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.

Commenting using the preprocessor

#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) */
...

/* */ 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
multi-line
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 => */

// delimited 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
}