sendmail, 4th Edition

m4 and dnl

Another problem with m4 is that it replaces its commands with empty lines. The earlier define commands, for example, will actually print like this:

                       ← a blank line
                       ← a blank line
 C B

To suppress this insertion of blank lines, you can use the special m4 command dnl (for Delete through New Line). That command looks like this:

define(`A',`B')dnl
define(`A',`C')dnl
A B

You can use dnl to remove blank lines where they might prove inconvenient or unsightly in a configuration file.

The dnl command can also be used to put comments into an mc file. Just be sure to put a blank line after the last dnl because each dnl gobbles both the text and the newline:

dnl This is a comment.
                       ← note the extra blank line