compiler warnings, -Werror, etc.

[Colin Walters <walters verbum org>]

Hi,

TL;DR version
=============

I'd like to avoid having to pass various different configure flags to
modules to disable -Werror.

For compiler warning defaults, I think something similar what Dan
Winship has in libsoup is what we should replicate across more GNOME
modules:

http://git.gnome.org/browse/libsoup/tree/configure.ac?id=f5902fce98ae0314f0d9ca6e544895548c94a456#n339

It's better than the GNOME_COMPILE_WARNINGS macro in gnome-common right
now, and *definitely* better than various modules having -Wall -Werror.

Longer version
==============

First, compiler warnings range widely in how important they are.  Some
of them, like -Wmissing-prototypes, are pretty much always indicative of
a serious problem in your code.  Others like -Wmaybe-uninitialzed are
actually *heuristics* - the GCC manual clearly states that GCC may in
fact be wrong, and that one even has "maybe" in the name.

This gets to the next problem, which is that -Wall includes
-Wmaybe-uninitialized and other heuristics like -Wstrict-aliasing.  Then
combine that with the fact that some people have got it in their head
that "-Wall -Werror" is the Right Thing, what actually ends up happening
is your code only compiles on a *particular version* of gcc.  That just
doesn't work in a distributed project like GNOME, where various bits get
reused by different projects and products on different schedules etc.

Also, -Wdeprecated-declarations with -Werror makes the entire concept of
deprecating things pointless.  The GNOME build shouldn't immediately
blow up because we deprecate something in GLib.

Finally, sadly -Wformat-security *isn't* in -Wall (at least in GCC 4.7),
but regardless, everyone should add -Werror=format-security like
basically now =)

So I think what Dan has is more the Right Thing - make the compiler
warnings that you should *never* hit into explicit errors.  That
establishes a minimum baseline.  Maintainers should feel free to make
some other things into errors as they wish (-Werror=unused-but-set), but
please don't use -Werror=deprecated-declarations or
-Werror=mabe-uninitialized.