21.4.1 Discarding input
A macro called
dnl discards text from the input. The
macro takes no arguments and expands to the empty string, but it has the
side effect of discarding all input up to and including the next newline
character. Here is an example of
dnl from the Autoconf source
# Restore the previous language.
[AC_FATAL([too many $0])])dnl
It is important to remember
dnl's behavior: it discards the
newline character, which can have unexpected effects on generated
`configure' scripts! If you want a newline to appear in the
output, you must add an extra blank line to compensate.
dnl need not appear in the first column of a given line -- it
will begin discarding input at any point that it is invoked in the input
file. However, be aware of the newline eating problem again! In the example
AC_TRY_LINK_FUNC above, note the deliberate use of
to remove surplus newline characters.
dnl makes sense for macro invocations that appear on
a single line, where you would expect the whole line to simply vanish
from the output. In the following subsections,
dnl will be used
to illustrate where it makes sense to use it.