MISRA C:2012 Rule 13.4

The result of an assignment operator should not be used

Description

Rule Definition

The result of an assignment operator should not be used¹ .

Rationale

The rule is violated if the following happens in the same expression:

• The assignment operator acts on a variable.

• Another read or operation is performed on the result of the assignment.

For example, the expression a[x]=a[x=y]; violates this rule. The [] operator acts on the result of the assignment x=y.

Troubleshooting

If you expect a rule violation but do not see it, refer to Diagnose Why Coding Standard Violations Do Not Appear as Expected.

Examples

Result of Assignment Used

int x, y, b, c, d;
int a[10];
unsigned int bool_var, false=0, true=1;

int foo(void) {

    x = y;            /* Compliant - x is not used */

    a[x] = a[x = y];  /* Non-compliant - Value of x=y is used */

    if ( bool_var = false )/* Non-compliant - bool_var=false is used */
{}


    if ( bool_var == false ) {}   /* Compliant */

    if ( ( 0u == 0u ) || ( bool_var = true ) )/* Non-compliant */
                    /*- even though (bool_var=true) is not evaluated */
 {} 
    

    if ( ( x = f () ) != 0 )/* Non-compliant - value of x=f() is used */
 {} 
    a[b += c] = a[b];/* Non-compliant - value of b += c is used */
    b = c = d = 0; /* Non-compliant - value of d=0 and c=d=0 are used */

}

In this example, the rule is violated when the result of an assignment is used.

Check Information

Group: Side Effects

Category: Advisory

AGC Category: Advisory

Version History

Introduced in R2014b

R2024b: More accurate analysis resulting in fewer false positive violations

Polyspace^® performs a more accurate analysis that reports fewer false positive violations. For example:

• Polyspace ignores compiler generated artifacts that previously resulted in false positives.

• Polyspace recognizes assignment operations in a ternary (?:) operator. In this example, the assignment operations (OBJ = X) and (OBJ_1 = X) are compliant.

  #define U8_T unsigned char
  #define S16 signed char
  
  #define SET_CC(obj,id,x) ( ((U8_T)id == (U8_T)0) ? ((obj)=x) : ((obj##_1)=x) ) /*Compliant*/
  
  #define ID 0
  
  S16 OBJ;
  S16 OBJ_1;
  S16 X;
  
  void g3245608_bis(S16 p) {
  	((U8_T)ID == (U8_T)0) ?
  	(OBJ = X)                        /*Compliant*/
  	:
  	(OBJ_1 = X);                     /*Compliant*/
  }

  The result of the assignment operators are not used in the ternary operation.