C standard library 

General topics 
Miscellaneous headers 
In the C programming language, data types constitute the semantics and characteristics of storage of data elements. They are expressed in the language syntax in form of declarations for memory locations or variables. Data types also determine the types of operations or methods of processing of data elements.
The C language provides basic arithmetic types, such as integer and real number types, and syntax to build array and compound types. Headers for the C standard library, to be used via include directives, contain definitions of support types, that have additional properties, such as providing storage with an exact size, independent of the language implementation on specific hardware platforms.^{[1]}^{[2]}
Contents
Basic types[edit]
Main types[edit]
The C language provides the four basic arithmetic type specifiers char, int, float and double, and the modifiers signed, unsigned, short, and long. The following table lists the permissible combinations in specifying a large set of storage sizespecific declarations.
Type  Explanation  Format specifier 

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. It contains CHAR_BIT bits.^{[3]}  %c 
signed char  Of the same size as char, but guaranteed to be signed. Capable of containing at least the [−127, +127] range;^{[3]}^{[note 1]}  %c (or %hhi for numerical output) 
unsigned char  Of the same size as char, but guaranteed to be unsigned. Contains at least the [0, 255] range.^{[4]}  %c (or %hhu for numerical output) 
short short int signed short signed short int  Short signed integer type. Capable of containing at least the [−32,767, +32,767] range;^{[3]}^{[note 1]} thus, it is at least 16 bits in size. The negative value is −32767 (not −32768) due to the one'scomplement and signmagnitude representations allowed by the standard, though the two'scomplement representation is much more common.^{[5]}  %hi 
unsigned short unsigned short int  Short unsigned integer type. Contains at least the [0, 65,535] range;^{[3]}  %hu 
int signed signed int  Basic signed integer type. Capable of containing at least the [−32,767, +32,767] range;^{[3]}^{[note 1]} thus, it is at least 16 bits in size.  %i or %d 
unsigned unsigned int  Basic unsigned integer type. Contains at least the [0, 65,535] range;^{[3]}  %u 
long long int signed long signed long int  Long signed integer type. Capable of containing at least the [−2,147,483,647, +2,147,483,647] range;^{[3]}^{[note 1]} thus, it is at least 32 bits in size.  %li 
unsigned long unsigned long int  Long unsigned integer type. Capable of containing at least the [0, 4,294,967,295] range;^{[3]}  %lu 
long long long long int signed long long signed long long int  Long long signed integer type. Capable of containing at least the [−9,223,372,036,854,775,807, +9,223,372,036,854,775,807] range;^{[3]}^{[note 1]} thus, it is at least 64 bits in size. Specified since the C99 version of the standard.  %lli 
unsigned long long unsigned long long int  Long long unsigned integer type. Contains at least the [0, +18,446,744,073,709,551,615] range;^{[3]} Specified since the C99 version of the standard.  %llu 
float  Real floatingpoint type, usually referred to as a singleprecision floatingpoint type. Actual properties unspecified (except minimum limits), however on most systems this is the IEEE 754 singleprecision binary floatingpoint format (32 bits). This format is required by the optional Annex F "IEC 60559 floatingpoint arithmetic".  Converting from text:^{[a]}

double  Real floatingpoint type, usually referred to as a doubleprecision floatingpoint type. Actual properties unspecified (except minimum limits), however on most systems this is the IEEE 754 doubleprecision binary floatingpoint format (64 bits). This format is required by the optional Annex F "IEC 60559 floatingpoint arithmetic". 

long double  Real floatingpoint type, usually mapped to an extended precision floatingpoint number format. Actual properties unspecified. It can be either x86 extendedprecision floatingpoint format (80 bits, but typically 96 bits or 128 bits in memory with padding bytes), the nonIEEE "doubledouble" (128 bits), IEEE 754 quadrupleprecision floatingpoint format (128 bits), or the same as double. See the article on long double for details.  %Lf %LF %Lg %LG %Le %LE %La %LA ^{[b]} 
The actual size of the integer types varies by implementation. The standard requires only 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, no other data types (except bitfields) can be smaller.
The minimum size for char
is 8 bits, the minimum size for short
and int
is 16 bits, for long
it is 32 bits and long long
must contain at least 64 bits.
The type int
should be the integer type that the target processor is most efficiently working with. This allows great flexibility: for example, all types can be 64bit. However, several different integer width schemes (data models) are popular. Because the data model defines how different programs communicate, a uniform data model is used within a given operating system application interface.^{[6]}
In practice, 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 MSDOS, modern Linux, and Microchip MCC18 for embedded 8bit PIC microcontrollers. POSIX requires char
to be exactly 8 bits in size.
Various rules in the C standard make unsigned char
the basic type used for arrays suitable to store arbitrary nonbitfield objects: its lack of padding bits and trap representations, the definition of object representation,^{[4]} and the possibility of aliasing.^{[7]}
The actual size and behavior of floatingpoint 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 32bit and 64bit IEEE 754 binary floatingpoint formats are used.
