Blog Archive for / 2007 / 10 /

10 Years of Programming with POSIX Threads

Monday, 29 October 2007

David Butenhof's Programming with POSIX Threads was published 10 years ago, in 1997. At the time, it was the definitive work on the POSIX thread API, and multi-threaded programming in general. Ten years is a long time in computing so how does it fare today?

New POSIX Standard

When the book was written, the latest version of the POSIX Standard was the 1996 edition (ISO/IEC 9945-1:1996). Since then, the standard has evolved. It is now maintained by a joint working group from The Open Group, the IEEE and ISO called The Austin Group. The new Standard is called the Single Unix Specification, Version 3 and the 2004 edition is available online.

The new standard has brought a few changes with it — many things that were part of extensions such as POSIX 1003.1j are now part of the main ISO Standard. This includes barriers and read-write locks, though barriers are still optional and the read-write locks have a slightly different interface. Programming with POSIX threads is therefore lacking a good description of the now-standard APIs — although Butenhof devotes a section in Chapter 7 to implementing read-write locks, this is now only of historical interest, as the semantics are different from those in the new standard.

Most things stay the same

Though there are inevitably some changes with the new standard, most of the APIs remain the same. Not only that, the fundamental concepts described in the book haven't changed — threads still work the same way, mutexes and condition variables still work the same way, and so forth. Not only that, but the rising numbers of multicore CPU desktop computers means that correct thread synchronization is more important than ever. Faulty assumptions about memory visibility that happened to be true for single core machines are often demonstrably false for multicore and multiprocessor machines, so the dangers of deadlock, livelock and race conditions are ever more present.

Still the definitive reference

Though it's probably worth downloading the new POSIX standard, or checking the man pages for the new functions, Programming with POSIX Threads is still a good reference to the POSIX thread APIs, and multi-threaded programming in general. It sits well alongside Patterns for Parallel Programming — whereas Patterns for Parallel Programming is mainly about designing programs for concurrency, Programming with POSIX Threads is very much focused on getting the implementation details right.

Highly Recommended.

Buy this book

Programming with POSIX Threads
David Butenhof
Published by Addison-Wesley
ISBN 0-201-63392-2

Buy from Amazon.co.uk
Buy from Amazon.com

Posted by Anthony Williams
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Tags: reviews, threads, POSIX, Butenhof, books
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Using CSS to Replace Text with Images

Monday, 29 October 2007

Lots has been said about ways to replace text with images so that users with a graphical browser get a nice pretty logo, whilst search engines and screen readers get to see the text version. Most recently, Eric Enge has posted A Comprehensive Guide to Hidden Text & Search Engines over at SEOmoz. In general, I think it's a fair summary of the techniques I've encountered.

However, I was surprised to see the order of entries in the "may be OK" list. Firstly, I'd have expected sIFR to be top of the list — this is a widely used technique, and just replaces existing text with the same text in a different font. I prefer to do without Flash where possible, and this only works where you want to change the font rather than use a logo, but I can certainly see the draw here.

Secondly, I was also surprised to see the suggestion that is top of the list is to position the text off screen. I think this is a really bad idea, for accessibility reasons. When I only had a dial-up connection, I often used to browse with images turned off in order to reduce download times. If the text is positioned off screen, I would have just got a blank space. Even now, I often check websites with images turned off, because I think it is important. It is for this reason that my preferred technique is "Fahrner Image Replacement" (FIR). Whilst Eric says this is a no-no according to the Google Guidelines, I can't really see how — it's not deceptive in intent, and the text is seen by users without image support (or with images turned off) as well as the search engine bots. Also, given the quote from Susan Moskwa, it seems fine. Here's a quick summary of how it works:

Overlaying text with an image in CSS

The key to this technique is to have a nested SPAN with no content, position it over the text, and set a background image on it. If the background image loads, it hides the original text.

