ARR02-C. Explicitly specify array bounds, even if implicitly defined by an initializer

The C standard allows an array variable to be declared both with a bound and with an initialization literal. The initialization literal also implies an array bound, in the number of elements specified.

The size implied by an initialization literal is usually specified by the number of elements

int array[] = {1, 2, 3}; /* 3-element array */

but it is also possible to use designators to initialize array elements in a noncontiguous fashion. C99, Section 6.7.8, [ISO/IEC 9899:1999] states

Space can be "allocated" from both ends of an array by using a single designator:

int a[MAX] = {
  1, 3, 5, 7, 9, [MAX-5] = 8, 6, 4, 2, 0
};

In the above, if MAX is greater than ten, there will be some zero-valued elements in the middle; if it is less than ten, some of the values provided by the first five initializers will be overridden by the second five.

C99 also dictates how array initialization is handled when the number of initialization elements does not equal the explicit array bound. C99, Section 6.7.8, "Initialization", paragraph 21 states

If there are fewer initializers in a brace-enclosed list than there are elements or members of an aggregate, or fewer characters in a string literal used to initialize an array of known size than there are elements in the array, the remainder of the aggregate shall be initialized implicitly the same as objects that have static storage duration.

And paragraph 22 states

If an array of unknown size is initialized, its size is determined by the largest indexed element with an explicit initializer. At the end of its initializer list, the array no longer has incomplete type.

While compilers can compute the size of an array based on its initialization list, explicitly specifying the size of the array provides a redundancy check, ensuring that the array's size is correct. It also enables compilers to emit warnings if the array's size is less than the size implied by the initialization.

Note that this recommendation does not apply (in all cases) to character arrays initialized with string literals. See guideline STR36-C. Do not specify the bound of a character array initialized with a string literal for more information.

Noncompliant Code Example (Incorrect Size)

This noncompliant code example initializes an array of integers using an initialization with too many elements for the array.

int a[3] = {1, 2, 3, 4};

The size of the array a is 3, although the size of the initialization is 4. The last element of the initialization (4) is ignored. Most compilers will diagnose this error.

Implementation Details

This noncompliant code example generates a warning in gcc. Microsoft Visual Studio 2008 generates a fatal diagnostic: error C2078: too many initializers.

Noncompliant Code Example (Implicit Size)

In this example, the compiler allocates an array of four integer elements, and because an array bound is not explicitly specified by the programmer, sets the array bound to 4. However, if the initializer changes, the array bound may also change, causing unexpected results.

int a[] = {1, 2, 3, 4};

Compliant Solution

This compliant solution explicitly specifies the array bound.

int a[4] = {1, 2, 3, 4};

Explicitly specifying the array bound, although it is implicitly defined by an initializer, allows a compiler or other static analysis tool to issue a diagnostic if these values do not agree.

Risk Assessment

Automated Detection

Related Vulnerabilities

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

Related Guidelines

C++ Secure Coding Standard: ARR02-CPP. Explicitly specify array bounds, even if implicitly defined by an initializer

Bibliography

[ISO/IEC 9899:1999] Section 6.7.8, "Initialization"
[MITRE 2007] CWE ID 665, "Incorrect or Incomplete Initialization"

      06. Arrays (ARR)      ARR30-C. Do not form or use out of bounds pointers or array subscripts