target processor is most efficient working with. This allows great flexibility: for example, all types can be 64-bit. However, several different integer width schemes (data models) are popular. This is because the data model defines how different programs communicate, a uniform data model is used within a given operating system application interface.[3]

In practice it should be noted that char is usually 8 bits in size and short is usually 16 bits in size (as are their unsigned counterparts). This holds true for platforms as diverse as 1990s SunOS 4 Unix, Microsoft MS-DOS, modern Linux, and Microchip MCC18 for embedded 8 bit PIC microcontrollers. POSIX requires char to be exactly 8 bits in size.

The actual size and behavior of floating-point types also vary by implementation. The only guarantee is that long double is not smaller than double, which is not smaller than float. Usually, the 32-bit and 64-bit IEEE 754 binary floating-point formats are used, if supported by hardware.

Boolean type[edit]

C99 added a boolean (true/false) type (_Bool) which is defined in the <stdbool.h> header. Additionally, the standard requires that macros are defined to alias the type as bool as well as providing macros for true and false.

Size and pointer difference types[edit]

The C language provides the separate types size_t and ptrdiff_t to represent memory-related quantities. Existing types were deemed insufficient, because their size is defined according to the target processor's arithmetic capabilities, not the memory capabilities, such as available address space. Both of these types are defined in the <stddef.h> header (cstddef header in C++).

size_t is used to represent the size of any object (including arrays) in the particular implementation. It is used as the return type of the sizeof operator. The maximum size of size_t is provided via SIZE_MAX, a macro constant which is defined in the <stdint.h> header (cstdint header in C++). As an unsigned type, size_t is guaranteed to be wide enough to accommodate at least the value of 65535. Signed sizes can be represented by ssize_t, which is a POSIX extension.

ptrdiff_t is used to represent the difference between pointers.

Interface to the properties of the basic types[edit]

Information about the actual properties, such as size, of the basic arithmetic types, is provided via macro constants in two headers: <limits.h> header (climits header in C++) defines macros for integer types and <float.h> header (cfloat header in C++) defines macros for floating-point types. The actual values depend on the implementation.

Properties of integer types
  • CHAR_BIT – size of the char type in bits (at least 8 bits)
  • SCHAR_MIN, SHRT_MIN, INT_MIN, LONG_MIN, LLONG_MIN(C99) – minimum possible value of signed integer types: signed char, signed short, signed int, signed long, signed long long
  • SCHAR_MAX, SHRT_MAX, INT_MAX, LONG_MAX, LLONG_MAX(C99) – maximum possible value of signed integer types: signed char, signed short, signed int, signed long, signed long long
  • UCHAR_MAX, USHRT_MAX, UINT_MAX, ULONG_MAX, ULLONG_MAX(C99) – maximum possible value of unsigned integer types: unsigned char, unsigned short, unsigned int, unsigned long, unsigned long long
  • CHAR_MIN – minimum possible value of char
  • CHAR_MAX – maximum possible value of char
  • MB_LEN_MAX – maximum number of bytes in a multibyte character
Properties of floating-point types
  • FLT_MIN, DBL_MIN, LDBL_MIN – minimum normalized positive value of float, double, long double respectively
  • FLT_TRUE_MIN, DBL_TRUE_MIN, LDBL_TRUE_MIN (C11) – minimum positive value of float, double, long double respectively
  • FLT_MAX, DBL_MAX, LDBL_MAX – maximum finite value of float, double, long double respectively
  • FLT_ROUNDS – rounding mode for floating-point operations
  • FLT_EVAL_METHOD (C99) – evaluation method of expressions involving different floating-point types
  • FLT_RADIX – radix of the exponent in the floating-point types
  • FLT_DIG, DBL_DIG, LDBL_DIG – number of decimal digits that can be represented without losing precision by float, double, long double respectively
  • FLT_EPSILON, DBL_EPSILON, LDBL_EPSILONdifference between 1.0 and the next representable value of float, double, long double respectively
  • FLT_MANT_DIG, DBL_MANT_DIG, LDBL_MANT_DIG – number of FLT_RADIX-base digits in the floating-point significand for types float, double, long double respectively
  • FLT_MIN_EXP, DBL_MIN_EXP, LDBL_MIN_EXP – minimum negative integer such that FLT_RADIX raised to a power one less than that number is a normalized float, double, long double respectively
  • FLT_MIN_10_EXP, DBL_MIN_10_EXP, LDBL_MIN_10_EXP – minimum negative integer such that 10 raised to a power one less than that number is a normalized float, double, long double respectively
  • FLT_MAX_EXP, DBL_MAX_EXP, LDBL_MAX_EXP – maximum positive integer such that FLT_RADIX raised to a power one more than that number is a normalized float, double, long double respectively
  • FLT_MAX_10_EXP, DBL_MAX_10_EXP, LDBL_MAX_10_EXP – maximum positive integer such that 10 raised to a power one more than that number is a normalized float, double, long double respectively
  • DECIMAL_DIG (C99) – minimum number of decimal digits such that any number of the widest supported floating-point type can be represented in decimal with a precision of DECIMAL_DIG digits and read back in the original floating-point type without changing its value. DECIMAL_DIG is at least 10.

