Local, automatic variables assume unexpected values if they are read before they are initialized. The C++ Standard, [dcl.init], paragraph 12 [ISO/IEC 14882-2014], states the following:
If no initializer is specified for an object, the object is default-initialized. When storage for an object with automatic or dynamic storage duration is obtained, the object has an indeterminate value, and if no initialization is performed for the object, that object retains an indeterminate value until that value is replaced. If an indeterminate value is produced by an evaluation, the behavior is undefined except in the following cases:
— If an indeterminate value of unsigned narrow character type is produced by the evaluation of:
— the second or third operand of a conditional expression,
— the right operand of a comma expression,
— the operand of a cast or conversion to an unsigned narrow character type, or
— a discarded-value expression,
then the result of the operation is an indeterminate value.
— If an indeterminate value of unsigned narrow character type is produced by the evaluation of the right operand of a simple assignment operator whose first operand is an lvalue of unsigned narrow character type, an indeterminate value replaces the value of the object referred to by the left operand.
— If an indeterminate value of unsigned narrow character type is produced by the evaluation of the initialization expression when initializing an object of unsigned narrow character type, that object is initialized to an indeterminate value.
The default initialization of an object is described by paragraph 7 of the same subclause:
To default-initialize an object of type
Tis a (possibly cv-qualified) class type, the default constructor for
Tis called (and the initialization is ill-formed if
Thas no default constructor or overload resolution results in an ambiguity or in a function that is deleted or inaccessible from the context of the initialization);
Tis an array type, each element is default-initialized;
— otherwise, no initialization is performed.
If a program calls for the default initialization of an object of a const-qualified type
Tshall be a class type with a user-provided default constructor.
As a result, objects of type
T with automatic or dynamic storage duration must be explicitly initialized before having their value read as part of an expression unless
T is a class type or an array thereof or is an unsigned narrow character type. If
T is an unsigned narrow character type, it may be used to initialize an object of unsigned narrow character type, which results in both objects having an indeterminate value. This technique can be used to implement copy operations such as
std::memcpy() without triggering undefined behavior.
Additionally, memory dynamically allocated with a
new expression is default-initialized when the new-initialized is omitted. Memory allocated by the standard library function
std::calloc() is zero-initialized. Memory allocated by the standard library function
std::realloc() assumes the values of the original pointer but may not initialize the full range of memory. Memory allocated by any other means (
std::malloc(), allocator objects,
operator new(), and so on) is assumed to be default-initialized.
Objects of static or thread storage duration are zero-initialized before any other initialization takes place [ISO/IEC 14882-2014] and need not be explicitly initialized before having their value read.
Reading uninitialized variables for creating entropy is problematic because these memory accesses can be removed by compiler optimization. is an example of a vulnerability caused by this coding error [VU#925211].
Noncompliant Code Example
In this noncompliant code example, an uninitialized local variable is evaluated as part of an expression to print its value, resulting in undefined behavior.
In this compliant solution, the object is initialized prior to printing its value.
Noncompliant Code Example
In this noncompliant code example, an
int * object is allocated by a new-expression, but the memory it points to is not initialized. The object's pointer value and the value it points to are printed to the standard output stream. Printing the pointer value is well-defined, but attempting to print the value pointed to yields an indeterminate value, resulting in undefined behavior.
In this compliant solution, the memory is direct-initialized to the value
12 prior to printing its value.
Initialization of an object produced by a new-expression is performed by placing (possibly empty) parenthesis or curly braces after the type being allocated. This causes direct initialization of the pointed-to object to occur, which will zero-initialize the object if the initialization omits a value, as illustrated by the following code.
Noncompliant Code Example
In this noncompliant code example, the class member variable
c is not explicitly initialized by a ctor-initializer in the default constructor. Despite the local variable
s being default-initialized, the use of
c within the call to
S::f() results in the evaluation of an object with indeterminate value, resulting in undefined behavior.
In this compliant solution,
S is given a default constructor that initializes the class member variable
Reading uninitialized variables is undefined behavior and can result in unexpected program behavior. In some cases, these security flaws may allow the execution of arbitrary code.
|Does not catch all instances of this rule, such as uninitialized values read from heap-allocated memory.|
|Return pointer to local|
|LDRA tool suite|
53 D, 69 D, 631 S, 652 S
|CERT_CPP-EXP53-a||Avoid use before initialization|
|Parasoft Insure++||Runtime detection|
|Polyspace Bug Finder|
|CERT C++: EXP53-CPP|
Rule partially covered.
2961, 2962, 2963, 2966, 2967, 2968,
2971, 2972, 2973, 2976, 2977, 2978
|V546, V573, V614, V670, V679, V730, V788, V1007|
|[ISO/IEC 14882-2014]||Clause 5, "Expressions"|
Subclause 5.3.4, "New"
Subclause 8.5, "Initializers"
Subclause 12.6.2, "Initializing Bases and Members"
|[Lockheed Martin 2005]||Rule 142, All variables shall be initialized before use|