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V2572. MISRA. Value of the expression s…
Analyzer diagnostics
General Analysis (C++)
General Analysis (C#)
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Diagnosis of micro-optimizations (C++)
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V2572. MISRA. Value of the expression should not be converted to the different essential type or the narrower essential type.

23 Déc 2019

This diagnostic rule is based on the software development guidelines developed by MISRA (Motor Industry Software Reliability Association).

This diagnostic rule applies only to code written in C.

The C language allows much freedom in casting between arithmetic types, but it can also lead to hidden problems such as loss of sign, value, or precision.

The MISRA standard defines an essential type model, where variables can have the following types:

  • Boolean for Boolean values true/false: '_Bool';
  • signed for signed integers or unnamed enums: 'signed char', 'signed short', 'signed int', 'signed long', 'signed long long', 'enum { .... };';
  • unsigned for unsigned integers: 'unsigned char', 'unsigned short', 'unsigned int', 'unsigned long', 'unsigned long long';
  • floating for floating-point values: 'float', 'double', 'long double';
  • character for characters only: 'char';
  • named enum for a named set of user-defined values: 'enum name { .... };'.

This model does not include pointers.

Following the essential type model can help to avoid many of the non-obvious issues mentioned above by assigning values of the same essential type to variables. Within this model, you are allowed to assign a value of a narrower essential type to a variable of a wider type. Implicit conversions between different essential types are forbidden.

Exceptions:

  • A non-negative constant expression of an essential signed type can be assigned to an object of an essential unsigned type if its value can be represented by this type.
  • The '{ 0 }' initializer can be used to initialize an aggregate type or union.

Example of non-compliant code:

typedef enum ENUM {ONE} ENUM;

void Positive(signed char x)
{
  unsigned char      uchr = x; // <=
  unsigned short     usht = x; // <=
  unsigned int        uit = x; // <=
  unsigned long       ulg = x; // <=
  unsigned long long ullg = x; // <=

  long double ld = 0.0;
  double d = ld;        // <=
  float f = d;          // <=

  ENUM e = x; // <=
}

Fixed code:

enum {ONE = 1, TWO, THREE, FOUR, FIVE, SIX, 
      MUCH = 123123, MORE = 0x7FFFFFFF-1};

void Negative()
{
  signed char c = ONE;        // ok
  signed short h = TWO;       // ok
  signed int i = THREE;       // ok
  signed long long ll = FOUR; // ok

  unsigned char uc = FIVE;       // ok
  unsigned short uh = SIX;       // ok
  unsigned int ui = MUCH;        // ok
  unsigned long long ull = MORE; // ok

  float f = 0.0f;     // ok
  double d = f;       // ok
  long double ld = d; // ok

  ENUM e = c; // ok
}

This diagnostic is classified as:

  • MISRA-C-10.3
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