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A better approach is to not specify the bound of a string initialized with a string literal because the compiler will automatically allocate sufficient space for the entire string literal, including the terminating null character. This rule is a specific exception to ARR02-C. Explicitly specify array bounds, even if implicitly defined by an initializer.

Noncompliant Code Example

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

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The size of the array s is 3, although the size of the string literal is 4. Any subsequent use of the array as a null-terminated byte string can result in a vulnerability, because s is not properly null-terminated. (See STR32-C. Null-terminate byte strings as requiredDo not pass a non-null-terminated character sequence to a library function that expects a string.)

Implementation Details

This code compiles with no warning in warning with Visual Studio 2012 2013 and GCC 4.38.31. It produces a three-character array with no terminating null character, as specified by the standard.

Compliant Solution

This compliant solution does not specify the bound of a character array in the array declaration. If the array bound is omitted, the compiler allocates sufficient storage to store the entire string literal, including the terminating null character.

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This approach is preferred because the size of the array can always be derived even if the size of the string literal changes.

Exceptions

STR36STR11-C-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:

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

STR36STR11-C-EX1EX2: If the character array must be larger than the string literal it is initialized with, you may explicitly specify an array bounds. This is particularly important if the array's contents might change during program execution.

#include <string.h>
 
void func(void) {
  char s[10] = "abc";
  strcpy(&s[3], "def");
}

Risk Assessment

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography

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