The C99 standard includes new real floatingpoint types float_t
and double_t
, defined in <math.h>
. They correspond to the types used for the intermediate results of floatingpoint expressions when FLT_EVAL_METHOD
is 0, 1, or 2. These types may be wider than long double
.
C99 also added complex types: float _Complex
, double _Complex
, long double _Complex
.
Boolean type[edit]
C99 added a boolean (true/false) type _Bool
. Additionally, the <stdbool.h>
header defines bool
as a convenient alias for this type, and also provides macros for true
and false
. _Bool
functions similarly to a normal integer type, with one exception: any assignments to a _Bool
that are not 0 (false) are stored as 1 (true). This behavior exists to avoid integer overflows in implicit narrowing conversions. For example, in the following code:
unsigned char b = 256; if (b) { /* do something */ }
Variable b
evaluates to false if unsigned char
has a size of 8 bits. This is because the value 256 does not fit in the data type, which results in the lower 8 bits of it being used, resulting in a zero value. However, changing the type causes the previous code to behave normally:
_Bool b = 256; if (b) { /* do something */ }
The type _Bool also ensures true values always compare equal to each other:
_Bool a = 1, b = 2; if (a == b) { /* do something */ }
Size and pointer difference types[edit]
The C language specification includes the typedefs size_t
and ptrdiff_t
to represent memoryrelated quantities. 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.h
in C++).
size_t
is an unsigned integer type used to represent the size of any object (including arrays) in the particular implementation. The operator sizeof yields a value of the type size_t
. 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++). size_t
is guaranteed to be at least 16 bits wide. Additionally, POSIX includes ssize_t
, which is a signed integer type of the same width as size_t
.
ptrdiff_t
is a signed integer type used to represent the difference between pointers. It is guaranteed to be valid only against pointers of the same type; subtraction of pointers consisting of different types is implementationdefined.
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 floatingpoint types. The actual values depend on the implementation.
Properties of integer types[edit]
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 longSCHAR_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 longUCHAR_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 longCHAR_MIN
– minimum possible value of charCHAR_MAX
– maximum possible value of charMB_LEN_MAX
– maximum number of bytes in a multibyte character
Properties of floatingpoint types[edit]
FLT_MIN
,DBL_MIN
,LDBL_MIN
– minimum normalized positive value of float, double, long double respectivelyFLT_TRUE_MIN
,DBL_TRUE_MIN
,LDBL_TRUE_MIN
(C11) – minimum positive value of float, double, long double respectivelyFLT_MAX
,DBL_MAX
,LDBL_MAX
– maximum finite value of float, double, long double, respectivelyFLT_ROUNDS
– rounding mode for floatingpoint operationsFLT_EVAL_METHOD
(C99) – evaluation method of expressions involving different floatingpoint typesFLT_RADIX
– radix of the exponent in the floatingpoint typesFLT_DIG
,DBL_DIG
,LDBL_DIG
– number of decimal digits that can be represented without losing precision by float, double, long double, respectivelyFLT_EPSILON
,DBL_EPSILON
,LDBL_EPSILON
– difference between 1.0 and the next representable value of float, double, long double, respectivelyFLT_MANT_DIG
,DBL_MANT_DIG
,LDBL_MANT_DIG
– number ofFLT_RADIX
base digits in the floatingpoint significand for types float, double, long double, respectivelyFLT_MIN_EXP
,DBL_MIN_EXP
,LDBL_MIN_EXP
– minimum negative integer such thatFLT_RADIX
raised to a power one less than that number is a normalized float, double, long double, respectivelyFLT_MIN_10_EXP
,DBL_MIN_10_EXP
,LDBL_MIN_10_EXP
– minimum negative integer such that 10 raised to that power is a normalized float, double, long double, respectivelyFLT_MAX_EXP
,DBL_MAX_EXP
,LDBL_MAX_EXP
– maximum positive integer such thatFLT_RADIX
raised to a power one less than that number is a normalized float, double, long double, respectivelyFLT_MAX_10_EXP
,DBL_MAX_10_EXP
,LDBL_MAX_10_EXP
– maximum positive integer such that 10 raised to that power is a normalized float, double, long double, respectivelyDECIMAL_DIG
(C99) – minimum number of decimal digits such that any number of the widest supported floatingpoint type can be represented in decimal with a precision ofDECIMAL_DIG
digits and read back in the original floatingpoint type without changing its value.DECIMAL_DIG
is at least 10.
Fixedwidth 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 usually supports only several types and that support varies between different environments. 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:
 Exactwidth integer types that are guaranteed to have the same number n of bits across all implementations. Included only if it is available in the implementation.
 Leastwidth integer types that 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 that 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 that are guaranteed to be able to hold a pointer. Included only if it is available in the implementation.
 Maximumwidth integer types that 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
in C++) provides features that enhance the functionality of the types defined in the <stdint.h>
header. It defines macros for printf format string and scanf format string specifiers corresponding to the types defined in <stdint.h>
and several functions for working with the intmax_t
and uintmax_t
types. This header was added in C99.
