Older blog entries for redi (starting at number 273)

Hey, C++ programmers, stop writing Strict Weak Orderings by hand for your classes.

For two members it's manageable:

bool operator<(const Widget& l, const Widget& r)
{
if (l.x < r.x)
return true;
if (r.x < l.x)
return false;
return l.y < r.y;
}

But for three members it starts to get tricky:
bool operator<(const Widget& l, const Widget& r)
{
if (l.x < r.x)
return true;
if (r.x < l.x)
return false;
if (l.y < r.y)
return true;
if (r.y < l.y)
return false;
return l.z < r.z;
}

If you think that's not tricky, are you sure I got it right? Maybe I put in a deliberate mistake. Maybe it wasn't deliberate. Now consider that everyone who writes that does it slightly differently e.g.

bool operator<(const Widget& l, const Widget& r)
{
return l.x < r.x ? true : r.x < l.x ? false : l.y < r.y ? true : r.y < l.y ? false : l.z < r.z;
}

Or even:
bool operator<(const Widget& l, const Widget& r)
{
return l.x < r.x || (!(r.x < l.x) && (l.y < r.y || (!(r.y < l.y) && l.z < r.z)));
}

Now do that for ten members.

Stop right now and do this instead:
bool operator<(const Widget& l, const Widget& r)
{
return std::tie(l.x, l.y, l.z) < std::tie(r.x, r.y, r.z);
}

This creates a tuple of references and does a lexicographical comparison, so it just does the Right Thing. All you have to do is list the data members in the same order in each call to the tie function template, and even PHP programmers could do that.

If you haven't caught up with C++ 2011 yet you can use Boost's tuple, just #include <boost/tuple/tuple_comparison.hpp> and then use boost::tie instead of std::tie.
29 Jan 2013 (updated 29 Jan 2013 at 19:04 UTC) »

Gah, stupid brain. I need to remember that the modified pattern space after a series of sed commands will only be printed and deleted right away if that modified pattern space doesn't match the addresses of any later commands. If it does match, it might get further modified and then not printed. Stick ;p;d after the series of commands to avoid that, and not spend an hour scratching your head.

In more interesting news, I've been having fun with new Fedora releases. I upgraded my desktop to F17, because it's not possible to upgrade straight to F18 from F16, but once there I found it so miraculously beefy I decided to keep it. On x86_64 F18 doesn't offer me much that isn't in F17, and what I really want is glibc 2.17 which includes the most excellent change I asked for, but isn't in F18. Following the advice of freenode's #fedora-devel I've got a mock chroot where I can use rawhide, so that I can make some changes to libstdc++ so that a high-resolution std::system_clock will be enabled by default on systems using glibc 2.17+

I have, however, installed F18 (beta) on the shiny ARM chromebook I got for xmas, thanks to lots of help from #fedora-arm. That's a huge improvement on my ageing eeepc netbook. Kudos to the Fedora ARM team on excellent work.

I've also got round to sending my paperwork to the FSF so I can contribute to GDB, which means I should resume working on my patch to teach GDB about C++11 rvalue references. That's been on hold while I sat on the paperwork, wrote a C++ standard proposal and started work on my talk for this year's ACCU conference. (If my wife wasn't working so hard on her studies she'd probably be asking me to stop all these extracurricular activites by now!)

I've often joked that instead of picking up Djikstra's cute acronym we should have called the basic synchronization object "the bottleneck". Bottlenecks are useful at times, sometimes indispensible -- but they're never GOOD.

David Butenhof on mutexes, from the illuminating Re: recursive mutexes
12 Nov 2012 (updated 30 Jan 2013 at 13:44 UTC) »

Gold linker + CentOS5 NFS client + Solaris 10 NFSd = ballache

I've spent a day and a half being completely bewildered by a weird NFS bug where ELF binaries (but not other files) written to an NFS mount show up on remote hosts with the correct file size but consisting entirely of nul zero bytes, but only when written from CentOS5 hosts, not from Solaris, Fedora or RHEL6 hosts.

I eventually narrowed it down to the Gold linker, which writes files using mmap, and the CentOS5 2.6.18 kernel has a bug when writing files with mmap to NFS mounts.

