From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from localhost (localhost [127.0.0.1]) by olra.theworths.org (Postfix) with ESMTP id 1FCE3431FAF for ; Mon, 22 Oct 2012 07:32:56 -0700 (PDT) X-Virus-Scanned: Debian amavisd-new at olra.theworths.org X-Spam-Flag: NO X-Spam-Score: 0 X-Spam-Level: X-Spam-Status: No, score=0 tagged_above=-999 required=5 tests=[none] autolearn=disabled Received: from olra.theworths.org ([127.0.0.1]) by localhost (olra.theworths.org [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id 0Hra137KXHE7 for ; Mon, 22 Oct 2012 07:32:54 -0700 (PDT) Received: from guru.guru-group.fi (guru.guru-group.fi [46.183.73.34]) by olra.theworths.org (Postfix) with ESMTP id 8D220431FAE for ; Mon, 22 Oct 2012 07:32:54 -0700 (PDT) Received: from guru.guru-group.fi (localhost [IPv6:::1]) by guru.guru-group.fi (Postfix) with ESMTP id CF8F410036F; Mon, 22 Oct 2012 17:32:53 +0300 (EEST) From: Tomi Ollila To: Jani Nikula , Ethan Glasser-Camp , notmuch@notmuchmail.org Subject: Re: [PATCH] lib: fix warnings when building with clang In-Reply-To: <87vce3vf5q.fsf@nikula.org> References: <1349076971-2065-1-git-send-email-jani@nikula.org> <87pq4c61hc.fsf@betacantrips.com> <87vce3vf5q.fsf@nikula.org> User-Agent: Notmuch/0.14+51~g62cd13b (http://notmuchmail.org) Emacs/24.2.1 (x86_64-unknown-linux-gnu) X-Face: HhBM'cA~ MIME-Version: 1.0 Content-Type: text/plain X-BeenThere: notmuch@notmuchmail.org X-Mailman-Version: 2.1.13 Precedence: list List-Id: "Use and development of the notmuch mail system." List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 22 Oct 2012 14:32:56 -0000 On Sun, Oct 21 2012, Jani Nikula wrote: > On Sun, 21 Oct 2012, Ethan Glasser-Camp wrote: >> Jani Nikula writes: >> >>> Building notmuch with CC=clang and CXX=clang++ produces the warnings: >>> >>> CC -O2 lib/tags.o >>> lib/tags.c:43:5: warning: expression result unused [-Wunused-value] >>> talloc_steal (tags, list); >>> ^~~~~~~~~~~~~~~~~~~~~~~~~ >>> /usr/include/talloc.h:345:143: note: expanded from: >>> ...__location__); __talloc_steal_ret; }) >>> ^~~~~~~~~~~~~~~~~~ >>> 1 warning generated. >>> >>> CXX -O2 lib/message.o >>> lib/message.cc:791:5: warning: expression result unused [-Wunused-value] >>> talloc_reference (message, message->tag_list); >>> ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ >>> /usr/include/talloc.h:932:36: note: expanded from: >>> ...(_TALLOC_TYPEOF(ptr))_talloc_reference_loc((ctx),(ptr), __location__) >>> ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ >>> 1 warning generated. >>> >>> Check talloc_reference() return value, and explicitly ignore >>> talloc_steal() return value as it has no failure modes, to silence the >>> warnings. >>> --- >>> lib/message.cc | 4 +++- >>> lib/tags.c | 2 +- >>> 2 files changed, 4 insertions(+), 2 deletions(-) >>> >>> diff --git a/lib/message.cc b/lib/message.cc >>> index 978de06..320901f 100644 >>> --- a/lib/message.cc >>> +++ b/lib/message.cc >>> @@ -788,7 +788,9 @@ notmuch_message_get_tags (notmuch_message_t *message) >>> * possible to modify the message tags (which talloc_unlink's the >>> * current list from the message) while still iterating because >>> * the iterator will keep the current list alive. */ >>> - talloc_reference (message, message->tag_list); >>> + if (!talloc_reference (message, message->tag_list)) >>> + return NULL; >>> + >>> return tags; >>> } >> >> Hi! What you did with talloc_steal is obviously fine. >> >> I'd be happier about what you did with talloc_reference() if there were >> precedent, or a clearly-articulated convention for notmuch. Instead this >> is the third use in the codebase that I can see, and the other two are >> each unique to themselves. In mime-node.c we print an "out-of-memory" >> error and in lib/filenames.c we cast (void) talloc_reference (...), I >> guess figuring that we're pretty much hosed anyhow if we run out of >> memory. >> >> Why return NULL here? It seems like if talloc_reference fails, we're >> going to crash eventually, so we should print an error to explain our >> impending doom. I'd guess you're uneasy printing anything from lib/, but >> still want to signal an error, and the only way you can do so is to >> return NULL. I guess that silences the compiler warning, but it's not >> really the correct way to handle the error IMO. On the other hand, it's >> such a weird corner case that I don't even think it merits a FIXME >> comment. >> >> How about an assert instead of a return NULL? > > No. I don't think a library should assert, exit, or print to stderr on > this sort of thing. It's up to the calling application. Even if it > probably doesn't have many choices left, given how much memory > talloc_reference needs (not much). > > Ignoring the talloc_reference return value with (void) is just wrong, > and the caller of notmuch_message_get_tags should anticipate a NULL > return. So IMHO that's the pragmatic thing to do in this mostly > theoretical situation, the biggest change being silencing the warning. I agree that the best library can do is to return NULL (if talloc had a place in ctx to store error indication that could be used but I did not see any in quick look -- and using global there is not a good idea) but, before returning NULL should 'tags' be freed. Additionally, should lib/filenames.c be changed to have code: if (unlikely (talloc_reference(filenames, list) == NULL)) { talloc_free (filenames); return NULL; } > BR, > Jani. (btw, what are the chances that program crashes before returning NULL due to page fault in stack frame allocation ???) Tomi