The type of a narrow string literal is an array of char, and the type of a wide string literal is an array of wchar_t. However, string literals (of both types) are notionally constant and should consequently be protected by const qualification. This recommendation is a specialization of DCL00-C. Const-qualify immutable objects and also supports STR30-C. Do not attempt to modify string literals.

Adding const qualification may propagate through a program; as const qualifiers are added, still more become necessary. This phenomenon is sometimes called const-poisoning. Const-poisoning can frequently lead to violations of EXP05-C. Do not cast away a const qualification. Although const qualification is a good idea, the costs may outweigh the value in the remediation of existing code.

Noncompliant Code Example (Narrow String Literal)

In this noncompliant code example, the const keyword has been omitted:

As string literals are constant, they should only be assigned to constant pointers. This recommendation supports rule STR30-C.

Non-Compliant Code Example 1

The const keyword is not included in this declaration.

char * c = "Hello"; // Bad: assigned to non-const
c[3] = 'a'; // Undefined (but compiles)

Compliant Solution 1

If a statement such as c[0] = 'C' were placed following the declaration in the noncompliant code example, the code is likely to compile cleanly, but the result of the assignment would be undefined because string literals are considered constant.

Compliant Solution (Immutable Strings)

In this compliant solution, the characters referred to by the pointer c are const-qualified, meaning that any attempt to assign them to different values is an error:If you properly assign string literals to const pointers, the compiler will not allow direct manipulation of the contents.

const 
char const * c = "Hello"; // Good
//c[3] = 'a'; would cause a compile error

Aside

Note that the following code is acceptable, as a and b do not actually point to string literals. They are char array objects which have had characters copied into them, and therefore are modifiable.

Compliant Solution (Mutable Strings)

In cases where the string is meant to be modified, use initialization instead of assignment. In this compliant solution, c is a modifiable char array that has been initialized using the contents of the corresponding string literal:

char c

char a[] = "abcHello";
char b[3] = "abc";

The above code is equivalent to:

char a[] = {'a', 'b', 'c', '\0'};
char b[] = {'a', 'b', 'c'};

Non-Compliant Coding Example 2.a

Consequently, a statement such as c[0] = 'C' is valid and behaves as expected.

Noncompliant Code Example (Wide String Literal)

In this noncompliant code example, the const keyword has been omitted:Though it is not compliant with the C Standard, this code executes correctly if the contents of CMUfullname are not modified.

wchar_t *c
char* CMUfullname = L"Carnegie Mellon";

/* get school from user input and validate */

if (strcmp(school,"CMU")) {
    school = CMUfullname;
}

Non-Compliant Coding Example 2.b

Adding in the const keyword will generate a compiler warning, as the assignment of CMUfullname to school discards the const qualifier. Any modifications to the contents of school after this assignment will lead to errors.

char const * CMUfullname = "Carnegie Mellon";

/* get school from user input and validate */

if (strcmp(school,"CMU")) {
    school = CMUfullname;
}

Compliant Solution 2

The compliant solution uses the const keyword to protect the string literal, as well as using strcpy to copy the value of CMUfullname into school, allowing future modification of school.

...

Hello";

If a statement such as c[0] = L'C' were placed following this declaration, the code is likely to compile cleanly, but the result of the assignment would be undefined because string literals are considered constant.

Compliant Solution (Immutable Strings)

In this compliant solution, the characters referred to by the pointer c are const-qualified, meaning that any attempt to assign them to different values is an error:

wchar_t const *c = L"Hello";

Compliant Solution (Mutable Strings)

In cases where the string is meant to be modified, use initialization instead of assignment. In this compliant solution, c is a modifiable wchar_t array that has been initialized using the contents of the corresponding string literal:

wchar_t c[] = L"Hello";

Consequently, a statement such as c[0] = L'C' is valid and behaves as expected.

...

Risk Assessment

Modifying string literals causes undefined behavior, resulting in abnormal program termination and denial-of-service vulnerabilities.

References:

Automated Detection

Related Vulnerabilities

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

Bibliography

...

Image Added Image Added Image Added Wiki Markup[http://www.open-std.org/jtc1/sc22/wg21/docs/papers/1993/N0389.asc] \[[ISO/IEC 9899-1999:TC2|AA. C References#ISO/IEC 9899-1999TC2]\] Section 6.7.8, "Initialization" \[Lockheed Martin 2005\] Lockheed Martin. Joint Strike Fighter Air Vehicle C+\+ Coding Standards for the System Development and Demonstration Program. Document Number 2RDU00001, Rev C. December 2005.     AV Rule 151.1