MISRA.ASSIGN.COND

Assignment operator is used in a condition.

MISRA C 2012 Rule 13.4: The result of an assignment operator should not be used

C90 [Unspecified 7, 8; Undefined 18], C99 [Unspecified 15, 18; Undefined 32] [Koenig 6]

Category: Advisory

Analysis: Decidable, Single Translation Unit

Applies to: C90, C99

Amplification

This rule applies even if the expression containing the assignment operator is not evaluated.

Rationale

Example

x = y;                          /* Compliant                                   */ 
a[ x ] = a[ x = y ];            /* Non-compliant - the value of x = y* is used */

/*
 * Non-compliant - value of bool_var = false is used but 
 * bool_var == false was probably intended 
 */ 
if ( bool_var = false ) 
{ 
}

/* Non-compliant even though bool_var = true isn't evaluated                    */ 
if ( ( 0u == 0u ) || (bool_var = true ) ) 
{ 
}

/* Non-compliant - value of x = f() is used                                     */ 
if ( ( x = f ( ) ) != 0 ) 
{ 
}

/* Non-compliant - value of b += c is used                                      */ 
a[ b += c ] = a[ b ];

/* Non-compliant - values of c = 0 and b = c = 0 are used                       */ 
a = b = c = 0;

See also

Rule 13.2

MISRA-C 2004 Rule 13.1 (required): Assignment operators shall not be used in expressions that yield a Boolean value.

This rule is also covered by MISRA.ASSIGN.SUBEXPR.

[Koenig 6]

No assignments are permitted in any expression which is considered to have a Boolean value. This precludes the use of both simple and compound assignment operators in the operands of a Boolean-valued expression. However, it does not preclude assigning a Boolean value to a variable.

If assignments are required in the operands of a Boolean-valued expression then they must be performed separately outside of those operands. This helps to avoid getting "=" and "==" confused, and assists the static detection of mistakes.

See "Boolean Expressions" in the glossary.

Example

For example write:

x = y;
if ( x != 0 )
{
   foo();
}

and not:

if ( ( x = y ) != 0 ) /* Boolean by context */
{
   foo();
}

or even worse:

if ( x = y )
{
foo();
}

MISRA-C++ Rule 6-2-1 (required): Assignment operators shall not be used in subexpressions.

This rule is also covered by MISRA.ASSIGN.SUBEXPR.

Rationale

Assignments used in a sub-expression add an additional side effect to that of the full expression, potentially resulting in a value inconsistent with developer expectations. In addition, this helps to avoid getting = and == confused.

Example

x = y;
x = y = z;                 // Non-compliant
if ( x != 0 )              // Compliant
{
   foo ( );
}
bool b1 = x != y;          // Compliant
bool b2;
b2 = x != y;               // Compliant
if ( ( x = y ) != 0 )      // Non-compliant
{
   foo ( );
}
if ( x = y )               // Non-compliant
{
   foo ( );
}
if ( int16_t i = foo ( ) ) // Compliant
{
}