 Printf format string
The macros are in the format PRI
. Here defines the output formatting and is one of d
(decimal), x
(hexadecimal), o
(octal), u
(unsigned) and i
(integer). 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
. Here defines the output formatting and is one of d
(decimal), x
(hexadecimal), o
(octal), u
(unsigned) and i
(integer). 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
This section needs expansion. You can help by adding to it. (October 2011) 
Additional floatingpoint types[edit]
Similarly to the fixedwidth integer types, ISO/IEC TS 18661 specifies floatingpoint types for IEEE 754 interchange and extended formats in binary and decimal:
_FloatN
for binary interchange formats;_DecimalN
for decimal interchange formats;_FloatNx
for binary extended formats;_DecimalNx
for decimal extended formats.
Structures[edit]
Structures aggregate the storage of multiple data items, of potentially differing data types, into one memory block referenced by a single variable. The following example declares the data type struct birthday
which contains the name and birthday of a person. The structure definition is followed by a declaration of the variable John
that allocates the needed storage.
struct birthday { char name[20]; int day; int month; int year; }; struct birthday John;
The memory layout of a structure is a language implementation issue for each platform, with a few restrictions. The memory address of the first member must be the same as the address of structure itself. Structures may be initialized or assigned to using compound literals. A function may directly return a structure, although this is often not efficient at runtime. Since C99, a structure may also end with a flexible array member.
A structure containing a pointer to a structure of its own type is commonly used to build linked data structures:
struct node { int val; struct node *next; };
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
. Here is a brief example:
int cat[10]; // array of 10 elements, each of type int
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 …", and all except the outermost dimension must have compiletime constant size:
int a[10][8]; // array of 10 elements, each of type 'array of 8 int elements'
Pointers[edit]
Every data type T
has a corresponding type pointer to T
. A pointer is a data type that contains the address of a storage location of a variable of a particular type. They are declared with the asterisk (*
) type declarator following the basic storage type and preceding the variable name. Whitespace before or after the asterisk is optional.
char *square; long *circle; int *oval;
Pointers may also be declared for pointer data types, thus creating multiple indirect pointers, such as char ** and int ***, including pointers to array types. The latter are less common than an array of pointers, and their syntax may be confusing:
char *pc[10]; // array of 10 elements of 'pointer to char' char (*pa)[10]; // pointer to a 10element array of char
The element pc
requires ten blocks of memory of the size of pointer to char
(usually 40 or 80 bytes on common platforms), but element pa
is only one pointer (size 4 or 8 bytes), and the data it refers to is an array of ten bytes (sizeof *pa == 10
).
Unions[edit]
A union type is a special construct that permits access to the same memory block by using a choice of differing type descriptions. For example, a union of data types may be declared to permit reading the same data either as an integer, a float, or any other user declared type:
union { int i; float f; struct { unsigned int u; double d; } s; } u;
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.
Type qualifiers[edit]
The aforementioned types can be characterized further by type qualifiers, yielding a qualified type. As of 2014^{[update]} and C11, there are four type qualifiers in standard C: const
(C89), volatile
(C89), restrict
(C99) and _Atomic
(C11) – the latter has a private name to avoid clashing with user names,^{[8]} but the more ordinary name atomic
can be used if the <stdatomic.h>
header is included. Of these, const
is by far the bestknown and most used, appearing in the standard library and encountered in any significant use of the C language, which must satisfy constcorrectness. The other qualifiers are used for lowlevel programming, and while widely used there, are rarely used by typical programmers.^{[citation needed]}
See also[edit]
The Wikibook C Programming has a page on the topic of: C Programming 
Notes[edit]
 ^ ^{a} ^{b} ^{c} ^{d} ^{e} The minimal ranges −(2^{n−1}−1) to 2^{n−1}−1 come from the various integer representations allowed by the standard. However, most platforms use two's complement, implying a range of the form −2^{m−1} to 2^{m−1}−1 with m ≥ n for these implementations, e.g. [−128,127] (SCHAR_MIN = −128 and SCHAR_MAX = 127) for an 8bit signed char.
References[edit]
 ^ Barr, Michael (2 December 2007). "Portable FixedWidth Integers in C". Retrieved 18 January 2016.
 ^ ^{a} ^{b} ISO/IEC 9899:1999 specification, TC3 (PDF). p. 255, § 7.18 Integer types <stdint.h>.
 ^ ^{a} ^{b} ^{c} ^{d} ^{e} ^{f} ^{g} ^{h} ^{i} ^{j} ISO/IEC 9899:1999 specification, TC3 (PDF). p. 22, § 5.2.4.2.1 Sizes of integer types <limits.h>.
 ^ ^{a} ^{b} ISO/IEC 9899:1999 specification, TC3 (PDF). p. 37, § 6.2.6.1 Representations of types — General.
 ^ Rationale for International Standard—Programming Languages—C Revision 5.10 (PDF). p. 25, § 5.2.4.2.1 Sizes of integer types <limits.h>.
 ^ "64Bit Programming Models: Why LP64?". The Open Group. Retrieved 9 November 2011.
 ^ ISO/IEC 9899:1999 specification, TC3 (PDF). p. 67, § 6.5 Expressions.
 ^ C11:The New C Standard, Thomas Plum