C Programming/C Reference/stdint.h

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stdint.h is a header file in the C standard library introduced in the C99 standard library section 7.18 to allow programmers to write more portable code by providing a set of typedefs that specify exact-width integer types, together with the defined minimum and maximum allowable values for each type, using macros[1] . This header is particularly useful for embedded programming which often involves considerable manipulation of hardware specific I/O registers requiring integer data of fixed widths, specific locations and exact alignments. stdint.h (for C or C++), and cstdint (for C++) can be downloaded or quickly created if they are not provided.

The naming convention for exact-width integer types is intN_t for signed int and uintN_t for unsigned int [1] . For example int8_t and uint64_t amongst others could be declared together with defining their corresponding ranges INT8_MIN to INT8_MAX and 0 (zero) to UINT64_MAX; again using a similar but upper case naming convention. In addition stdint.h defines limits of integer types capable of holding object pointers such as UINTPTR_MAX, the value of which depends on the processor and its address range[1].

The exact-width types and their corresponding ranges are only included in that header if they exist for that specific compiler/processor. Note that even on the same processor, two different compiler implementations can differ. The use of #if or #ifdef would allow the inclusion or exclusion of types by the use of compilers preprocessor so that the correct exact-width set is selected for a compiler and its processor target.

The related include file <limits.h> provides macros values for the range limits of common integer variable types. In C <limits.h> is already included in <stdint.h>, but in contrast to <stdint.h> which is implementation independent; all maximum and minimum integer values defined in <limits.h> are compiler implementation specific. For example a compiler generating 32 bit executables will define LONG_MIN as −2,147,483,648 [−231] however for 64 bit processors targets, LONG_MIN can be −9,223,372,036,854,775,808 [−263].

Background[edit]

Corresponding integer types[edit]

The C standard has a notion of "corresponding integer types". Informally, what this means is for any integer type T:

typedef   signed T A;
typedef unsigned T B;

the type A and the type B are said to be corresponding integer types (note: typedef doesn't create a new type, it creates a new identifier as a synonym for the given type). This is important for two reasons:

  • corresponding types are friendly to aliasing and type puns
  • corresponding types have a similar object representation

Both of these combined require code like:

A a = 1;
B b = 1;
*(B*)&a = 0;
*(A*)&b = 0;

to have defined behavior by the standard (as opposed to being undefined in the general case). There are many caveats to how far you can push this, so it's important to actually read the C standard to see what's legal or not (the bulk of this has to deal with padding bits and out of range representations).

Representation[edit]

The C99 standard elaborated the difference between value representations and object representations.

The object representation of an integer consists of 0 or more padding bits, 1 or more value bits[1], and either 0 or 1 sign bits (this doesn't count as a value bit) depending on the signedness of the integer type.

The value representation is a conceptual representation of an integer. The value representation ignores any padding bits and does a (possible) rearrangement to the bits so that the integer is ordered sequentially from most significant value bit to least significant value bit. Most programmers deal with this representation because it allows them easily to write portable code by only dealing with −0 and out of range values as opposed to both of those in addition to tricky aliasing rules and trap representations if they choose to deal with the object representation directly.

Signed representation[edit]

The C standard allows for only three signed integer representations specified by the compiler writer:

Integer types[edit]

The types <something>_t and u<something>_t are required to be corresponding signed and unsigned integer types. For the types that are marked optional, an implementation must either define both <something>_t and u<something>_t or neither of the two. The limits of these types shall be defined with macros with a similar name in the same fashion as described below.

If a type is of the form [u]<something>N_t (or similarly for a preprocessor define), N must be a positive decimal integer with no leading 0's.

Exact-width integer types[edit]

These are of the form intN_t and uintN_t. Both types must be represented by exactly N bits with no padding bits. intN_t must be encoded as a two's complement signed integer and uintN_t as an unsigned integer. These types are optional unless the implementation supports types with widths of 8, 16, 32 or 64, then it shall typedef them to the corresponding types with corresponding N. Any other N is optional[1] .

