In C++, modifying an object, calling a library I/O function, accessing a volatile-qualified value, or calling a function that performs one of these actions are ways to modify the state of the execution environment. These actions are called side effects. All relationships between value computations and side effects can be described in terms of sequencing of their evaluations. The C++ Standard, [intro.execution], paragraph 13 [ISO/IEC 14882-2014], establishes three sequencing terms:

Sequenced before is an asymmetric, transitive, pair-wise relation between evaluations executed by a single thread, which induces a partial order among those evaluations. Given any two evaluations A and B, if A is sequenced before B, then the execution of A shall precede the execution of B. If A is not sequenced before B and B is not sequenced before A, then A and B are unsequenced. [Note: The execution of unsequenced evaluations can overlap. — end note] Evaluations A and B are indeterminately sequenced when either A is sequenced before B or B is sequenced before A, but it is unspecified which. [Note: Indeterminately sequenced evaluations cannot overlap, but either could be executed first. — end note]

Paragraph 15 further states (nonnormative text removed for brevity) the following:

Except where noted, evaluations of operands of individual operators and of subexpressions of individual expressions are unsequenced. ... The value computations of the operands of an operator are sequenced before the value computation of the result of the operator. If a side effect on a scalar object is unsequenced relative to either another side effect on the same scalar object or a value computation using the value of the same scalar object, and they are not potentially concurrent, the behavior is undefined. ... When calling a function (whether or not the function is inline), every value computation and side effect associated with any argument expression, or with the postfix expression designating the called function, is sequenced before execution of every expression or statement in the body of the called function. ... Every evaluation in the calling function (including other function calls) that is not otherwise specifically sequenced before or after the execution of the body of the called function is indeterminately sequenced with respect to the execution of the called function. Several contexts in C++ cause evaluation of a function call, even though no corresponding function call syntax appears in the translation unit. ... The sequencing constraints on the execution of the called function (as described above) are features of the function calls as evaluated, whatever the syntax of the expression that calls the function might be.

Do not allow the same scalar object to appear in side effects or value computations in both halves of an unsequenced or indeterminately sequenced operation.

The following expressions have sequencing restrictions that deviate from the usual unsequenced ordering [ISO/IEC 14882-2014]:

This rule means that statements such as

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

have defined behavior, and statements such as the following do not.

// i is modified twice in the same full expression
i = ++i + 1;  

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

Not all instances of a comma in C++ code denote use of the comma operator. For example, the comma between arguments in a function call is not the comma operator. Additionally, overloaded operators behave the same as a function call, with the operands to the operator acting as arguments to a function call.

This entire rule will need to be re-evalulated for C++17, because the order of evaluation has been tightened there. For instance, the example of a[i++] = i; will now be well-defined.

See http://en.cppreference.com/w/cpp/language/eval_order, http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0145r3.pdf, and http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0400r0.html.

Noncompliant Code Example

In this noncompliant code example, i is evaluated more than once in an unsequenced manner, so the behavior of the expression is undefined.

void f(int i, const int *b) {
  int a = i + b[++i];
  // ...
}

Compliant Solution

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

void f(int i, const int *b) {
  ++i;
  int a = i + b[i];
  // ...
}
void f(int i, const int *b) {
  int a = i + b[i + 1];
  ++i;
  // ...
}

Noncompliant Code Example

The call to func() in this noncompliant code example has undefined behavior because the argument expressions are unsequenced.

extern void func(int i, int j);
 
void f(int i) {
  func(i++, i);
}

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

extern void func(int i, int j);
 
void f(int i) {
  i++;
  func(i, i);
}

This compliant solution is appropriate when the programmer intends for the second argument to be 1 greater than the first.

extern void func(int i, int j);
 
void f(int i) {
  int j = i++;
  func(j, i);
}

Noncompliant Code Example

This noncompliant code example is similar to the previous noncompliant code example. However, instead of calling a function directly, this code calls an overloaded operator<<(). Overloaded operators are equivalent to a function call and have the same restrictions regarding the sequencing of the function call arguments. This means that the operands are not evaluated left-to-right, but are unsequenced with respect to one another. Consequently, this noncompliant code example has undefined behavior.

