C supports two different styles of comments: Block Comments and Single Line Comments.

Block Comments

Block Comments begin with a /* and end with a */ and may span multiple lines. For example:

Lines 1-4 are comprised of one long comment that spans four lines. The comment begins with the /* at the beginning of line 1 and ends with the */ at the end of line 4. Every other character between those two delimiters is ignored by the compiler.

Line 7 shows another example where a block comment is used for just a single line. While line 10 shows a similar construct to add a comment at the end of a line of code.

Single Line Comments

Single line comments begin with a // and run to the end of the line. These are sometimes called "C++ style comments" because they were developed for C++ and were not originally part of the ANSI C standard. They may not span multiple lines. For example:

Here we have replaced each block comment from the first example with an equivalent form using a line comment.

Lines 1-4 are now four separate comments with each line starting with //.

Lines 7 and 10 were also converted into single-line comments that are more appropriate for their situations.

Nesting Comments

Block comments may not be nested within other block comments. However, single-line comments may be nested within a block comment.

Lines 1-3 show that it is perfectly valid to nest a single line comment within a block comment. In fact, this is a widespread practice when testing programs. From time to time, you may want to temporarily remove a block of code from your program. Using block comment delimiters as shown here will have that effect as long as the only other comments between the delimiters are single-line comments.

Lines 5-8 show what can go wrong when block comments are nested within one another. A comment begins with the /* at the beginning of line 5. Because of line 5, the opening delimiter /* on line 6 is completely ignored and the closing delimiter */ at the end of line 6 is matched with the opening delimiter on line 5. This leaves the code on line 7 uncommented, followed by a complete comment at the end of line 7.

Therefore, at the beginning of line 8, we have a closing delimiter */ that is unmatched with an opening delimiter which will cause a syntax error.

Best Practices

Ideally, you should use single-line comments throughout most of your code, reserving block comments for times when you want to temporarily remove a block of code or for when you really need multi-line comments. Below is an example of how many programmers use the two comment styles.