Blog Archive for / 2008 / 07 /

July 2008 C++ Standards Committee Mailing Published

Wednesday, 30 July 2008

The July 2008 mailing for the C++ Standards Committee was published today. Primarily this is just an update on the "state of evolution" papers, and the issue lists. However, there are a couple of new and revised papers. Amongst them is my revised paper on packaged_task: N2709: Packaging Tasks for Asynchronous Execution.

As I mentioned when the most recent C++0x draft was published, futures are still under discussion, and the LWG requested that packaged_task be moved to a separate paper, with a few minor changes. N2709 is this separate paper. Hopefully the LWG will approve this paper at the September meeting of the C++ committee in San Francisco; if they don't, then packaged_task will join the long list of proposals that have missed the boat for C++0x.

Posted by Anthony Williams
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Tags: C++0x, C++, standards
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How Search Engines See Keywords

Friday, 25 July 2008

Jennifer Laycock's recent post on How Search Engines See Keywords over at Search Engine Guide really surprised me. It harks back to the 1990s, with talk of keyword density, and doesn't match my understanding of modern search engines at all. It especially surprised me given the author: I felt that Jennifer was pretty savvy about these things. Maybe I'm just missing something really crucial.

Anyway, my understanding is that the search engines index each and every word on your page, and store a count of each word and phrase. If you say "rubber balls" three times, it doesn't matter if you also say "red marbles" three times: the engines don't assign "keywords" to a page, they find pages that match what the user types. This is why if I include a random phrase on a web page exactly once, and then search for that phrase then my page will likely show up in the results (assuming my phrase was sufficiently uncommon), even though other phrases might appear more often on the same page.

Once the engine has found the pages that contain the phrase that users have searched for (whether in content, or in links to that page), the search engine then ranks those pages to decide what to show. The ranking will use things like the number of times the phrase appears on the page, whether it appears in the title, in headings, links, <strong> tags or just in plain text, how many other pages link to that page with that phrase, and all the usual stuff.

Here, let's put it to the test. At the time of writing, a search on Google for "wibble flibble splodge bucket" with quotes returns no results, and a search without quotes returns just three entries. Given Google's crawl rate for my website, I expect this blog entry will turn up in the search results for that phrase within a few days, even though it only appears the once and other phrases such as "search engines" appear far more often. Of course, I may be wrong, but only time will tell.

Posted by Anthony Williams
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Tags: seo, search engines, keywords
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Review of the Windows Installer XML (WiX) Toolset

Tuesday, 15 July 2008

A couple of weeks ago, one of the products I maintain for a client needed a new installer. This application is the server part of a client-server suite, and runs as a Service on Microsoft Windows. The old installer was built using the simple installer-builder that is part of Microsoft Visual Studio, and could not easily be extended to handle the desired functionality. Enter the WiX toolset.

As its name suggests, the Windows Installer XML Toolset is a set of tools for building Windows Installer (MSI) packages, using XML to describe the installer. At first glance, this doesn't look much easier than populating the MSI tables directly using Microsoft's Orca tool, but it's actually really straightforward, especially with the help of the WiX tutorial.

It's all about the <XML>

The perceived complexity comes from several factors. First up, every individual file, directory and registry entry that is to be installed must be specified as a <Component> and given a GUID which can be used to uniquely identify that item. The details of what to install where, combined with the inherent verbosity of XML makes the installer build file quite large. Thankfully, being XML, whitespace such as indenting and blank lines can be used to separate things into logical groups, and XML comments can be used if necessary. The installer GUI is also built using XML, which can make things very complicated. Thankfully WiX comes with a set of pre-designed GUIs which can be referenced from your installer build file — you can even provide your own images in order to brand the installer with your company logo, for example.

Once you've got over the initial shock of the XML syntax, the toolkit is actually really easy to use. The file and directory structure to be used by the installed application is described using nested XML elements to mimic the directory structure, with special predefined directory identifiers for things like the Windows directory, the Program Files directory or the user's My Documents folder. You can also use nested <Feature> tags to create a tree of features which the user can choose to install (or not) if they opt for a custom install. Each "feature" is a group of one or more components which are identified with nested tags in the feature tags.

