Re: warning: module to simplify adding compiler warnings

[Bruno Haible]

Paolo Bonzini wrote:
> By the way, I just noticed an (unused?) warning.m4 file.  Ok to delete it?

As far as I understood the modivation for Simon's new module, the old
warning.m4 file does not match these requirements, therefore yes it is OK
to delete it; only Bison uses it.

Here's what I understood:
-------------------------------------------------------------------------
The @code{warnings} module allows to regularly build a package with more
GCC warnings than the default warnings emitted by GCC.

It provides the following functionality:

@itemize @bullet
@item
You can select some warning options, such as @samp{-Wall}, to be enabled
whenever building with a GCC version that supports these options.  The
user can choose to override these warning options by providing the
opposite options in the @code{CFLAGS} variable at configuration time.

@item
You can make these warnings apply to selected directories only.  In
projects where subprojects are maintained by different people, or where
parts of the source code are imported from external sources -- for example
from gnulib --, it is useful to apply different warning options to
different directories.

@item
It allows to use @samp{-Werror} at @samp{make distcheck} time, to verify
that on the maintainer's system, no warnings remain.  (Note that use of
@samp{-Werror} in @code{CFLAGS} does not work in general, because it may
break autoconfiguration.)
@end itemize

To use this module, you need the following:

@enumerate
@item
In @file{configure.ac}, use for example
@smallexample
gl_WARN_ADD([-Wall], [WARN_CFLAGS])
gl_WARN_ADD([-Wpointer-arith], [WARN_CFLAGS])
@end smallexample

@item
In the directories which shall use @code{WARN_CFLAGS}, use it in the
definition of @code{AM_CFLAGS}, like this:
@smallexample
AM_CFLAGS = $(WARN_CFLAGS)
@end smallexample

Note that the @code{AM_CFLAGS} is used in combination with @code{CFLAGS}
and before @code{CFLAGS} in build rules emitted by Automake.  This allows
the user to provide @code{CFLAGS} that override the @code{WARN_CFLAGS}.
@end enumerate

Note that it is a bad idea to use @samp{gl_WARN_ADD([-Werror])}.  The
warnings emitted by GCC depend, to some extent, on the contents of the
system header files, on the size and signedness of built-in types, etc.
Use of @samp{-Werror} would cause frustration to all users on platforms
that the maintainer has not tested before the release.  It is better if
maintainers use @samp{-Werror} only for themselves (for example, during
@samp{make distcheck}, as mentioned above).
-------------------------------------------------------------------------

Simon, maybe you can add a piece of text like this as documentation?

Bruno