MISRA C:2023 Rule 13.4

The result of an assignment operator should not be used

Since R2024a

Description

Rule Definition

The result of an assignment operator should not be used.

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 R2024a

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.