Custom Actions

What if you're not just installing a simple desktop application? Windows Installer provides support for custom actions which can be run as part of the installation, and WiX allows you to specify these. The old installer for the server application used custom actions to install the service, but these weren't actually necessary — the Windows Installer can automatically configure services, it was just that the Visual Studio Installer builder didn't support that. With WiX it's just a matter of adding a <ServiceInstall> tag to one of your components.

The custom actions we did require were easy to add with the <CustomAction> tags — you can write custom actions as DLLs or EXEs, or even as simple VBScript included directly in the installer build file. These can then be added to the <InstallExecuteSequence> at the appropriate point, taking care to specify the right conditions (such as whether to run on install or uninstall, and whether or not to run depending on which features are being installed).

Conclusion

The WiX toolset is very powerful, and gives you full control over everything the Windows Installer can do. Though the XML syntax is a little cumbersome, it is actually quite simple to use. Once you get used to the syntax, it is easy to see what the installer is doing, and what you need to do to get the desired effect. Though designing installer GUIs is quite hard, the supplied options will be sufficient in many cases and it is easy to customize them to your needs.

It's a free tool, so you can't beat it on price, and the tutorial really reduces the learning curve. Next time you need to build an installer, I recommend you give WiX a try.

Posted by Anthony Williams
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Tags: windows, installer, msi, wix, review
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C++ Concurrency Papers Still Under Discussion

Monday, 07 July 2008

Last week I wrote about the new working draft of the C++0x standard and the newly-accepted concurrency papers, but I didn't mention the papers still under discussion. There's a few of those, listed below. The committee intends to publish a formal Committee Draft of the C++0x Standard at the end of the September meeting, so anything not voted into the WP at that meeting has missed the boat (though of course defects will still be fixed).

N2671: An Asynchronous Future Value: Proposed Wording

For those of you who've been following my postings about asynchronous future values for C++, this is the latest proposal on futures. Though it was discussed at the June meeting, the LWG didn't feel it was ready to be voted in to the working draft yet. At the request of the LWG, packaged_task has been removed; I should have a separate proposal for that ready before the next meeting.

N2668: Concurrency Modifications to Basic String

Yes, I listed this as approved last week, but I misread the minutes of the votes: it is still under discussion. The changes in this paper ensure that it is safe for two threads to access the same std::string object at the same time, provided they both perform only read operations. They also ensure that copying a string object and then modifying that copy is safe, even if another thread is accessing the original. This essentially disallows copy-on-write implementations since the benefits are now severely limited.

N2633: Improved support for bidirectional fences

This paper aims to simplify and improve the support for fences (also known as memory barriers) when writing code using the new atomic types. As the paper points out, the current atomic_variable.fence(memory_ordering) semantics can mean that compilers have to issue stronger-than-necessary fences in many cases. By making the fences free functions that are not tied to an individual variable, they will map much better to the underlying instructions, and should lead to clearer (and more optimal) code.

N2643: C++ Data-Dependency Ordering: Function Annotation

This paper is a counterpart to N2664: C++ Data-Dependency Ordering: Atomics and Memory Model, and extends the list of cases where dependencies are carried by allowing you to annotate functions. The new style [[annotation syntax]] is used to indicate which parameters carry a dependency into a function, and whether or not the return type carries a dependency to the call site.

Posted by Anthony Williams
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Tags: C++, concurrency, C++0x
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New C++ Working Draft and Concurrency Papers Now Available

Friday, 04 July 2008

The post-meeting mailing following June's C++ Standards committee meeting in France is now available. This includes a new Working Draft for the C++0x standard, and a few concurrency-related papers.

