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It is common for an array variable to be initialized by a string literal and declared with an explicit bound that matches the number of characters in the string literal. Subclause 6.7.9, paragraph 14, of the C Standard [ISO/IEC 9899:2011], says:

An array of character type may be initialized by a character string literal or UTF−8 string literal, optionally enclosed in braces. Successive bytes of the string literal (including the terminating null character if there is room or if the array is of unknown size) initialize the elements of the array.

However, if the string is intended to be used as a null-terminated byte string, then the array will have one too few characters to hold the string because it does not account for the terminating null character. Such a sequence of characters has limited utility and has the potential to cause vulnerabilities if a null-terminated byte string is assumed.

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This noncompliant code example initializes an array of characters using a string literal that defines one character more character (counting the terminating '\0') than the array can hold:

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STR36-EX1: If the intention is to create a character array and not a null-terminated byte string, initializing to fit exactly without a null byte is allowed but not recommended. The preferred approach to create an array containing just the three characters 'a', 'b', and 'c', for example, is to declare each character literal as a separate element as follows:

char s[3] = { 'a', 'b', 'c' }; /* NOT a string. */

Also, you should make clear in comments or documentation if a character array is, in fact, not a null-terminated byte string.

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Automated Detection

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