In autotools, what is dnl'ed?

Question

I see a lot in autoconf code about what's happening, not dnl'ed. What is dnl'ed?

Answer 1

from GNU M4 1.4.13 macro processor :

8.1 Removing spaces in input files

Built-in dnlmeans "Undo the following line":

- Built-in: dnl

All characters, up to the next new line, are discarded without performing a macro exchange. A warning is issued if the end of the file occurs without a new line.

     

Decomposition is dnlnot valid.

define, , define. ,

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

, (. Comments).

[...]

Answer 2

IIRC, dnl - do-next-line. . M4.

dnl preprocessor will eat this
but not this dnl though this

but not this

(, ).