Fixed-width integer types[edit]

The C99 standard includes definitions of several new integer types to enhance the portability of programs.[2] The already available basic integer types were deemed insufficient, because their actual sizes are implementation defined and may vary across different systems. The new types are especially useful in embedded environments where hardware supports usually only several types and that support varies from system to system. All new types are defined in <inttypes.h> header (cinttypes header in C++) and also are available at <stdint.h> header (cstdint header in C++). The types can be grouped into the following categories:

  • Exact-width integer types which are guaranteed to have the same number N of bits across all implementations. Included only if it is available in the implementation.
  • Least-width integer types which are guaranteed to be the smallest type available in the implementation, that has at least specified number N of bits. Guaranteed to be specified for at least N=8,16,32,64.
  • Fastest integer types which are guaranteed to be the fastest integer type available in the implementation, that has at least specified number N of bits. Guaranteed to be specified for at least N=8,16,32,64.
  • Pointer integer types which are guaranteed to be able to hold a pointer
  • Maximum-width integer types which are guaranteed to be the largest integer type in the implementation

The following table summarizes the types and the interface to acquire the implementation details (N refers to the number of bits):

Type category Signed types Unsigned types
Type Minimum value Maximum value Type Minimum value Maximum value
Exact width intN_t INTN_MIN INTN_MAX uintN_t 0 UINTN_MAX
Least width int_leastN_t INT_LEASTN_MIN INT_LEASTN_MAX uint_leastN_t 0 UINT_LEASTN_MAX
Fastest int_fastN_t INT_FASTN_MIN INT_FASTN_MAX uint_fastN_t 0 UINT_FASTN_MAX
Pointer intptr_t INTPTR_MIN INTPTR_MAX uintptr_t 0 UINTPTR_MAX
Maximum width intmax_t INTMAX_MIN INTMAX_MAX uintmax_t 0 UINTMAX_MAX

Printf and scanf format specifiers[edit]

The <inttypes.h> header (cinttypes header in C++) provides features that enhance the functionality of the types defined in <stdint.h> header. Included are macros that define printf format string and scanf format string specifiers corresponding to the <stdint.h> types and several functions for working with intmax_t and uintmax_t types. This header was added in C99.

Printf format string

The macros are in the format PRI{fmt}{type}. Here {fmt} defines the output formatting and is one of d (decimal), x (hexadecimal), o (octal), u (unsigned) and i (integer). {type} defines the type of the argument and is one of N, FASTN, LEASTN, PTR, MAX, where N corresponds to the number of bits in the argument.

