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

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

According to C99, Section 6.5 [ISO/IEC 9899:1999]

Between the previous and next sequence point an object can only have its stored value modified once by the evaluation of an expression. Additionally, the prior value can be read only to determine the value to be stored.

(See also undefined behavior 32 of Annex J.)

This requirement must be met for each allowable ordering of the subexpressions of a full expression; otherwise, the behavior is undefined.

The following sequence points are defined in Annex C, Sequence Points, of C99 [ISO/IEC 9899-1999]:

• The call to a function, after the arguments have been evaluated.
• The end of the first operand of the following operators:
  • logical AND: &&
  • logical OR: ||
  • conditional: ?
  • comma operator: ,
• The end of a full declarator.
• The end of a full expression:
  • an initializer
  • the expression in an expression statement (that is, at the semicolon)
  • the controlling expression of a selection statement (if or switch)
  • the controlling expression of a while or do statement
  • each of the expressions of a for statement
  • the expression in a return statement.
• Immediately before a C standard library function returns.
• After the actions associated with each formatted input/output function conversion specifier.
• Immediately before and immediately after each call to a comparison function, by a standard searching or sorting function, and between any call to a comparison function and any movement of the objects passed as arguments to that call.

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 a sequence point.

This rule means that statements such as

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

have well-defined behavior, 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;   

do 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;

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

Related Vulnerabilities

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

Related Guidelines

CERT C++ Secure Coding Standard: EXP30-CPP. Do not depend on order of evaluation between sequence points

The CERT Oracle Secure Coding Standard for Java: EXP05-J. Do not write more than once to the same variable within an expression

ISO/IEC 9899:1999] Section 5.1.2.3, "Program execution," Section 6.5, "Expressions," and Annex C, "Sequence points"

ISO/IEC TR 24772 "JCW Operator precedence/Order of Evaluation" and "SAM Side-effects and order of evaluation"

MISRA Rule 12.1

Bibliography

[Summit 2005] Questions 3.1, 3.2, 3.3, 3.3b, 3.7, 3.8, 3.9, 3.10a, 3.10b, and 3.11
[Saks 2007]

      03. Expressions (EXP)      EXP31-C. Avoid side effects in assertions