Re: Autoheader should ignore m4 "dnl" comments

[Eric Blake]

On 02/21/2018 08:07 PM, Kang-Che Sung wrote:
> This bug is a funny one, I think.
>
> In flex (lexer) configure.ac, I use "dnl" comments within
> AC_CHECK_FUNCS for inline notes, like this:
>
>      # --------------------------------
>      AC_CHECK_FUNCS([dnl
>      pow dnl Used only by "examples/manual/expr"
>      setlocale dnl Needed only if NLS is enabled
>      reallocarr dnl NetBSD function. Use reallocarray if not available.
>      reallocarray dnl OpenBSD function. We have replacement if not available.
>      ])

Rather than waiting for an updated autoconf with the fix, you could 
instead workaround it in the meantime, by making it more obvious that 
your comments are just that:

AC_CHECK_FUNCS(
[pow ]dnl Used only by "examples/manual/expr"
[seclocale ]dnl Needed only if NLS is enabled
[reallocarr ]dnl NetBSD function. Use reallocarray if not available.
[reallocarray ]dnl OpenBSD function. We have replacement if not available.
)

>      # --------------------------------
>
> But this syntax will make autoheader generate unneeded HAVE_ macros in
> config.h.in:
>
>      /* Define to 1 if you have the `available.' function. */
>      #undef HAVE_AVAILABLE_
>
>      /* Define to 1 if you have the `by' function. */
>      #undef HAVE_BY
>
>      /* Define to 1 if you have the `dnl' function. */
>      #undef HAVE_DNL
>
> The dnl comments should be ignored when generating config.h.in header,
> or at least document that users should not add comments from within
> AC_CHECK_FUNCS.

Yeah, that's an unfortunate side effect of the current implementation, 
which treats AC_CHECK_FUNCS as a whitespace separated list of words and 
does NOT perform macro expansion on that list as you would expect.  It 
may be possible to fix autoconf, but the fix may be hairy; or it might 
not even be possible to fix, at which point, documentation would be the 
only thing to improve.

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
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