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

/* commentComment with end comment marker unintentionally omitted
security_critical_function();
/* someSome other comment */

In this example, the call to the security-critical function is not executed. It is possible that a A reviewer examining this page may could incorrectly assume that the code is executed.

In cases where this If execution failure 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 delimitorsdelimiters.

Because missing end delimitors delimiters are error prone and often viewed as a mistake, this approach is not recommended for commenting out code.

Compliant Solution (

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Preprocessor)

Instead of using /* and */ to comment out blocks of code, comment out blocks of code using use conditional compilation (e.g.for example, #if, #ifdef, or #ifndef).:

#if 0  /*
        * useUse of critical security function no
        * longer necessary.
        */
security_critical_function();
/* someSome 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. Consequently, natural-language comments and pseudocode should always be written between the comment delimiters /* and */ or following //.

Compliant Solution (

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Compiler)

This compliant solution takes advantage of the compiler's ability to remove unreachable (dead) code. The code inside the if block must remain acceptable to the compiler. If other parts of the program, such as macros, types, or function prototypes, later change later in a way that would cause syntax errors, the unexecuted code must be brought up to date to correct the problem. Then, if it is needed again in the future, all that must be done is to the programmer need only remove the surrounding if statement and the NOTREACHED comment.

The NOTREACHED comment tells some compilers and static analysis tools not to complain about this unreachable code.   It also serves as documentation. 

if (0) {  /* use
           * Use of critical security function no
           * longer necessary, for now.
           */
  /*NOTREACHED*/
  security_critical_function();
  /* someSome other comment */
}

This code is an instance of exception MSC07-C-EX2 to MSC07-C. Detect and remove dead code.

Noncompliant Code Example

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

// */          /* commentComment, not syntax error */

f = g/**//h;   /* equivalentEquivalent to f = g / h; */

//\
i();           /* partPart of a two-line comment */

/\
/ j();         /* partPart of a two-line comment */


/*//*/ l();    /* equivalentEquivalent to l(); */

m = n//**/o
+ p;           /* equivalentEquivalent to m = n + p; */

a = b //*divisor:*/c
+d;            /* interpreted
                * Interpreted as a = b/c + d; in c90
                * compiler and a = b + d; in c99 compiler.
                */

Compliant Solution

Use a consistent style of commenting:

/* Nice simple comment */

int i; /* counterCounter */

Risk Assessment

Confusion over which instructions are executed and which are not can lead to serious programming errors and vulnerabilities, including denial of service, abnormal program termination, and data integrity violation. This problem is mitigated by the use of interactive development environments (IDEs) 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 on a black-and-white printer.

Automated Detection

The LDRA tool suite V 7.6.0 can detect violations of this recommendation.

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Tool	Version	Checker	Description
Astrée	Include Page Astrée_V Astrée_V	mmline-comment sline-comment sline-splicing smline-comment	Partially checked
GCC	Include Page GCC_V GCC_V		Can detect violations of this rule when the -Wcomment flag is used
ECLAIR	Include Page ECLAIR_V ECLAIR_V	CC2.MSC04	Fully implemented
Helix QAC	Include Page Helix QAC_V Helix QAC_V	C3108
LDRA tool suite	Include Page LDRA_V LDRA_V	119 S, 302 S, 611 S	Partially implemented
Parasoft C/C++test	Include Page Parasoft_V Parasoft_V	CERT_C-MSC04-a CERT_C-MSC04-b CERT_C-MSC04-c CERT_C-MSC04-d	The character sequence /* shall not be used within a C-style comment The character sequence // shall not be used within a C-style comment The character sequence /* shall not be used within a C++-style comment Line-splicing shall not be used in // comments
PC-lint Plus	Include Page PC-lint Plus_V PC-lint Plus_V	1, 427, 602, 689, 853, 9059, 9060, 9066, 9259	Fully supported
Polyspace Bug Finder	Include Page Polyspace Bug Finder_V Polyspace Bug Finder_V	CERT C: Rec. MSC04-C	Checks for use of /* and // within a comment (rule partially covered)
RuleChecker	Include Page RuleChecker_V RuleChecker_V	mmline-comment sline-comment sline-splicing smline-comment	Partially checked

Related Vulnerabilities

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

References

\[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.4.9, "Comments," and Section 6.10.1, "Conditional inclusion"
\[[MISRA 04|AA. C References#MISRA 04]\] Rule 2.2, "Source code shall only use /\* ... \*/ style comments," Rule 2.3, "The character sequence /\* shall not be used within a comment," and Rule 2.4, "Sections of code should not be "commented out"
\[[Summit 05|AA. C References#Summit 05]\] [Question 11.19|http://c-faq.com/ansi/ifdefsyntax.html]

Related Guidelines

Bibliography

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Image Added Image Added MSC03-C. Avoid errors of addition      13. Miscellaneous (MSC)       Image Modified