There was a very similar RHEL4 bug that should be fixed in my kernel, but for some reason the kernel-2.6.18-redhat.patch file in the SRPM comments out the fix. I don't know why.

Maybe this post will show up for anyone else searching for the symptoms, because I didn't have much luck searching the web for it.

My solution is to avoid Gold on CentOS5 (since we can't easily stop using NFS, unfortunately) but I wish I could get that day of my life back.

Update: The distcc FAQ (search for Files written to NFS filesystems are corrupt) mentions this problem and refers to a post to the distcc list and a post to the linux-nfs list where a workaround using the no_subtree_check option for nfsd is given, but that assumes the NFS server is linux, and mine isn't

Wishbone and the Illustrated Classics series have gotten me through every conversation I've ever had about Dickens, Treasure Island, The Anarchist's Cookbook, and Our Bodies, Ourselves.
-- Randall Munroe

Similarly, XKCD and The Daily Mash get me through every conversation I ever have about ... pretty much anything really.


We seem to have a new type of syndicated blog that breaks advogato's recentlog, Firefox doesn't render anything after the Gist embedded in oubiwann's post. Maybe the <script> tag is the problem.
"Well, the feature was in the language so I figured I
should use it.". It is our belief that this is not a
sufficient criteria for using a feature of C++. A feature
should be used only when it can be demonstrated to be
of benefit. A mountain is climbed "because it is there".
The same should not hold true for C++ features. Their
mere existence is not justification for use.

-- Evan Adams, The Old Man and the C
The Future of C++: Live broadcast this Friday by Herb Sutter.

If you are interested in C++, on any platform, you’ll want to tune in.
26 Oct 2012 (updated 26 Oct 2012 at 13:16 UTC) »
"Don't put all your information on websites you don't trust." that's Andy Smith, a very sensible govt. official on internet security, stating the obvious.

On the other hand, Helen Goodman, MP for Bishop-Auckland, is a moron.

Andy Smith, an internet security chief at the Cabinet Office, said people should only give accurate details to trusted sites such as government ones.

He said names and addresses posted on social networking sites "can be used against you" by criminals.

His advice was described by Labour MP Helen Goodman as "totally outrageous".

Ms Goodman, shadow culture minister, told BBC News: "This is the kind of behaviour that, in the end, promotes crime.

"It is exactly what we don't want. We want more security online. It's anonymity which facilitates cyber-bullying, the abuse of children."


What a fucking idiot.

Giving a fake date of birth to some e-commerce site that has no valid need for my identifying details and just wants to send me marketing mail no more promotes crime than buying a pair of tights for my wife promotes bank robbery.

(Aside: this image comes from armedrobberyadvice.com ... WTF?)

If Helen Goodman, shadow cabinet minister for talking crap about subjects she doesn't understand, has her way I'll be forced to show my passport to get a Facebook account ... oh wait, they already think they're entitled to that.

Thankfully until the day I can't open a bank account without a validated faecesbook account I can continue being a facebook-refusenik and give fake DOBs to marketers, and can call Helen Goodman MP a fucking moron (and I don't even need the benefit of anonymity to facilitate doing that.) Moron.
25 Oct 2012 (updated 6 Feb 2013 at 10:55 UTC) »

Recently I've had time to work on some C++11 library additions for libstdc++ which had been brewing in my Git tree for months. std::forward_list meets the C++11 allocator container requirements now. std::scoped_allocator_adaptor is 99% complete now. I'm trying to provide std::this_thread::yield(), std::this_thread::sleep_*() and a high-resolution std::system_clock even when GCC isn't built with --enable-libstdcxx-time (with huge thanks to Roland McGrath for help from glibc.) I need to write tests for my implementations of std::wstring_convert and std::wbuffer_convert so I can commit them.

Some stuff I've looked at today:

Mirage, a good reason to learn OCaml.

Parallela, a supercomputer for everyone.

From a few entries down on Advogato's recentlog: the Ada Initiative is running a fundraising campaign, you should donate.

6 Sep 2012 (updated 6 Sep 2012 at 11:40 UTC) »

I hate Istanbul airport.

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