-Werror considered harmful

[Behdad Esfahbod <behdad behdad org>]

[I think I'm not on d-d-l, CC is appreciated.]

<rant>

Hi,

I'm sending this email instead of filing a bug because I think I need to say
this as widely as I can: modules adding -Werror (or -Werror=... variants
thereof) are harmful

Background:

A few years ago Javier opened this bug against gnome-common:

  https://bugzilla.gnome.org/show_bug.cgi?id=608953

which references my blogpost:

  http://mces.blogspot.ca/2008/12/year-end-cleaning-ie-on-warning-options.html

The bug title is "Add some more compiler warning options".  Something that I
thinks is good hygiene.  However, in the course of that happening, things like
these ended up going in:

    dnl These compiler flags typically indicate very broken or suspicious
    dnl code.  Some of them such as implicit-function-declaration are
    dnl just not default because gcc compiles a lot of legacy code.
    dnl We choose to make this set into explicit errors.
    base_error_flags=" \
        -Werror=missing-prototypes \
        -Werror=implicit-function-declaration \
        -Werror=pointer-arith \
        -Werror=init-self \
        -Werror=format-security \
        -Werror=format=2 \
        -Werror=missing-include-dirs \
    "

which then made it into glib as part of bug 687385.

Now, can you spot the problem?  Hint: the comment above says "*typically*
indicate very broken".  I translate: "typically this is fine, in other
situations we are breaking people's build, but that's fine".

Indeed, the email that went out with the change [1] acknowledged the problem:

"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."

but goes on to conclude:

"So I think what Dan has is more the Right Thing - make the compiler
warnings that you should *never* hit into explicit errors."

Interesting.  So some omniscient hacker decided that compilers compiling GNOME
shall never ever see code that generates a missing-prototypes warning, err,
error.  Not in the cryptic system headers of whatever weird cross compiler I
may be using.  I want your crystal ball seized!

Here I am, trying to cross-compile glib using mingw32, so I can cross-compile
pango, so I can fix a pangowin32 bug, and was stopped getting this:

behdad:libcharset 2$ make V=1
/bin/sh ../../libtool  --tag=CC   --mode=compile i586-mingw32msvc-gcc
-DHAVE_CONFIG_H -I. -I../../../glib/libcharset -I../..
-DLIBDIR=\"/home/behdad/.local/i586-mingw32msvc/lib\" -I../..
-I/home/behdad/.local/i586-mingw32msvc/include   -g -O2 -mms-bitfields  -Wall
-Wstrict-prototypes -Werror=declaration-after-statement
-Werror=missing-prototypes -Werror=implicit-function-declaration
-Werror=pointer-arith -Werror=init-self -Werror=format=2
-Werror=missing-include-dirs -MT localcharset.lo -MD -MP -MF
.deps/localcharset.Tpo -c -o localcharset.lo
../../../glib/libcharset/localcharset.c
libtool: compile:  i586-mingw32msvc-gcc -DHAVE_CONFIG_H -I.
-I../../../glib/libcharset -I../..
-DLIBDIR=\"/home/behdad/.local/i586-mingw32msvc/lib\" -I../..
-I/home/behdad/.local/i586-mingw32msvc/include -g -O2 -mms-bitfields -Wall
-Wstrict-prototypes -Werror=declaration-after-statement
-Werror=missing-prototypes -Werror=implicit-function-declaration
-Werror=pointer-arith -Werror=init-self -Werror=format=2
-Werror=missing-include-dirs -MT localcharset.lo -MD -MP -MF
.deps/localcharset.Tpo -c ../../../glib/libcharset/localcharset.c
-DDLL_EXPORT -DPIC -o .libs/localcharset.o
In file included from ../../../glib/libcharset/localcharset.c:28:
/usr/lib/gcc/i586-mingw32msvc/4.2.1-sjlj/../../../../i586-mingw32msvc/include/stdio.h:372:
error: no previous prototype for 'getc'
/usr/lib/gcc/i586-mingw32msvc/4.2.1-sjlj/../../../../i586-mingw32msvc/include/stdio.h:379:
error: no previous prototype for 'putc'
/usr/lib/gcc/i586-mingw32msvc/4.2.1-sjlj/../../../../i586-mingw32msvc/include/stdio.h:386:
error: no previous prototype for 'getchar'
/usr/lib/gcc/i586-mingw32msvc/4.2.1-sjlj/../../../../i586-mingw32msvc/include/stdio.h:393:
error: no previous prototype for 'putchar'
In file included from ../../../glib/libcharset/localcharset.c:28:
/usr/lib/gcc/i586-mingw32msvc/4.2.1-sjlj/../../../../i586-mingw32msvc/include/stdio.h:535:
error: no previous prototype for 'fopen64'
/usr/lib/gcc/i586-mingw32msvc/4.2.1-sjlj/../../../../i586-mingw32msvc/include/stdio.h:547:
error: no previous prototype for 'ftello64'
/usr/lib/gcc/i586-mingw32msvc/4.2.1-sjlj/../../../../i586-mingw32msvc/include/stdio.h:606:
error: no previous prototype for 'vsnwprintf'


Please, please, please, really, stop acting as if you know my build
environment...  If I want errors, I'll enable them myself.

</rant>

[1] https://mail.gnome.org/archives/desktop-devel-list/2012-July/msg00100.html

-- 
behdad
http://behdad.org/