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++

Developing Lightweight, Statically Initializable C++ Mutexes


While working on a solution for a fairly narrow and specific problem, I discovered a new synchronization mechanism. This mechanism not only provided a novel and effective solution to my problem, but turns out to have a number of properties making it useful for a wide range of important applications.

My original problem had to do with the implementation of Singletons. The issues with Singletons involve the order and time of their initialization and destruction, as well as the thread safety of these operations. Despite the multitude of techniques developed to deal with these issues, there does not seem to be one universal solution—let alone a simple one.

The particular Singleton I was working on had to be located in a low-level library, which was used by many applications and other libraries. This constraint ruled out the use of a C++ file-scope variable as a mechanism for initializing the Singleton because there is no way to ensure that initialization of such an object is done properly before its first use. To avoid the active collaboration that would result from explicit initialization, I concluded that the Singleton had to be initialized on first access. The Meyers Singleton pattern (Listing One) fits this requirement. When this pattern is employed, the Singleton is initialized the first time it is used, and automatically destroyed when the process terminates. A word of caution: It is sometimes necessary not only to ensure that a Singleton is destroyed at process termination, but also to establish a particular relative order; for my Singleton, such was not the case.

class Singleton {
    Singleton();
    ~Singleton();
  public:
    static Singleton& instance();
};

Singleton& Singleton::instance() {
    static Singleton singleton;
    return singleton;
}
Listing One

This implementation of a Singleton leaves the issue of thread safety unresolved. If multiple threads enter the instance routine simultaneously and if this routine has never been executed before, various problems may occur, depending on the compiler and platform. For example, the constructor of my Singleton may be executed multiple times or not at all. Alternatively, the constructor may be executed exactly once, which is correct, but one of the instance routines may return before the construction is completed—again, leading to unpredictable and hard-to-debug failures. As you'd expect, you can eliminate this race condition by using one of a number of various synchronization mechanisms. Listing Two, for example, employs a mutex. In Listing Two, I did not show how the mutex variable is initialized. The mutex itself is a Singleton! If this mutex has a nontrivial constructor or destructor, we have a chicken-and-egg problem.

class Singleton {
    Singleton();
    ~Singleton();
    static Singleton *s_singleton;
    static void init();
  public:
    static Singleton& instance();
};
Singleton *Singleton::s_singleton = 0;
void Singleton::init() {
    static Singleton singleton;
    s_singleton = &singleton;
}
Singleton& Singleton::instance() {
    lock(&mutex);
    if (!s_singleton) {
        init();
    }
    unlock(&mutex);
    return *s_singleton;
}

Listing Two

UNIX: pthread Solutions

Solving the problem of initializing the mutex on UNIX platforms isn't difficult. The pthread library available on these platforms provides a mutex that is a Plain Old Data (POD) type requiring only static (nonruntime) initialization, as in Listing Three. Additionally, the pthread library provides a mechanism called "once routines" that can also be used to solve this initialization problem (Listing Four).

Singleton& Singleton::instance() {
    static pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
    pthread_mutex_lock(&mutex);
    if (!s_singleton) {
        init();
    }
    pthread_mutex_unlock(&mutex);
    return *s_singleton;
}

Listing Three

Singleton& Singleton::instance() {
    static pthread_once_t once_control = PTHREAD_ONCE_INIT;
    pthread_once(&once_control, &Singleton::init);
    return *s_singleton;
}
Listing Four


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.