Re: AC_CHECK_FUNCS and gcc with -Werror

[Steffen Dettmer]

On Wed, Mar 3, 2010 at 3:59 PM, Eric Blake <address@hidden> wrote:
>> I'm afraid again I just fail to see the obvious and it is very
>> easy to do?
>
> Yes.  Here's how m4 does it.  It uses the gl_WARN_ADD macro from the
> gnulib module manywarnings; but if you choose not to use gnulib, it is a
> simple enough macro that you can probably just inline it directly into
> configure.ac:
>
> # gl_WARN_ADD(PARAMETER, [VARIABLE = WARN_CFLAGS])

(ohh and this piece of M4-language-magic you call `very easy'...
will I ever be able to understand a bit of it uhh-ohh...)

> # ------------------------------------------------
> # Adds parameter to WARN_CFLAGS if the compiler supports it.  For example,
> # gl_WARN_ADD([-Wparentheses]).
> AC_DEFUN([gl_WARN_ADD],

(If I understand correctly, this macro checks if the compiler
still runs when the flag is given.)

> Then in configure.ac, you give the user the option to request extra
> compiler flags, and create an AC_SUBST variable containing the result of
> the supported flags:
>
> AC_ARG_ENABLE([gcc-warnings],

ahh, you already wrote but I just notice now, it is gcc-option.
So instead of --enable-errors we could use --enable-gcc-errors
and --enable-cl-errors.

> Finally, in Makefile.am, you use those flags.  That way, CFLAGS was
> unchanged during configure (except temporarily, during gl_WARN_ADD, when
> sniffing whether the flag works), but make gets the benefit of the flag:
>
> AM_CFLAGS = $(WERROR_CFLAGS)

ahh ok this way.
Before I change my 352 Makefile.am's:
- Can this do harm (e.g. in makefiles for packages without C
  sources or so)?
- Can I simply append `AM_CFLAGS += $(WERROR_CFLAGS)' to each
  Makefile.am and be done?
- should I better use something like `-include warnings.mak' or
  alike to be more flexible in case I'll need it later?

- Alternatively, by accident, ts there some way to tell configure
  something like: right just before writing config.status, set
  `CFLAGS=$CFLAGS\ $WERROR_CFLAGS' or so?
  As Peter suggests?
- Instead of changing 352 Makefile.am's, should I change 42
  configure.{in,ac} to have a line like
  CFLAGS=$CFLAGS\ $WERROR_CFLAGS
  right before AC_OUTPUT? Wouldn't this work, too? But as far as
  I know the order of macros/statements in configure.in does not
  mean that the actions are really executed in the order they
  appear...
  Would this be an automatic way of what Peter suggests?


> > We now have in configure.in (or actually an included .m4
> > file) `CFLAGS="$CFLAGS -Werror";'
>
> That's where you are going wrong.  The recommended approach is
> to modify CFLAGS in the Makefile, using an AC_SUBST, rather
> than hard-coding the modification into configure.

ohh I though configure should configure the compiler flags?

We do not want that developers have to know about compiler flags
(i.e. they should not need to run make CFLAGS=longListOfStrangeThings),
especially when they cross-compile just to check for warnings
with compilers for platforms they don't use.

oki,

Steffen