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 or #ifdef).

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

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 */

Priority: P6 Level: L2

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.

References

• ISO/IEC 9899-1999 6.4.9 Comments
• 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"