In autotools, what is dnl'ed?

Question

I see a lot in autoconf code about stuff being dnl'ed and not dnl'ed. What is dnl'ed?

Answer 1

from the GNU M4 1.4.13 macro processor manual:

8.1 Deleting whitespace in input

The builtin dnl stands for “Discard to Next Line”:

— Builtin: dnl

All characters, up to and including the next newline, are discarded without performing any macro expansion. A warning is issued if the end of the file is encountered without a newline.

The expansion of dnl is void.

It is often used in connection with define, to remove the newline that follows the call to define. Thus

   define(`foo', `Macro `foo'.')dnl A very simple macro, indeed.
   foo
   ⇒Macro foo.

The input up to and including the next newline is discarded, as opposed to the way comments are treated (see Comments).

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