Channels ▼


Writing Lock-Free Code: A Corrected Queue

Do Work, Then Publish

You might also have noticed that the original code in [2] did the equivalent of lines C (copy) and D (divider update) in the reverse order. You should always be alert and suspicious when you see code that tries to do things backwards: Remember, we're supposed to do all the work off to the side (line C) and only then publish that we did it (line D), as previously shown.

I'm sure someone is about to point out that we could actually get away with writing D then C in this code. Yes, but don't; it's a bad habit. It's true that, in this particular case and now that divider is an ordered atomic variable (which wasn't true in the original code), it just so happens that we could get away with writing D then C due to the happy accident of a detail of the implementation combining with a design restriction:

    We always maintain one placeholder divider element between the producer and the consumer, so "publishing" the change to divider what would otherwise be one step too soon, so that refers to an unconsumed node rather than to a consumed node, happens to be innocuous as long as we're only one step ahead. There's exactly one consumer thread, so multiple calls to Consume must run in sequence and can never get two steps ahead.

But it's still a bad habit to get into. It's not a good idea to cut corners by relying on "happy" accidents, especially because there's not much to be gained here from breaking the correct pattern. Besides, even if we wrote D then C now, it might be just another thing we'd have to change anyway next month, because...

Coming Up

Next month, we will consider how to generalize the queue for multiple producer and consumer threads. Your homework: What new issues does this raise? What parts of the code we just considered would be broken in the presence of multiple consumers alone and why? What about multiple producers? What about both? Once you've discovered the problems, what would you need to change in the code and in the queue data structure itself to address them?

You have one month. Think of how you would approach it, and we'll take up the challenge when we return.


[1] H. Sutter. “Lock-Free Code: A False Sense of Security” (DDJ, September 2008). (

[2] P. Marginean. "Lock-Free Queues" (DDJ, July 2008). (

[3] This is just like a canonical exception safety pattern—do all the work off to the side, then commit to accept the new state using nonthrowing operations only. "Think in transactions" applies everywhere, and should be ubiquitous in the way we write our code.

[4] Compare-and-swap (CAS) is the most widely available fundamental lock-free operation and so I'll focus on it here. However, some systems instead provide the equivalently powerful load-linked/store-conditional (LL/SC) instead.


Thanks to Tim Harris for his comments on drafts of this article.

Herb is a software development consultant, a software architect at Microsoft, and chair of the ISO C++ Standards committee. He can be contacted at

Related Reading

More Insights

Currently we allow the following HTML tags in comments:

Single tags

These tags can be used alone and don't need an ending tag.

<br> Defines a single line break

<hr> Defines a horizontal line

Matching tags

These require an ending tag - e.g. <i>italic text</i>

<a> Defines an anchor

<b> Defines bold text

<big> Defines big text

<blockquote> Defines a long quotation

<caption> Defines a table caption

<cite> Defines a citation

<code> Defines computer code text

<em> Defines emphasized text

<fieldset> Defines a border around elements in a form

<h1> This is heading 1

<h2> This is heading 2

<h3> This is heading 3

<h4> This is heading 4

<h5> This is heading 5

<h6> This is heading 6

<i> Defines italic text

<p> Defines a paragraph

<pre> Defines preformatted text

<q> Defines a short quotation

<samp> Defines sample computer code text

<small> Defines small text

<span> Defines a section in a document

<s> Defines strikethrough text

<strike> Defines strikethrough text

<strong> Defines strong text

<sub> Defines subscripted text

<sup> Defines superscripted text

<u> Defines underlined text

Dr. Dobb's encourages readers to engage in spirited, healthy debate, including taking us to task. However, Dr. Dobb's moderates all comments posted to our site, and reserves the right to modify or remove any content that it determines to be derogatory, offensive, inflammatory, vulgar, irrelevant/off-topic, racist or obvious marketing or spam. Dr. Dobb's further reserves the right to disable the profile of any commenter participating in said activities.

Disqus Tips To upload an avatar photo, first complete your Disqus profile. | View the list of supported HTML tags you can use to style comments. | Please read our commenting policy.