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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 (preprocessor)

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 //.

Compliant Solution (compiler)

This compliant solution takes advantage of the compiler's ability to remove dead code.  The code inside the if block must still stay acceptable to the compiler.  If macros or function prototypes change later in a way that would cause the unexecuted code to contain syntax errors, the 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 remove the surrounding if statement.

if (0) {  /* use of critical security function no longer necessary, at least for now */
  security_critical_function();
  /* some other comment */
}

Non-Compliant Code Example

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// */          /* 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; */

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

Compliant Solution

Use a consistent style of commenting:

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