C Language Comments


Comments are used to indicate something to the person reading the code. Comments are treated like a blank by the compiler and do not change anything in the code's actual meaning. There are two syntaxes used for comments in C, the original /* */ and the slightly newer //. Some documentation systems use specially formatted comments to help produce the documentation for code.


  • /*...*/
  • //... (C99 and later only)

/* */ delimited comments

A comment starts with a forward slash followed immediately by an asterisk (/*), and ends as soon as an asterisk immediately followed by a forward slash (*/) is encountered. Everything in between these character combinations is a comment and is treated as a blank (basically ignored) by the compiler.

/* this is a comment */

The comment above is a single line comment. Comments of this /* type can span multiple lines, like so:

/* this is a
comment */

Though it is not strictly necessary, a common style convention with multi-line comments is to put leading spaces and asterisks on the lines subsequent to the first, and the /* and */ on new lines, such that they all line up:

 * this is a
 * multi-line
 * comment

The extra asterisks do not have any functional effect on the comment as none of them have a related forward slash.

These /* type of comments can be used on their own line, at the end of a code line, or even within lines of code:

/* this comment is on its own line */
if (x && y) { /*this comment is at the end of a line */
    if ((complexCondition1) /* this comment is within a line of code */
            && (complexCondition2)) {
        /* this comment is within an if, on its own line */

Comments cannot be nested. This is because any subsequent /* will be ignored (as part of the comment) and the first */ reached will be treated as ending the comment. The comment in the following example will not work:

/* outer comment, means this is ignored => /* attempted inner comment */ <= ends the comment, not this one => */

To comment blocks of code that contain comments of this type, that would otherwise be nested, see the Commenting using the preprocessor example below

// delimited comments


C99 introduced the use of C++-style single-line comments. This type of comment starts with two forward slashes and runs to the end of a line:

// this is a comment

This type of comment does not allow multi-line comments, though it is possible to make a comment block by adding several single line comments one after the other:

// each of these lines are a single-line comment
// note how each must start with
// the double forward-slash

This type of comment may be used on its own line or at the end of a code line. However, because they run to the end of the line, they may not be used within a code line

// this comment is on its own line
if (x && y) { // this comment is at the end of a line
    // this comment is within an if, on its own line

Commenting using the preprocessor

Large chunks of code can also be "commented out" using the preprocessor directives #if 0 and #endif. This is useful when the code contains multi-line comments that otherwise would not nest.

#if 0 /* Starts the "comment", anything from here on is removed by preprocessor */ 

/* A large amount of code with multi-line comments */  
int foo()
    /* lots of code */

    /* ... some comment describing the if statement ... */
    if (someTest) {
        /* some more comments */
        return 1;

    return 0;

#endif /* 0 */

/* code from here on is "uncommented" (included in compiled executable) */

Possible pitfall due to trigraphs


While writing // delimited comments, it is possible to make a typographical error that affects their expected operation. If one types:

int x = 20;  // Why did I do this??/

The / at the end was a typo but now will get interpreted into \. This is because the ??/ forms a trigraph.

The ??/ trigraph is actually a longhand notation for \, which is the line continuation symbol. This means that the compiler thinks the next line is a continuation of the current line, that is, a continuation of the comment, which may not be what is intended.

int foo = 20; // Start at 20 ??/
int bar = 0;

// The following will cause a compilation error (undeclared variable 'bar')
// because 'int bar = 0;' is part of the comment on the preceding line
bar += foo;