<h1 id="title"><span></span>Some Title Text</h1>

It is important to set the size of the enclosing tag to match the image, so that the hidden text doesn't leak out round the edges at large font sizes. The CSS is simple:

#title
{
    position: relative;
    width: 200px;
    height: 100px;
    margin: 0px;
    padding: 0px;
    overflow: hidden;
}

#title span
{
    position: absolute;
    top: 0px;
    left: 0px;
    width: 200px;
    height: 100px;
    background-image: url(/images/title-image.png);
}

This simple technique works in all the major browsers, including Internet Explorer, and gracefully degrades. Obviously, you can't select text from the image, but you can generally select the hidden text (though it's hard to see what you're doing), and copying the whole page will include the hidden text. Check it out — how does the title above ("Overlaying text with an image in CSS") appear in your browser?

Update: It has been pointed out in a comment on the linked SEOmoz article by bjornjohansen that you need to be aware of the potential for browsers with a different font size. This is definitely important — that's why we specify the exact dimensions for the enclosing element, and use overflow: hidden to avoid overhang. It's also important to ensure that the raw text (without the image) fits the specified space when rendered in at least one font size larger than "normal", so that people who use larger fonts can still read it with images disabled, without getting the text clipped.

Update: In another comment over on the SEOmoz article, MarioFr suggested that for headings the A tag could be used instead of SPAN — since empty A tags can be used as a link target in the heading, it works as a suitable replacement. I've changed the heading above to use an A tag for both purposes as an example.

Posted by Anthony Williams
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Tags: css, web design, image replacement
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Review of Fit for Developing Software by Rick Mugridge and Ward Cunningham

Monday, 22 October 2007

As the subtitle of this book says, Fit is the Framework for Integrated Tests, which was originally written by Ward. This is a testing framework that allows tests to be written in the form of Excel spreadsheets or HTML tables, which makes it easy for non-programmers to write tests. This book is divided into several parts. Parts 1 and 2 give an in-depth overview of how to use Fit effectively, and how it enables non-programmers to specify the tests, whereas parts 3-5 provide details that programmers will need for how to set up their code to be run from Fit.

Though I have been aware of Fit for a long time, I have never entirely grasped how to use it; reading this book gave me a strong urge to give it a go. It is very clear, with plenty of examples. I thought the sections on good/bad test structure, and how to restructure your tests to be clearer and easy to maintain were especially valuable — though they are obviously focused on Fit, many of the suggestions are applicable to testing through any framework.

Fit was developed as a Java framework, and so all the programming examples are in Java. However, as stated in the appendix, there are ports for many languages including C#, Python and C++. The way of structuring the fixtures that link the Fit tests to the code under test varies with each language, but the overall principles still apply.

The book didn't quite succeed in convincing me to spend time working with Fit or Fitnesse to try and integrate it with any of my existing projects, but I still think it's worth a look, and will try and use it on my next greenfield project.

Recommended.

Buy this book

At Amazon.co.uk
At Amazon.com

Posted by Anthony Williams
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Tags: reviews, fit, books, testing
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Reduce Bandwidth Usage by Compressing Pages in PHP

Monday, 15 October 2007

In Reduce Bandwidth Usage by Supporting If-Modified-Since in PHP, I identified one way to reduce your bandwidth usage — use the appropriate HTTP headers to avoid sending content that hasn't changed. Another way to reduce your bandwidth usage is to compress your pages.

HTTP headers

The Accept-Encoding HTTP header is used by browsers to specify potential encodings for a requested web page. For Firefox, this is generally set to "gzip, deflate", meaning that the browser will accept (and decompress) web pages compressed with the gzip or deflate compression algorithms. The web server can then use the Content-Encoding header to indicate that it has used a particular encoding for the served page. The Vary header is used to tell the browser or proxy that different encodings can be used. For example, if the server compresses the page using gzip, then it will return headers that say

    Content-Encoding: gzip
    Vary: Accept-Encoding

Handling compression in PHP

For static pages, compression is handled by your web server (though you might have to configure it to do so). For pages generated with PHP you are in charge. However, supporting compression is really easy. Just add:

    ob_start('ob_gzhandler');

to the start of the script. It is important that this comes before any output has been written as in order to compress the output, all output has to be passed through the filter, and the headers have to be set. If any content has already been sent to the browser, then this won't work, which is why I put it at the start of the script — that way, there's not much chance of anything interfering.

Tags: PHP, web design, HTTP, compression, reducing bandwidth

Posted by Anthony Williams
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