EXP18-C. Do not perform assignments in selection statements

Do not use the assignment operator in the following cases because it typically indicates programmer error and can result in unexpected behavior:

• Controlling expression of if, switch (selection statement)
• Controlling expression of while, do...while (iteration statement) 
• Second operand of for (iteration statement) 
• First operand of  ?: (selection statement)
• Either operand of || or && (logical operators) 
• Second operand of comma operator when the comma expression is used in any of these contexts
• Second and third operands  of ?:  (selection statement) where the ternary expression is used in any of these contexts

Noncompliant Code Example

In this noncompliant code example, an assignment expression is the outermost expression in an if statement.

if (a = b) {
  /* ... */
}

Although the intent of the code may be to assign b to a and test the value of the result for equality to zero, it is very frequently a case of the programmer mistakenly using the assignment operator = instead of the equals operator ==. Consequently, many compilers will warn about this condition, making this coding error detectable by adhering to MSC00-C. Compile cleanly at high warning levels.

Compliant Solution

When the assignment of b to a is not intended, this conditional block is now executed when a is equal to b.

if (a == b) {
  /* ... */
}

When the assignment is intended, the following is an alternative compliant solution:

if ((a = b) != 0) {
  /* ... */
}

It is less desirable in general, depending on what was intended, because it mixes the assignment in the condition, but it is clear that the programmer intended the assignment to occur.

Noncompliant Code Example

In this noncompliant example, the expression x = y is used as the controlling expression of the while statement.

 do { /* ... */ } while ( foo(), x = y ) ;

Compliant Solution

When the assignment of y to x is not intended, this conditional block is now executed when x is equal to y.

do { /* ... */ } while ( foo(), x == y ) ; 

When the assignment is intended, the following is an alternative compliant solution:

do { /* ... */ } while ( foo(), (x = y) != 0) ;

Noncompliant Code Example

In this noncompliant example, the expression p = q is used as the controlling expression of the while statement.

 do { /* ... */ } while ( x = y, p = q ) ;

Compliant Solution

This is a compliant example because the expression x = y is not used as the controlling expression of the while statement.

do { /* ... */ } while ( x = y, p == q ) ; 

Exceptions

EXP18-EX1: Assignment can be used where the result of the assignment is itself a parameter to a comparison expression or relational expression.

In this compliant example, the expression x = y  is itself a parameter to a comparison operation. 

if ( ( x = y )  != 0  ) { /* ... */ } 

EXP18-EX2: Assignment can be used where the expression consists of a single primary expression.

In this compliant example, the expression  x = y is a single primary expression.

if ( ( x = y ) ) { /* ... */ } 

EXP18-EX3: Assignment can be used in the above contexts if it occurs in a function argument or array index.

In this compliant example, the expression x = y is used in a function argument.

if ( foo( x = y ) ) { /* ... */ } 

if ( ( v = w ) && flag ) { /* ... */ }

if ( ( v == w ) && flag ) { /* ... */ }; 

if ( ( (v = w) != 0 ) && flag ) { /* ... */ };

Errors of omission can result in unintended program flow.

Automated Detection

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

Bibliography