C data types

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.
The C language provides the four basic arithmetic type specifiers char, int, float and double (as well as the boolean type bool), and the modifiers signed, unsigned, short, and long. The following table lists the permissible combinations in specifying a large set of storage size-specific declarations.
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 bit-fields) 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 64-bit. 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.
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 MS-DOS, modern Linux, and Microchip MCC18 for embedded 8-bit 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 non-bit-field objects: its lack of padding bits and trap representations, the definition of object representation, and the possibility of aliasing.
The actual size and behavior of floating-point types also vary by implementation. The only requirement 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 for float and double respectively.
The C99 standard includes new real 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.
C99 also added complex types: float _Complex, double _Complex, long double _Complex. C11 added imaginary types (which were described in an informative annex of C99): float _Imaginary, double _Imaginary, long double _Imaginary. Including the header <complex.h> allows all these types to be accessed with using complex and imaginary respectively.
C99 added a Boolean data 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:
In C23, the boolean type was moved to bool, making the <stdbool.h> header now useless.
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:
The type _Bool also ensures true values always compare equal to each other:
In C23, bool became a core functionality of the language, allowing for the following examples of code:
Since C23, the language allows the programmer to define integers that have a width of an arbitrary number of bits. Those types are specified as _BitInt(N), where N is an integer constant expression that denotes the number of bits, including the sign bit for signed types, represented in two's complement. The maximum value of N is provided by BITINT_MAXWIDTH and is at least ULLONG_WIDTH. Therefore, the type _BitInt(2) (or signed _BitInt(2)) takes values from −2 to 1 while unsigned _BitInt(2) takes values from 0 to 3. The type unsigned _BitInt(1) also exists, being either 0 or 1 and has no equivalent signed type.
The C language specification includes the typedefs size_t and ptrdiff_t to represent memory-related 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 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 implementation-defined.
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.
The C99 standard includes definitions of several new integer types to enhance the portability of programs. 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:
The following table summarizes the types and the interface to acquire the implementation details (n refers to the number of bits):
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.
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.
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.
Similarly to the fixed-width integer types, ISO/IEC TS 18661 specifies floating-point types for IEEE 754 interchange and extended formats in binary and decimal:
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.
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 run-time. 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:
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:
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 compile-time constant size:
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.
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:
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).
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:
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 allow referencing functions with a particular signature. For example, to store the address of the standard function abs in the variable my_int_f:
Function pointers are invoked by name just like normal function calls. Function pointers are separate from pointers and void pointers.
The aforementioned types can be characterized further by type qualifiers, yielding a qualified type. As of 2014 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, but the more ordinary name atomic can be used if the <stdatomic.h> header is included. Of these, const is by far the best-known and most used, appearing in the standard library and encountered in any significant use of the C language, which must satisfy const-correctness. The other qualifiers are used for low-level programming, and while widely used there, are rarely used by typical programmers.