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

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 0, it is 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, the conditional block is now executed when a is equal to b:

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

Compliant Solution (Intentional Assignment)

When the assignment is intended, this 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);

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:

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

Compliant Solution (Unintentional Assignment)

When the assignment of y to x is not intended, the conditional block 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, this compliant solution can be used:

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

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

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

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

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

if ((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

Tool	Version	Checker	Description
Astrée			Supported, but no explicit checker
Axivion Bauhaus Suite		CertC-EXP45
Clang		-Wparentheses	Can detect some instances of this rule, but does not detect all
CodeSonar		LANG.STRUCT.CONDASSIG LANG.STRUCT.SE.COND LANG.STRUCT.USEASSIGN	Assignment in conditional Condition contains side effects Assignment result in expression
Compass/ROSE			Could detect violations of this recommendation by identifying any assignment expression as the top-level expression in an if or while statement
ECLAIR		CC2.EXP18 CC2.EXP21	Fully implemented
GCC			Can detect violations of this recommendation when the -Wall flag is used
Klocwork		ASSIGCOND.CALL ASSIGCOND.GEN MISRA.ASSIGN.COND
LDRA tool suite		114 S, 132 S	Enhanced Enforcement
Parasoft C/C++test		CERT_C-EXP45-a CERT_C-EXP45-b CERT_C-EXP45-c CERT_C-EXP45-d	Avoid conditions that always evaluate to the same value Assignment operators shall not be used in conditions without brackets A function identifier shall only be used with either a preceding '&', or with a parenthesised parameter list, which may be empty Assignment operators shall not be used in expressions that yield a Boolean value
Polyspace Bug Finder		Invalid use of = operator	Assignment in conditional statement
PRQA QA-C		3314, 3326, 3344, 3416	Partially implemented
PRQA QA-C++	4.2	4071, 4074
PVS-Studio		V559, V633, V699
SonarQube C/C++ Plugin		AssignmentInSubExpression

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)

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