V2543. MISRA. Value of the essential character type should be used appropriately in the addition/subtraction operations.

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

This diagnostic rule is only relevant for C. Values of the essential character type should not be used in arithmetic expressions.

The MISRA standard defines the following essential type model, in which a variable may have a type:

• Boolean, if it operates true/false values: '_Bool';
• signed, if operates signed integer numbers, or is an unnamed enum: 'signed char', 'signed short', 'signed int', 'signed long', 'signed long long', 'enum { .... }';
• unsigned, if operates unsigned integer numbers: 'unsigned char', 'unsigned short', 'unsigned int', 'unsigned long', 'unsigned long long';
• floating, if operates floating point numbers: 'float', 'double', 'long double';
• character, if operates only characters: 'char';
• Named enum, if operates a named set of user-specific values: 'enum name { .... };'.

There are no pointers in this model.

According to the essential type model, essential character type values mustn't be used in arithmetic expressions, as they are represented by a non-numerical type.

Let's see the list of correct ways of using character-type variables in arithmetic expressions:

• When adding, one operand must have the character type, another one - of signed or unsigned integer type. The result of such operation has the character type:
  • character + [un]signed => character; (1)
  • [un]signed + character => character; (2)
• When subtracting, the left operand must have character type and the right operand - the signed or unsigned integer type. The result of this operation will be a value of character type:
  • character - [un]signed => character; (3)
• When subtracting, both operands must have character type. The result of such operation will be a value of signed integer type:
  • character - character => signed; (4)

An example of the code for which the analyzer will issue warnings:

void foo(char ch, unsigned ui, float f, _Bool b, enum A eA)
{
  ch + f; // Essential character type should not be used in
// the addition operation with expression
// of the essential floating type

ch + b; // Also relates to the essential Boolean

ch + eA; // Also relates to the essential enum <A> type

(ch + ui) + (ch - 6); // After the expressions in parentheses
// have been executed, both operands of the
// essential character type are used
// in addition operation
}

This diagnostic is classified as: