Mixing the use of traditional or block comments (starting with /* and ending with */) and end-of-line comments (from // to the end of the line) can lead to misleading and confusing code, which may result in errors.
Noncompliant Code Example
The following examples show mixed comments that may be misunderstood:
// */ /* Comment, not syntax error */ f = g/**//h; /* Equivalent to f = g / h; */ /*//*/ l(); /* Equivalent to l(); */ m = n//**/o + p; /* Equivalent to m = n + p; */ a = b //*divisor:*/c + d; /* Equivalent to a = b + d; */
Compliant Solution
Use a consistent style of commenting:
// Nice simple comment int i; // Counter
Noncompliant Code Example
There are other misuses of comments that should be avoided. The following noncompliant example uses the character sequence /* to begin a comment but neglects to use the delimiter */ to end the comment. Consequently, the call to the security-critical method is not executed. A reviewer examining this page could incorrectly assume that the code is executed.
/* Comment with end comment marker unintentionally omitted security_critical_method(); /* Some other comment */
Using an editor that provides syntax highlighting or that formats the code to identify issues like missing end comment delimiters can help detect accidental omissions.
Because missing end delimiters are error prone and often viewed as a mistake, this approach is not recommended for commenting out code.
Compliant Solution
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 later change 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 the programmer must do is remove the surrounding if statement and the NOTREACHED comment.
The NOTREACHED comment could tell some compilers and static analysis tools not to complain about this unreachable code. It also serves as documentation.
if (false) { /* Use of critical security method no
* longer necessary, for now */
/* NOTREACHED */
security_critical_method();
/* Some other comment */
}
This is an instance of an exceptional situation described in 63. Detect and remove superfluous code and values.
Applicability
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, for example, when reviewing source code printed on a black-and-white printer.
Nested block comments and inconsistent use of comments could be detected by suitable static analysis tools.
Bibliography