A "comment" is a sequence of characters beginning with a forward slash/asterisk combination (/*) that is treated as a single white-space character by the compiler and is otherwise ignored. A comment can include any combination of characters from the representable character set, including newline characters, but excluding the "end comment" delimiter (*/). Comments can occupy more than one line but can't be nested.

Comments can appear anywhere a white-space character is allowed. Since the compiler treats a comment as a single white-space character, you can't include comments within tokens. The compiler ignores the characters in the comment.

Use comments to document your code. This example is a comment accepted by the compiler:

/* Comments can contain keywords such as
   for and while without generating errors. */

Comments can appear on the same line as a code statement:

printf( "Hello\n" );  /* Comments can go here */

You can choose to precede functions or program modules with a descriptive comment block:

/* MATHERR.C illustrates writing an error routine
* for math functions.
*/

Since comments can't contain nested comments, this example causes an error:

/* Comment out this routine for testing

   /* Open file */
    fh = _open( "myfile.c", _O_RDONLY );
    .
    .
    .
*/

The error occurs because the compiler recognizes the first */, after the words Open file, as the end of the comment. It tries to process the remaining text and produces an error when it finds the */ outside a comment.

While you can use comments to render certain lines of code inactive for test purposes, the preprocessor directives #if and #endif and conditional compilation are a useful alternative for this task. For more information, see Preprocessor Directives in the Preprocessor Reference.

Microsoft Specific

The Microsoft compiler also supports single-line comments preceded by two forward slashes (//). These comments can't extend to a second line.

// This is a valid comment

Comments beginning with two forward slashes (//) are terminated by the next newline character that isn't preceded by an escape character. In the next example, the newline character is preceded by a backslash (\), creating an "escape sequence." This escape sequence causes the compiler to treat the next line as part of the previous line. (For more information, see Escape Sequences.)

// my comment \
    i++;

Therefore, the i++; statement is commented out.

The default for Microsoft C is that the Microsoft extensions are enabled. Use /Za to disable these extensions.

END Microsoft Specific

C Tokens