Scanf format string

The macros are in the format SCN{fmt}{type}. Here {fmt} defines the output formatting and is one of d (decimal), x (hexadecimal), o (octal), u (unsigned) and i (integer). {type} defines the type of the argument and is one of N, FASTN, LEASTN, PTR, MAX, where N corresponds to the number of bits in the argument.

Functions

Additional floating-point types[edit]

The C99 standard includes new floating-point types float_t and double_t, defined in <math.h>. They correspond to the types used for the intermediate results of floating-point expressions when FLT_EVAL_METHOD is 0, 1, or 2. These types may be wider than long double.

Structures[edit]

Structures are a way of storing multiple pieces of data in one variable. For example, say we wanted to store the name and birthday of a person in strings, in one variable. We could use a structure to house that data:

struct birthday
{
    char name[20];
    int day;
    int month;
    int year;
};

Structures may contain pointers to structs of its own type, which is common in linked data structures.

A C implementation has freedom to design the memory layout of the struct, with few restrictions; one being that the memory address of the first member will be the same as the address of struct itself. Structs may be initialized or assigned to using compound literals.

A user-written function can directly return a structure, though it will often not be very efficient at run-time.

Arrays[edit]

For every type T, except void and function types, there exist the types “array of N elements of type T”.

An array is a collection of values, all of the same type, stored contiguously in memory. An array of size N is indexed by integers from 0 up to and including N-1.

There are also "arrays of unspecified size" where the number of elements is not known by the compiler.

For example:

int cat[10];  // array of 10 elements, each of type int
int bob[];    // array of an unspecified number of 'int' elements.

Arrays can be initialized with a compound initializer, but not assigned. Arrays are passed to functions by passing a pointer to the first element.

Multidimensional arrays are defined as "array of array …". All but the outermost dimension must have compile-time constant size:

int a[10][8];  // array of 10 elements, each of type 'array of 8 int elements'
float f[][32]; // array of unspecified number of 'array of 32 float elements'

Pointer types[edit]

For every type T there exists a type “pointer to T”.

Variables can be declared as being pointers to values of various types, by means of the * type declarator. To declare a variable as a pointer, precede its name with an asterisk.

char *square;
long *circle;

Hence "for every type T" also applies to pointer types there exists multi-indirect pointers like char** or int*** and so on. There exists also "pointer to array" types, but they are less common than "array of pointer", and their syntax is quite confusing:

char *pc[10]; // array of 10 elements of 'pointer to char'
char (*pa)[10]; // pointer to a 10-element array of char

pc consumes 10×sizeof(char*) bytes (usually 40 or 80 bytes on common platforms), but pa is only one pointer, so sizeof(pa) is usually 4 or 8, and the data it refers to is an array of 10 bytes: sizeof(*pa) == 10.

Unions[edit]

Union types are special structures which allow access to the same memory using different type descriptions; one could, for example, describe a union of data types which would allow reading the same data as an integer, a float or a user declared type:

union
{
    int i;
    float f;
    struct
    {
        unsigned int u;
        double d;
    } s;
} u;

In the above example the total size of u is the size of u.s (which happens to be the sum of the sizes of u.s.u and u.s.d), since s is larger than both i and f. When assigning something to u.i, some parts of u.f may be preserved if u.i is smaller than u.f.

Reading from a union member is not the same as casting since the value of the member is not converted, but merely read.

Function pointers[edit]

Function pointers allow referencing functions with a particular signature. For example, to store the address of the standard function abs in the variable my_int_f:

int (*my_int_f)(int) = &abs;
// the & operator can be omitted, but makes clear that the "address of" abs is used here

Function pointers are invoked by name just like normal function calls. Function pointers are separate from pointers and void pointers.

See also[edit]

References[edit]

  1. ^ Barr, Michael (2 December 2007). "Portable Fixed-Width Integers in C". Retrieved 8 November 2011. 
  2. ^ a b ISO/IEC 9899:1999 specification. p. 264, § 7.18 Integer types. 
  3. ^ "64-Bit Programming Models: Why LP64?". The Open Group. Retrieved 9 November 2011.