From a concurrency point of view, there are several papers of interest. Firstly, a few have been accepted into the working draft, notably:

N2661: A Foundation to Sleep On

This paper provides a generalised time point and duration library, which is used by the thread functions that take times or durations. These have been updated to use these new types and renamed to make their purpose clearer: functions that wait for a duration are now called xxx_for, and take a value of type std::chrono::duration<Rep,Period>, whereas those that take absolute time points are now called xxx_until and take a value of type std::chrono::time_point<Clock,Duration>.

N2668: Concurrency Modifications to Basic String

Update: This paper has not actually been approved, I was mistaken. Though the majority of the committee were in favour, there was not consensus, so this paper will be discussed at a future meeting. Thanks to Herb Sutter for picking me up on this.
The changes in this paper ensure that it is safe for two threads to access the same std::string object at the same time, provided they both perform only read operations. They also ensure that copying a string object and then modifying that copy is safe, even if another thread is accessing the original. This essentially disallows copy-on-write implementations since the benefits are now severely limited.

N2660: Dynamic Initialization and Destruction with Concurrency

With the changes from this paper, if an application uses multiple threads then the initialization and destruction of objects with static storage duration (such as global variables) may run concurrently on separate threads. This can provide faster start-up and shut-down times for an application, but it can also introduce the possibility of race conditions where none existed previously. If you use threads in your application, it is now even more important to check the initialization order of objects with static storage duration.

N2514: Implicit Conversion Operators for Atomics

With this change, the atomic types such as std::atomic_int are implicitly convertible to their corresponding fundamental types. This means, for example, that:

std::atomic_int x;
int y=x;

is well-formed where it wasn't previously. The implicit conversions are equivalent to calling the load() member function, and have memory_order_seq_cst ordering semantics.

N2674: Shared_ptr atomic access, revision 1

This paper introduces a new set of overloads of the free functions for atomic operations (such as atomic_load and atomic_store), which operate on instances of std::shared_ptr<>. This allows one thread to read an instance of std::shared_ptr whilst another thread is modifying that same instance if they both use the new atomic functions.
This paper also renames atomic_swap operations to atomic_exchange (and likewise for atomic_compare_swap and the corresponding member functions) for all atomic types, in order to avoid confusion with other types that provide swap functions. The atomic exchange operations only alter the value of a single object, replacing the old value with a new one, they do not exchange the values of two objects in the way that std::swap does.

N2664: C++ Data-Dependency Ordering: Atomics and Memory Model

With the adoption of this paper the memory model gets a new ordering option: memory_order_consume. This is a limited form of memory_order_acquire which allows for data-dependent ordering. If a thread uses memory_order_consume, then it is not guaranteed to see modifications to other variables made by the thread that performed the releasing operation unless those variables are accessed in conjunction with the consumed variable. This means, for example, that member variables of an object are visible if the consumed value is a pointer to that object, but that values of independent objects are not necessarily visible. This allows the compiler to perform some optimizations that are forbidden by memory_order_acquire, and reduces the synchronization overhead on some hardware architectures.

N2678: Error Handling Specification for Chapter 30 (Threads)

This paper brings the exceptions thrown by the thread under the new system_error umbrella, with corresponding error codes and error categories.

N2669: Thread-Safety in the Standard Library (Rev 2)

Now the standard supports threads, we need to say which standard library operations are thread-safe, and which are not. This paper basically says that non-modifying operations on the same object are safe, and any operations on separate objects are also safe. Also, separate threads may call the same library functions on separate objects without problems. As you might expect, concurrent modifications to the same object are data races and undefined behaviour.

The committee also voted to include N2659: Thread-Local Storage in C++0x, but it doesn't appear to be in the current draft. This paper introduces the thread_local keyword to indicate that each thread should have its own copy of a given object.

Finally, N2657: Local and Unnamed Types as Template Arguments has been incorporated in the working paper. Though this isn't directly concurrency related, it is something I've been campaigning for since N1427 back in 2003.

Apart from N2657, I've only listed the concurrency changes: check out the Working Draft for the C++0x standard, and the State of C++ Evolution for more details on the changes.

Posted by Anthony Williams
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Tags: concurrency, c++
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