固定宽度浮动类型在哪里？

发布于 2025-02-02 19:00:47 字数 833 浏览 3 评论 0原文

C标准固定宽度浮点类型是什么？它们在哪里定义？

Misra-C：2004，规则6.3：
typedefs指示大小和签名的应代替基本数值类型。

Misra-C：2004，规则6.3 ISO（POSIX）TYPEDEFS是：

typedef          char   char_t;
typedef signed   char   int8_t;
typedef signed   short  int16_t;  
typedef signed   int    int32_t;
typedef signed   long   int64_t;
typedef unsigned char   uint8_t;
typedef unsigned short  uint16_t;
typedef unsigned int    uint32_t;
typedef unsigned long   uint64_t;
typedef          float  float32_t;
typedef          double float64_t;
typedef long     double float128_t;

在我的公司我们使用C-99。
IAR电子工作台，第8.4版

我们在Misra-C：2004，因为他们投票不升级Misra，这将需要创建验证测试协议并运行协议。

平台是运行Microcos操作系统的ARM Cortex M。

这是详细的问题：

固定宽度浮点类型的类型是什么？
什么包括文件内部（或由编译器定义为默认值）？

原文

What are the C standard fixed width floating point types and where are they defined?

From MISRA-C:2004, Rule 6.3:
typedefs that indicate size and signedness should be used in place of the basic numerical types.

The MISRA-C:2004, Rule 6.3 quotes that ISO (POSIX) typedefs are:

typedef          char   char_t;
typedef signed   char   int8_t;
typedef signed   short  int16_t;  
typedef signed   int    int32_t;
typedef signed   long   int64_t;
typedef unsigned char   uint8_t;
typedef unsigned short  uint16_t;
typedef unsigned int    uint32_t;
typedef unsigned long   uint64_t;
typedef          float  float32_t;
typedef          double float64_t;
typedef long     double float128_t;

At my company we are using C-99.
IAR Electronic Workbench, version 8.4

We are at MISRA-C:2004 because they voted not to upgrade MISRA, which would require create a validation test protocol and running the protocol.

Platform is ARM Cortex M running MicroCOS operating system.

Here are the detailed questions:

What are the typedefs for the fixed width floating point types?
What include file are they inside (or are they defined as default by the compiler)?

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握住你手 2025-02-09 19:00:47

c标准固定宽度浮点类型未定义

c浮点（FP）目标旨在包含变化和许多实现。
MISRA FP目标是限制多样性。

固定尺寸的FP类型不会导致均匀的位编码或其他一致的FP属性。它们在C中的实用性有限 - 因此它们不属于C标准或库的一部分。

fall-back

代码可以在下面使用，_STATIC_ASSERT（自C11）或C99 替代。

typedef      float  float32_t;
typedef      double float64_t;
typedef long double float128_t;

_Static_assert(sizeof(float32_t)*CHAR_BIT == 32, "float 32");
_Static_assert(sizeof(float64_t)*CHAR_BIT == 64, "float 64");
_Static_assert(sizeof(float128_t)*CHAR_BIT == 128, "float 128");

进一步的注释

兼容C可能没有所有32、64、128位FP类型，因此无法定义所有float32_t，float64_t，float128_t。

2种不同的C实现可能具有32位FP类型，但编码不同。比较 float32 。

2不同的C实现可能具有32位FP类型，即使他们的整数Endians同意，但相同的编码也可能具有不同的Endians。

规则6.3（咨询）：应用指示尺寸和签名的类型和签名使用基本的数值类型。：该目标“有助于阐明存储的大小”而不是其他。

规则1.5（咨询）：浮点数应符合定义的浮点点标准。特别难以实现。即使实现使用相同的fp编码与 ieee 754 ，C允许操作足够定义的操作定义了与IEE 754不同的行为。

理想情况下，在C中，符合IEEE 754的实现定义了__ stdc_iec_559 __ __。然而，证明和维持的合格足够挑战，以至于实现可以放弃定义__ stdc_iec_559 __，因为它可能仅为99.999％。

C standard fixed width floating point types are not defined

C floating point (FP) goals are designed to embrace variations and many implementations.
MISRA FP goals are to restrict variety.

Fixed size FP types do not result in uniform bit encoding nor other consistent FP properties. They have limited usefulness in C - hence they are not part of the C standard or library.

Fall-back

Code could use below and a _Static_assert (since C11) or a C99 substitute.

typedef      float  float32_t;
typedef      double float64_t;
typedef long double float128_t;

_Static_assert(sizeof(float32_t)*CHAR_BIT == 32, "float 32");
_Static_assert(sizeof(float64_t)*CHAR_BIT == 64, "float 64");
_Static_assert(sizeof(float128_t)*CHAR_BIT == 128, "float 128");

Further notes

Compliant C may not have all 32, 64, 128 bit FP types, thus unable to define all float32_t, float64_t, float128_t.

2 different Compliant C implementations may have a 32-bit FP types, but different encoding. Compare float32 vs. CCSI resulting in different range, precision and sub-normal support.

2 different Compliant C implementations may have a 32-bit FP types with the same encoding, but different endians, even if their integer endians agree.

Rule 6.3 (advisory): typedefs that indicate size and signedness should be used in place of the basic numerical types.: that goal "helps to clarify the size of the storage" and not much else.

Rule 1.5 (advisory): Floating-point implementations should comply with a defined floating-point standard. is particularly difficult to achieve. Even if an implementation uses the same FP encoding as IEEE 754, C allows the operations enough implementation defined behavior to differ from IEE 754.

Ideally, in C, an implementation that conforms to IEEE 754 defines __STDC_IEC_559__. Yet proving and maintaining conformity is challenging enough that an implementation may forego defining __STDC_IEC_559__ as it may only be 99.999% conforming.

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