CertC-EXP45

Do not perform assignments in selection statements

Required inputs: IR

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.

Operator Context 
if Controlling expression
while Controlling expression
do ... while Controlling expression
for Second operand
?: First operand
?: Second or third operands, where the ternary expression is used in any of these contexts
&&  Either operand 
||  either operand 
, Second operand, when the comma expression is used in any of these contexts



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);
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);
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

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

This noncompliant code example has a typo that results in an assignment rather than a comparison.

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

Many compilers will warn about this condition. This coding error would typically be eliminated by adherence to  MSC00-C. Compile cleanly at high warning levels. Although this code compiles, it will cause  unexpected behavior to an unsuspecting programmer. If the intent was to verify a string such as a password, user name, or group user ID, the code may produce significant  vulnerabilities and require significant debugging.

Compliant Solution (RHS Variable)

When comparisons are made between a variable and a literal or const-qualified variable, placing the variable on the right of the comparison operation can prevent a spurious assignment.

In this code example, the literals are placed on the left-hand side of each comparison. If the programmer were to inadvertently use an assignment operator, the statement would assign  ch to  '\t', which is invalid and produces a diagnostic message. 

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

Due to the diagnostic, the typo will be easily spotted and fixed.

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

As a result, any mistaken use of the assignment operator that could otherwise create a  vulnerability for operations such as string verification will result in a compiler diagnostic regardless of compiler, warning level, or  implementation.

Exceptions

EXP45-C-EX1: Assignment can be used where the result of the assignment is itself an operand to a comparison expression or relational expression. In this compliant example, the expression  x = y  is itself an operand to a comparison operation: 

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

EXP45-C-EX2: Assignment can be used where the expression consists of a single primary expression. The following code is compliant because the expression   x = y is a single primary expression: 

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

The following controlling expression is noncompliant because  && is not a comparison or relational operator and the entire expression is not primary:

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

When the assignment of  v to  w is not intended, the following controlling expression can be used to execute the conditional block when  v is equal to  w:

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

When the assignment is intended, the following controlling expression can be used:

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

EXP45-C-EX3: Assignment can be used in a function argument or array index. In this compliant solution, the expression  x = y is used in a function argument: 


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


Risk Assessment

Errors of omission can result in unintended program flow.

Recommendation Severity Likelihood Remediation Cost Priority Level
EXP45-C Low Likely Medium P6 L2
Related Guidelines
Taxonomy Taxonomy item Relationship
CERT C EXP19-CPP. Do not perform assignments in conditional expressions Prior to 2018-01-12: CERT: Unspecified Relationship
CERT Oracle Secure Coding Standard for Java EXP51-J. Do not perform assignments in conditional expressions Prior to 2018-01-12: CERT: Unspecified Relationship
ISO/IEC TR 24772:2013 Likely Incorrect Expression [KOA] Prior to 2018-01-12: CERT: Unspecified Relationship
ISO/IEC TS 17961 No assignment in conditional expressions [boolasgn] Prior to 2018-01-12: CERT: Unspecified Relationship
CWE 2.11 CWE-480, Use of Incorrect Operator 2017-07-05: CERT: Rule subset of CWE
CWE 2.11 CWE-481 2017-07-05: CERT: Rule subset of CWE
Bibliography
[ Dutta 03] "Best Practices for Programming in C"
[ Hatton 1995] Section 2.7.2, "Errors of Omission and Addition"
Excerpt from SEI CERT C Coding Standard: Rules for Developing Safe, Reliable, and Secure Systems (2016 Edition) and SEI CERT C Coding Standard [https://cmu-sei.github.io/secure-coding-standards/sei-cert-c-coding-standard/rules/expressions-exp/exp45-c], Copyright (C) 1995-2026 Carnegie Mellon University. See section 9.4. "3rd-Party Licenses" in the documentation for full details.

Possible Messages

Key

Text

Severity

Disabled

assignment_inside_bool

Assignment inside boolean expression.

None

False

Options

allow_in_extra_parens

allow_in_extra_parens : bool = True

Whether if ((x = y)) should be allowed (note the extra parens)
 

allow_in_relational_operator

allow_in_relational_operator : bool = True

Whether an assignment is allowed as operand of a comparison or relational operator, e.g. (x = y) != 0