MSC04-A. Use comments consistently and in a readable fashion

Non-Compliant Code Example

Do not use the character sequence /* within a comment:

/* comment with end comment marker unintentionally omitted
...
security_critical_function();
/* some other comment */

In this example, the call to the security critical function is not executed. It is possible that, in reviewing this page, a reviewer may assume that the code is executed.

In cases where this is the result of an accidental omission, it is useful to use an editor that provides syntax highlighting or formats the code to help identify issues like missing end comment deliminators.

Because missing end deliminators is error prone and often viewed as a mistake, it is recommended that this approach not be used to comment out code.

Compliant Solution

Comment out blocks of code using conditional compilation (e.g., #if, #ifdef), or #ifndef).

#if 0  /* use of critical security function no longer necessary */
security_critical_function();
/* some other comment */
#endif

The text inside a block of code commented-out using #if, #ifdef), or #ifndef must still consist of valid preprocessing tokens. This means that the characters " and ' must each be paired just as in real C code, and the pairs must not cross line boundaries. In particular, an apostrophe within a contracted word looks like the beginning of a character constant. Therefore, natural-language comments and pseudocode should always be written between the comment delimiters /* and */ or following //.

Non-Compliant Code Example

These are some additional examples of comment styles that are confusing and should be avoided:

// */          // comment, not syntax error
...
f = g/**//h;   // equivalent to f = g / h;
...
//\
i();           // part of a two-line comment
...
/\
/ j();         // part of a two-line comment
...
/*//*/ l();    // equivalent to l();
...
m = n//**/o
+ p;           // equivalent to m = n + p;

Compliant Solution 1

Use a consistent style of commenting:

// Nice simple comment

int i; // counter

Compliant Solution 2

Use a consistent style of commenting:

/* Nice simple comment */

int i; /* counter */

Risk Assessment

Confusion over which instructions are executed and which are not can lead to serious programming errors and vulnerabilities. This problem is mitigated by the use of interactive development environments (IDE) and editors that use fonts, colors, or other mechanisms to differentiate between comments and code. However, the problem can still manifest itself, for example, when reviewing source code printed at a black and white printer.

Examples of vulnerabilities resulting from the violation of this recommendation can be found on the CERT website.

References

[[ISO/IEC 9899-1999]] Sections 6.4.9 Comments, 6.10.1 Conditional inclusion
[[MISRA 04]] Rule 2.3: The character sequence /* shall not be used within a comment; Rule 2.4: Sections of code should not be "commented out"
[[Summit 05]] Question 11.19