Specific integral type limits
Specifier Signing Bits Bytes Minimum Value Maximum Value
int8_t Signed 8 1 −27 which equals −128 27 − 1 which is equal to 127
uint8_t Unsigned 8 1 0 28 − 1 which equals 255
int16_t Signed 16 2 −215 which equals −32,768 215 − 1 which equals 32,767
uint16_t Unsigned 16 2 0 216 − 1 which equals 65,535
int32_t Signed 32 4 −231 which equals −2,147,483,648 231 − 1 which equals 2,147,483,647
uint32_t Unsigned 32 4 0 232 − 1 which equals 4,294,967,295
int64_t Signed 64 8 −263 which equals −9,223,372,036,854,775,808 263 − 1 which equals 9,223,372,036,854,775,807
uint64_t Unsigned 64 8 0 264 − 1 which equals 18,446,744,073,709,551,615

The limits of these types are defined with macros with the following formats:

  • INTN_MAX is the maximum value (2N−1 − 1) of the signed version of intN_t.
  • INTN_MIN is the minimum value (−2N−1) of the signed version of intN_t.
  • UINTN_MAX is the maximum value (2N – 1) of the unsigned version of uintN_t.

Minimum-width integer types[edit]

These are of the form int_leastN_t and uint_leastN_t. int_leastN_t is a signed integer and uint_leastN_t is an unsigned integer[1] .

The standard mandates that these have widths greater than or equal to N, and that no smaller type with the same signedness has N or more bits. For example, if a system provided only a uint32_t and uint64_t, uint_least16_t must be equivalent to uint32_t.

An implementation is required to define these for the following N: 8, 16, 32, 64. Any other N is optional.

The limits of these types are defined with macros with the following formats:

  • INT_LEASTN_MAX is the maximum value (2N−1 − 1 or greater) of the signed version of int_leastN_t.
  • INT_LEASTN_MIN is the minimum value (−2N−1 + 1 or less) of the signed version of int_leastN_t.
  • UINT_LEASTN_MAX is the maximum value (2N − 1 or greater) of the unsigned version of uint_leastN_t .

stdint.h should also define macros which will convert constant decimal, octal or hexadecimal value which are guaranteed to be suitable for the corresponding types and to be usable with the #if:

  • INTN_C(value) is substituted for a value suitable for int_leastN_t. For example if int_least64_t is "typedefed" to signed long long int, INT64_C(123) corresponds to 123LL.
  • UINTN_C(value) is substituted for a value suitable for uint_leastN_t.

Fastest minimum-width integer types[edit]

These are of the form int_fastN_t and uint_fastN_t.

The standard does not mandate anything about these types except that their widths must be greater than or equal to N. It also leaves it up to the implementor to decide what it means to be a "fast" integer type.

An implementation is required to define these for the following N: 8, 16, 32, 64[2].

The limits of these types are defined with macros with the following formats:

  • INT_FASTN_MAX is the maximum value (2N−1 − 1 or greater) of the signed version of int_fastN_t.
  • INT_FASTN_MIN is the minimum value (−2N−1 + 1 or less) of the signed version of int_fastN_t.
  • UINT_FASTN_MAX is the maximum value (2N − 1 or greater) of the unsigned version of uint_fastN_t [1] .

Integers wide enough to hold pointers[edit]

intptr_t and uintptr_t is a signed and unsigned integer which are guaranteed to hold the value of a pointer. These two types are optional.

The limits of these types are defined with the following macros:

  • INTPTR_MIN is the minimum value (−32,767 [−215 + 1] or less) of intptr_t.
  • INTPTR_MAX is the maximum value (32,767 [215 − 1] or greater) of intptr_t.
  • UINTPTR_MAX is the maximum value (65,535 [216 − 1] or greater) of uintptr_t[3].

Greatest-width integer types[edit]

intmax_t and uintmax_t is a signed and unsigned integer which are of the greatest supported width. They are in other words the integer types which have the greatest limits.

The limits of these types are defined with macros with the following formats:

  • INTMAX_MAX is the maximum value (9,223,372,036,854,775,807 [263 − 1] or greater) of the signed version of intmax_t.
  • INTMAX_MIN is the minimum value (−9,223,372,036,854,775,807 [−263 + 1] or less) of the signed version of intmax_t.
  • UINTMAX_MAX is the maximum value (18,446,744,073,709,551,615 [264 − 1] or greater) of the unsigned version of uintmax_t.

