The C standard allows an array variable to be declared both with a dimension index bound and with an initialization literal. The initialization literal also implies an array dimensionbound, in the number of elements specified.
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C99 also dictates how array initialization is handled when the number of initialization elements does not equal the explicit array dimensionbound. C99 Section 6.7.8, "Initialization", paragraph 21 states:
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Note that this recommendation does not apply to character arrays initialized with string literals, see STR36-C. Do not specify the dimension of a character array initialized with a string literal for more information.
Non-Compliant Code Example (Incorrect Size)
This non-compliant code example initializes an array of integers using an initialization with too many elements for the array.
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The size of the array a is three, although the size of the initialization is four. The last element of the initialization (4) is ignored. Most compilers will diagnose this error.
Implementation Details
This non-compliant code example generates a warning in gcc. Microsoft Visual Studio 2008 generates a fatal diagnostic: error C2078: too many initializers.
Non-Compliant 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.
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int a[] = {1, 2, 3, 4};
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Compliant Solution
This compliant solution explicitly specifies the array dimensionbound.
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int a[4] = {1, 2, 3, 4};
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Explicitly specifying the array dimension 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
Recommendation | Severity | Likelihood | Remediation Cost | Priority | Level |
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ARR02-A | medium | unlikely | low | P6 | L2 |
Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
References
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\[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.7.8, "Initialization" |
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ARR01-A. Do not apply the sizeof operator to a pointer when taking the size of an array 06. Arrays (ARR) ARR30-C. Guarantee that array indices are within the valid range