EXP30-CPP. Do not depend on order of evaluation between sequence points

Evaluation of an expression may produce side effects. At specific points during execution called sequence points, all side effects of previous evaluations have completed and no side effects of subsequent evaluations have yet taken place.

The order in which operands in an expression are evaluated is unspecified in C++. The only guarantee is that they will all be completely evaluated at the next sequence point. According to ISO/IEC 14882-2003:

The following are the sequence points defined by ISO/IEC 14882-2003:

• at the completion of evaluation of each full-expression;
• after the evaluation of all function arguments (if any) and before execution of any expressions or statements in the function body;
• after the copying of a returned value and before the execution of any expressions outside the function;
• after the evaluation of the first operand of the following operators: && (logical AND); || (logical OR); ? (conditional); , (comma, but see the note immediately following);
• after the initialization of each base and member in a class.

Note that not all instances of a comma in C++ code denote a usage of the comma operator. For example, the comma between arguments in a function call is NOT the comma operator.

According to ISO/IEC 14882-2003:

Between the previous and next sequence point a scalar object shall have its stored value modified at most once by the evaluation of an expression. Furthermore, the prior value shall be accessed only to determine the value to be stored.

This rule means that statements such as

i = i + 1;
a[i] = i;

are allowed, while statements like

/* i is modified twice between sequence points */
i = ++i + 1;  

/* i is read other than to determine the value to be stored */
a[i++] = i;   

are not.

Noncompliant Code Example

Programs cannot safely rely on the order of evaluation of operands between sequence points. In this noncompliant code example, the order of evaluation of the operands to the + operator is unspecified.

a = i + b[++i];

If i was equal to 0 before the statement, the statement may result in the following outcome:

a = 0 + b[1];

Or it may result in the following outcome:

a = 1 + b[1];

Compliant Solution

These examples are independent of the order of evaluation of the operands and can only be interpreted in one way.

++i;
a = i + b[i];

Or alternatively:

a = i + b[i+1];
++i;

Non-Compliant Code Example

Both of these statements violate the rule concerning sequence points stated above, so the behavior of these statements is undefined.

i = ++i + 1;  // an attempt is made to modify the value of i twice between sequence points
a[i++] = i;   // an attempt is made to read the value of i other than to determine the value to be stored

Compliant Solution

These statements are allowed by the standard.

i = i + 1;
a[i] = i;

Noncompliant Code Example

The order of evaluation for function arguments is unspecified.

func(i++, i);

The call to func() has undefined behavior because there are no sequence points between the argument expressions. The first (left) argument expression reads the value of i (to determine the value to be stored) and then modifies i. The second (right) argument expression reads the value of i between the same pair of sequence points as the first argument, but not to determine the value to be stored in i. This additional attempt to read the value of i has undefined behavior.

Compliant Solution

This solution is appropriate when the programmer intends for both arguments to func() to be equivalent.

i++;
func(i, i);

This solution is appropriate when the programmer intends for the second argument to be one greater than the first.

j = i++;
func(j, i);

Risk Assessment

Attempting to modify an object multiple times between sequence points may cause that object to take on an unexpected value. This can lead to unexpected program behavior.

Automated Detection

Splint Version 3.1.1 can detect violations of this rule.

GCC Compiler can detect violations of this rule when the -Wsequence-point flag is used.

Compass/ROSE can detect simple violations of this rule. It needs to examine each expression and make sure that no variable is modified twice in the expression. Also no variable is modified once, and read elsewhere, with the single exception that a variable may appear on both the left and right of an assignment operator.

The Coverity Prevent Version 5.0 EVALUATION_ORDER checker can detect the specific instance where Statement contains multiple side-effects on the same value with an undefined evaluation order because with different compiler flags or different compilers or platforms, the statement may behave differently.

Related Vulnerabilities

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

Other Languages

This rule appears in the C Secure Coding Standard as EXP30-C. Do not depend on order of evaluation between sequence points.

This rule appears in the Java Secure Coding Standard as EXP09-J. Do not write more than once to the same variable within an expression.

Bibliography

[ISO/IEC 14882-2003] Sections 1.9 Program execution, 5 Expressions, 12.6.2 Initializing bases and members.
[ISO/IEC 14882-2003] Sections 1.9 Program execution, 5 Expressions, 12.6.2 Initializing bases and members.
[Summit 05] Questions 3.1, 3.2, 3.3, 3.3b, 3.7, 3.8, 3.9, 3.10a, 3.10b, 3.11.
[Lockheed Martin 05] AV Rule 204.1 The value of an expression shall be the same under any order of evaluation that the standard permits.
[Saks 07]

EXP17-CPP. Treat relational and equality operators as if they were nonassociative      03. Expressions (EXP)      EXP31-CPP. Avoid side effects in assertions