In C, character string literals are stored in arrays of type char. However, string literals are notionally constant and should consequently be protected by const qualification. This recommendation is a specialization of DCL00-A. Const-qualify immutable objects and also supports rule STR30-C. Do not attempt to modify string literals.

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

Non-Compliant Code Example

In the following non-compliant code, the const keyword has been omitted.

char *c = "Hello";

If a statement such as {{c\[0\] = 'C'}} were placed following the above declaration, the code is likely to compile cleanly, but the result of the assignment is undefined as 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 attempts to assign them to different values is an error.

const char *c = "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 char array which has been initialized using the contents of the corresponding string literal.

char c[] = "Hello";

Consequently, a statement such as {{c\[0\] = '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.

Automated Detection

The LDRA tool suite V 7.6.0 is able to detect violations of this recommendation.

Related Vulnerabilities

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

References:

\[[Corfield 93|AA. C References#Corfield 93]\]
\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.7.8, "Initialization"
\[[Lockheed Martin 2005|AA. C References#Lockheed Martin 05]\] AV Rule 151.1

STR04-A. Use plain char for character data      07. Characters and Strings (STR)       STR06-A. Do not assume that strtok() leaves the parse string unchanged