#include <iostream>
 
void f(int i) {
  std::cout << i++ << i << std::endl;
}

Compliant Solution

In this compliant solution, two calls are made to operator<<(), ensuring that the arguments are printed in a well-defined order.

#include <iostream>
 
void f(int i) {
  std::cout << i++;
  std::cout << i << std::endl;
}

Noncompliant Code Example

The order of evaluation for function arguments is unspecified. This noncompliant code example exhibits unspecified behavior but not undefined behavior.

extern void c(int i, int j);
int glob;
 
int a() {
  return glob + 10;
}

int b() {
  glob = 42;
  return glob;
}
 
void f() {
  c(a(), b());
}

The order in which a() and b() are called is unspecified; the only guarantee is that both a() and b() will be called before c() is called. If a() or b() rely on shared state when calculating their return value, as they do in this example, the resulting arguments passed to c() may differ between compilers or architectures.

Compliant Solution

In this compliant solution, the order of evaluation for a() and b() is fixed, and so no unspecified behavior occurs.

extern void c(int i, int j);
int glob;
 
int a() {
  return glob + 10;
}
 
int b() {
  glob = 42;
  return glob;
}
 
void f() {
  int a_val, b_val;
 
  a_val = a();
  b_val = b();

  c(a_val, b_val);
}

Risk Assessment

Attempting to modify an object in an unsequenced or indeterminately sequenced evaluation may cause that object to take on an unexpected value, which can lead to unexpected program behavior.

Rule

Severity

Likelihood

Remediation Cost

Priority

Level

EXP50-CPP

Medium

Probable

Medium

P8

L2

Automated Detection

Tool

Version

Checker

Description

Axivion Bauhaus Suite

CertC++-EXP50
Clang
-WunsequencedCan detect simple violations of this rule where path-sensitive analysis is not required
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. It also must check that no variable is modified once, then read elsewhere, with the single exception that a variable may appear on both the left and right of an assignment operator

Coverity

EVALUATION_ORDER

Can detect the specific instance where a 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

ECLAIR

CC2.EXP30

Fully implemented

GCC


Can detect violations of this rule when the -Wsequence-point flag is used

Klocwork

PORTING.VAR.EFFECTS
MISRA.EXPR.PARENS
MISRA.EXPR.PARENS.INSUFFICIENT
MISRA.EXPR.PARENS.REDUNDANT
MISRA.INCR_DECR.OTHER


LDRA tool suite

35 D, 1 Q, 9 S, 134 S, 67 D, 72 D

Partially implemented

Parasoft C/C++test

CERT_CPP-EXP50-a
CERT_CPP-EXP50-b
CERT_CPP-EXP50-c
CERT_CPP-EXP50-d
CERT_CPP-EXP50-e
CERT_CPP-EXP50-f

The value of an expression shall be the same under any order of evaluation that the standard permits
Don't write code that depends on the order of evaluation of function arguments
Don't write code that depends on the order of evaluation of function designator and function arguments
Don't write code that depends on the order of evaluation of expression that involves a function call
Between sequence points an object shall have its stored value modified at most once by the evaluation of an expression
Don't write code that depends on the order of evaluation of function calls

Polyspace Bug Finder

CERT C++: EXP50-CPPChecks for situations where expression value depends on order of evaluation (rule fully covered).
PRQA QA-C++
3220, 3221, 3222, 3223, 3228
PVS-Studio

V521, V708
SonarQube C/C++ Plugin
IncAndDecMixedWithOtherOperatorsPartially implemented
Splint



Related Vulnerabilities

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

Related Guidelines

SEI CERT C Coding StandardEXP30-C. Do not depend on the order of evaluation for side effects

Bibliography

[ISO/IEC 14882-2014]Subclause 1.9, "Program Execution"
[MISRA 2008]Rule 5-0-1 (Required)