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Do not use the assignment operator in the following cases because it contexts listed in the following table because doing so typically indicates programmer error and can result in unexpected behavior:.

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Performing assignment statements in other contexts do not violate this rule. However, they may violate other rules, such as EXP30-C. Do not depend on the order of evaluation for side effects.

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 zero0, 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 the 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 solutionthis compliant solution explicitly uses inequality as the outermost expression while performing the assignment in the inner expression:

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 code 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 the conditional block is now should be executed only when x is equal to y., as in this compliant solution:

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

Compliant Solution (Intentional Assignment)

When the assignment is intended, the following is an alternative this compliant solution can be used:

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

Compliant Solution (for statement)

The same result can be obtained using the for statement, which is specifically designed to evaluate an expression on each iteration of the loop, just before performing the test in its controlling expression. Remember that its controlling expression is the second operand, where the assignment occurs in its third operand:

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

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 In this compliant solution, the expression x = y is not used as the controlling expression of the while statement.:

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

Noncompliant Code Example

while 

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(ch = '\t' || ch == ' ' || ch == '\n') {
  /* ... */
}

while ('\t' = ch || ' ' == ch || '\n' == ch) {
  /* ... */
}

while ('\t' == ch || ' ' == ch || '\n' == ch) {
  

Exceptions

EXP18-EXP1: 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. 

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/* ... */

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}

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if (

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(

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x = y) 

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!= 0) { /* ... */ } 

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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) { /* ... */ }; 

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

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

Key here (explains table format and definitions)

Bibliography

CERT-CWE Mapping Notes

Key here for mapping notes

CWE-480 and EXP45-C

Intersection( EXP45-C, EXP46-C) = Ø

CWE-480 = Union( EXP45-C, list) where list =

• Usage of incorrect operator besides s/=/==/

CWE-569 and EXP45-C

CWE-480 = Subset( CWE-569)

Bibliography

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