EXP45-C. Do not perform assignments in selection statements

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

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 0, 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 (Unintentional Assignment)

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) {
  /* ... */
}

Compliant Solution (Intentional Assignment)

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 (Unintentional Assignment)

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); 

Compliant Solution (Intentional Assignment)

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

EXP45-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) { /* ... */ } 

EXP45-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)) { /* ... */ } 

EXP45-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