Re: AC_CHECK_FUNCS and gcc with -Werror

[Eric Blake]

According to Steffen Dettmer on 3/3/2010 7:27 AM:
> I'm sorry, I didn't express well what we want. We do not
> need/want -Werror inside configure. Just inside make.
> 
> 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])
# ------------------------------------------------
# Adds parameter to WARN_CFLAGS if the compiler supports it.  For example,
# gl_WARN_ADD([-Wparentheses]).
AC_DEFUN([gl_WARN_ADD],
[AS_VAR_PUSHDEF([gl_Warn], [gl_cv_warn_$1])dnl
AC_CACHE_CHECK([whether compiler handles $1], [gl_Warn], [
  save_CPPFLAGS="$CPPFLAGS"
  CPPFLAGS="${CPPFLAGS} $1"
  AC_PREPROC_IFELSE([AC_LANG_PROGRAM([])],
                    [AS_VAR_SET([gl_Warn], [yes])],
                    [AS_VAR_SET([gl_Warn], [no])])
  CPPFLAGS="$save_CPPFLAGS"
])
AS_VAR_PUSHDEF([gl_Flags], m4_if([$2], [], [[WARN_CFLAGS]], [[$2]]))dnl
AS_VAR_IF([gl_Warn], [yes], [gl_AS_VAR_APPEND([gl_Flags], [" $1"])])
AS_VAR_POPDEF([gl_Flags])dnl
AS_VAR_POPDEF([gl_Warn])dnl
m4_ifval([$2], [AS_LITERAL_IF([$2], [AC_SUBST([$2])], [])])dnl
])



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],
  [AS_HELP_STRING([--enable-gcc-warnings],
                  [turn on lots of GCC warnings (for developers)])],
  [case $enableval in
     yes|no) ;;
     *)      AC_MSG_ERROR([bad value $enableval for gcc-warnings option]) ;;
   esac
   gl_gcc_warnings=$enableval],
  [gl_gcc_warnings=no]
)

if test "$gl_gcc_warnings" = yes; then
  gl_WARN_ADD([-Werror], [WERROR_CFLAGS])
  AC_SUBST([WERROR_CFLAGS])
# ... more here, for selecting particular -W warnings
fi


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)


> 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.

-- 
Eric Blake   address@hidden    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

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