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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. Subclause 6.7.9, Example 12, of the C Standard [ISO/IEC 9899:2011] 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.

The C Standard also dictates how array initialization is handled when the number of initialization elements does not equal the explicit array bound. Subclause 6.7.9, paragraphs 21 and 22, state:

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.
If an array of unknown size is initialized, its size is determined by the largest indexed element with an explicit initializer. The array type is completed at the end of its initializer list.

Although compilers can compute the size of an array on the basis of 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 STR11-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 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.

Exceptions

ARR02-C-EX1: STR11-C. Do not specify the bound of a character array initialized with a string literal is a specific exception to this recommendation; it requires that the bound of a character array initialized with a string literal is unspecified.

Risk Assessment

Recommendation

Severity

Likelihood

Remediation Cost

Priority

Level

ARR02-C

Medium

Unlikely

Low

P6

L2

Automated Detection

Tool

Version

Checker

Description

Astrée
19.04
array-size-globalPartially checked
Axivion Bauhaus Suite

6.9.0

CertC-ARR02Fully implemented
CodeSonar
5.1p0
LANG.STRUCT.DECL.FAMDeclaration of flexible array member
Compass/ROSE




ECLAIR

1.2

CC2.ARR02

Fully implemented

LDRA tool suite
9.7.1

127 S
397 S
404 S

Fully  implemented

Parasoft C/C++test

10.4.2

CERT_C-ARR02-a

Explicitly specify array bounds in array declarations with initializers

Polyspace Bug Finder

R2018a

Improper array initialization

MISRA C:2012 Rule 8.11

MISRA C:2012 Rule 9.5

Incorrect array initialization when using initializers

When an array with external linkage is declared, its size should be explicitly specified

Where designated initializers are used to initialize an array object the size of the array shall be specified explicitly

PRQA QA-C
9.5

0688,3674,3684, 0678

Fully implemented
PVS-Studio

6.23

V798
RuleChecker
19.04
array-size-globalPartially checked
SonarQube C/C++ Plugin
3.11
S834

Related Vulnerabilities

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

Related Guidelines

Key here (explains table format and definitions)

Taxonomy

Taxonomy item

Relationship

CERT CCTR02-CPP. Explicitly specify array bounds, even if implicitly defined by an initializerPrior to 2018-01-12: CERT: Unspecified Relationship
CWE 2.11CWE-665, Incorrect or incomplete initializationPrior to 2018-01-12: CERT:
MISRA C:2012Rule 8.11 (advisory)Prior to 2018-01-12: CERT: Unspecified Relationship
MISRA C:2012Rule 9.5 (required)Prior to 2018-01-12: CERT: Unspecified Relationship

Bibliography

[ISO/IEC 9899:2011]Subclause 6.7.9, "Initialization"



4 Comments


  1. sometype_t varname[] = { .... };
    int const varname_ct = sizeof(varname)/sizeof(varname[0]);
    // I have trouble seeing why this style would be discouraged, as it seems to be.
    


    1. This recommendation came out of the safety community. The general notion is that by specifying the bound and the initialization, the compiler can issue a warning if the two sizes disagree, decreasing the changes of an error occurring.

      1. I also have trouble envisioning the type of an error this recommendation intends to prevent given that C99 doesn't allow initializing an array with more initializers than there are elements. In fact, in my experience, it's the redundancy that this guideline recommends that can lead to problems. For example:

        #define NSTRINGS 5
        const char* const a[NSTRINGS] = { "foo", "bar", "baz", "foobar" };
        for (size_t i = 0; i != NSTRINGS; ++i)
            puts(a[i]);   /* whoops! undefined behavior when (i == 4) */
        

        This type of an error would be avoided if Bruce's approach had been used:

        const char* const a[] = { "foo", "bar", "baz", "foobar" };
        const size_t NSTRINGS = sizeof a / sizeof *a;
        for (size_t i = 0; i != NSTRINGS; ++i)
            puts(a[i]);
        

        In light of this common programming mistake I tend to rather strongly disagree with this recommendation and suggest one to the contrary: i.e., Avoid explicitly specifying array size in array declarations with initializers.

  2. This rule conflicts with flexible array members.