Autoconf, Automake, and Libtool: Discarding input

| ### 21.4.1 Discarding input A macro called `dnl` discards text from the input. The `dnl` 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 code:
    # AC_LANG_POP
    # -----------
    # Restore the previous language.
    define([AC_LANG_POP],
    [popdef([_AC_LANG])dnl
    ifelse(_AC_LANG, [_AC_LANG],
            [AC_FATAL([too many $0])])dnl
    AC_LANG(_AC_LANG)])
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 of `AC_TRY_LINK_FUNC` above, note the deliberate use of `dnl` to remove surplus newline characters. In general, `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.

This document was generated by Gary V. Vaughan on February, 8 2006 using texi2html