Dr. Dobb's is part of the Informa Tech Division of Informa PLC

This site is operated by a business or businesses owned by Informa PLC and all copyright resides with them. Informa PLC's registered office is 5 Howick Place, London SW1P 1WG. Registered in England and Wales. Number 8860726.


Channels ▼
RSS

C/C++

C++ and the Perils of Double-Checked Locking: Part II


volatile: A Brief History

To find the roots of volatile, let's go back to the 1970s, when Gordon Bell (of PDP-11 fame) introduced the concept of memory-mapped I/O (MMIO). Before that, processors allocated pins and defined special instructions for performing port I/O. The idea behind MMIO is to use the same pins and instructions for both memory and port access. Hardware outside the processor intercepts specific memory addresses and transforms them into I/O requests; so dealing with ports became simply reading from and writing to machine-specific memory addresses.

What a great idea. Reducing pin count is good—pins slow down signal, increase defect rate, and complicate packaging. Also, MMIO doesn't require special instructions for ports. Programs just use the memory, and the hardware takes care of the rest.

Or almost.

To see why MMIO needs volatile variables, consider the following code:

unsigned int *p = GetMagicAddress();
unsigned int a, b;
a = *p;
b = *p;

If p refers to a port, a and b should receive two consecutive words read from that port. However, if p points to a bona fide memory location, then a and b load the same location twice and, hence, will compare equal. Compilers exploit this assumption in the copy propagation optimization that transforms b=*p; into the more efficient b = a;. Similarly, for the same p, a, and b, consider:

*p = a;
*p = b;

The code writes two words to *p, but the optimizer might assume that *p is memory and perform the dead assignment elimination optimization by eliminating the first assignment.

So, when dealing with ports, some optimizations must be suspended. volatile exists for specifying special treatment for ports, specifically: The content of a volatile variable is unstable (can change by means unknown to the compiler); all writes to volatile data are observable, so they must be executed religiously; and all operations on volatile data are executed in the sequence in which they appear in the source code. The first two rules ensure proper reading and writing. The last one allows implementation of I/O protocols that mix input and output.

This is informally what C and C++'s volatile guarantees. Java took volatile a step further by guaranteeing the aforementioned properties across multiple threads. This was an important step, but it wasn't enough to make volatile usable for thread synchronization: The relative ordering of volatile and nonvolatile operations remained unspecified. This omission forces many variables to be volatile to ensure proper ordering.

Java 1.5's volatile has the more restrictive, but simpler, acquire/release semantics: Any read of a volatile is guaranteed to occur prior to any memory reference (volatile or not) in the statements that follow, and any write to a volatile is guaranteed to occur after all memory references in the statements preceding it. .NET defines volatile to incorporate multithreaded semantics as well, which are similar to the currently proposed Java semantics. We know of no similar work being done on C's or C++'s volatile.

— S.M. and A.A.


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.