Macros which will convert constant decimal, octal or hexadecimal value which will suit the corresponding type are also defined:

  • INTMAX_C(value) is substituted for a value suitable for intmax_t.
  • UINTMAX_C(value) is substituted for a value suitable for uintmax_t[1] .

Other integer limits[edit]

  • PTRDIFF_MIN is the minimum value of ptrdiff_t.
  • PTRDIFF_MAX is the maximum value of ptrdiff_t.
  • SIZE_MAX is the maximum value (216 − 1 or greater) of size_t.
  • WCHAR_MIN is the minimum value of wchar_t.
  • WCHAR_MAX is the maximum value of wchar_t.
  • WINT_MIN is the minimum value of wint_t.
  • WINT_MAX is the maximum value of wint_t.
  • SIG_ATOMIC_MIN is the minimum value of sig_atomic_t.
  • SIG_ATOMIC_MAX is the maximum value of sig_atomic_t.

Criticisms and caveats[edit]

  • Some (non-conforming) implementations tack C99 support on top of a C89 runtime library.[citation needed] One of the consequences of this is that the new printf and scanf specifiers aren't recognized and will probably lead to something undefined. The typical ways of working around this are:
    • The most common (and the most wrong) way is to use the long or unsigned long types as an intermediate step and pass these types into printf or scanf. This works reasonably well for the exact, minimum, and fast integer types less than 32-bits but may cause trouble with ptrdiff_t and size_t and the types larger than 32-bits, typically on platforms that use 32-bit longs and 64-bit pointers.
    • Not using scanf directly but manually reading in a buffer, calling strto[i|u]max, and then converting it to the desired type. This doesn't help with printing out integers though.
    • Using a 3rd-party printf and scanf library that is C99 compatible.
    • Using the C99 standard printing format specifiers. PRId64 for example. These are declared in inttypes.h.
  • The rules for integer rank and corresponding integer types may force implementers to choose the lesser of two evils in not supporting an integer type, making a bad compromise, or supporting an integer type a non-conforming way.
    • For example, there are machines that either have special support for an extremely large signed integer register or an extremely large unsigned integer register without supporting the other type.[citation needed] An implementation can either choose not to expose this to a C implementation, synthesize a slow type as the corresponding integer type, synthesizing a weird corresponding integer type, or expose the integer to the programmer without setting it to the [u]intmax_t types or synthesizing a corresponding integer type.
  • The [u]intN_t types are a compromise between the desire to have guaranteed two's complement integer types and the desire to have guaranteed types with no padding bits (as opposed to a more fine grained approach which would define more types). Because of the "all or nothing" approach to the [u]intN_t types, an implementation might have to play the same sort of games described above depending on whether they care about speed, programmer convenience, or standards conformance.

See also[edit]

Notes and references[edit]

  1. a b c d e f g h technically, it actually allows 0 or more value bits, but the only way you can construct this is with a single-bit bit-field of a signed integer
  2. http://www.tuxgraphics.org/common/src2/article09043/avr-libc-user-manual-1.6.4/group__avr__stdint.html
  3. http://linux.die.net/man/3/intptr_t

External links[edit]

As stdint.h is not shipped with older C++ compilers and Visual Studio C++ products prior to Visual Studio 2010, third-party implementations are available:

  • pstdint.h – A cross-platform, free implementation from Paul Hsieh. This implementation was tested on the following compilers, all with 0 warnings at their highest respective settings: Borland Turbo C 2.0, WATCOM C/C++ 11.0 (16 bits and 32bits), Microsoft Visual C++ 6.0 (32 bit), Microsoft Visual Studio.net (VC7), Intel C++ 4.0, GNU gcc v3.3.3.
  • msinttypes – ISO C9x compliant stdint.h and inttypes.h for Microsoft Visual Studio are available on Google Code. This implementation was tested with Microsoft Visual Studio 6.0, Microsoft Visual Studio .NET 2003, Microsoft Visual Studio 2005, and Microsoft Visual Studio 2008.
  • boost/cstdint.hpp – This file from the Boost library contains data types which are in stdint.h. The advantage of the use of <boost/cstdint.hpp> is that it can be used on many platforms. The code becomes naturally portable, and can be compiled on any platform without changes whenever the boost library can be used.