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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Concurrency</title><meta name="generator" content="DocBook XSL Stylesheets V1.75.2" /><meta name="keywords" content=" ISO C++ , library " /><link rel="home" href="../spine.html" title="The GNU C++ Library Documentation" /><link rel="up" href="using.html" title="Chapter 3. Using" /><link rel="prev" href="using_dynamic_or_shared.html" title="Linking" /><link rel="next" href="using_exceptions.html" title="Exceptions" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Concurrency</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="using_dynamic_or_shared.html">Prev</a> </td><th width="60%" align="center">Chapter 3. Using</th><td width="20%" align="right"> <a accesskey="n" href="using_exceptions.html">Next</a></td></tr></table><hr /></div><div class="sect1" title="Concurrency"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="manual.intro.using.concurrency"></a>Concurrency</h2></div></div></div><p>This section discusses issues surrounding the proper compilation
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of multithreaded applications which use the Standard C++
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library. This information is GCC-specific since the C++
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standard does not address matters of multithreaded applications.
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</p><div class="sect2" title="Prerequisites"><div class="titlepage"><div><div><h3 class="title"><a id="manual.intro.using.concurrency.prereq"></a>Prerequisites</h3></div></div></div><p>All normal disclaimers aside, multithreaded C++ application are
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only supported when libstdc++ and all user code was built with
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compilers which report (via <code class="code"> gcc/g++ -v </code>) the same thread
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model and that model is not <span class="emphasis"><em>single</em></span>. As long as your
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final application is actually single-threaded, then it should be
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safe to mix user code built with a thread model of
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<span class="emphasis"><em>single</em></span> with a libstdc++ and other C++ libraries built
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with another thread model useful on the platform. Other mixes
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may or may not work but are not considered supported. (Thus, if
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you distribute a shared C++ library in binary form only, it may
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be best to compile it with a GCC configured with
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--enable-threads for maximal interchangeability and usefulness
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with a user population that may have built GCC with either
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--enable-threads or --disable-threads.)
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</p><p>When you link a multithreaded application, you will probably
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need to add a library or flag to g++. This is a very
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non-standardized area of GCC across ports. Some ports support a
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special flag (the spelling isn't even standardized yet) to add
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all required macros to a compilation (if any such flags are
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required then you must provide the flag for all compilations not
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just linking) and link-library additions and/or replacements at
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link time. The documentation is weak. Here is a quick summary
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to display how ad hoc this is: On Solaris, both -pthreads and
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-threads (with subtly different meanings) are honored. On OSF,
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-pthread and -threads (with subtly different meanings) are
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honored. On Linux/i386, -pthread is honored. On FreeBSD,
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-pthread is honored. Some other ports use other switches.
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AFAIK, none of this is properly documented anywhere other than
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in ``gcc -dumpspecs'' (look at lib and cpp entries).
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</p></div><div class="sect2" title="Thread Safety"><div class="titlepage"><div><div><h3 class="title"><a id="manual.intro.using.concurrency.thread_safety"></a>Thread Safety</h3></div></div></div><p>
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We currently use the <a class="ulink" href="http://www.sgi.com/tech/stl/thread_safety.html" target="_top">SGI STL</a> definition of thread safety.
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</p><p>The library strives to be thread-safe when all of the following
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conditions are met:
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</p><div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem"><p>The system's libc is itself thread-safe,
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</p></li><li class="listitem"><p>
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The compiler in use reports a thread model other than
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'single'. This can be tested via output from <code class="code">gcc
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-v</code>. Multi-thread capable versions of gcc output
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something like this:
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</p><pre class="programlisting">
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%gcc -v
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Using built-in specs.
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...
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Thread model: posix
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gcc version 4.1.2 20070925 (Red Hat 4.1.2-33)
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</pre><p>Look for "Thread model" lines that aren't equal to "single."</p></li><li class="listitem"><p>
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Requisite command-line flags are used for atomic operations
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and threading. Examples of this include <code class="code">-pthread</code>
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and <code class="code">-march=native</code>, although specifics vary
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depending on the host environment. See <a class="ulink" href="http://gcc.gnu.org/onlinedocs/gcc/Option-Summary.html" target="_top">Machine
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Dependent Options</a>.
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</p></li><li class="listitem"><p>
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An implementation of atomicity.h functions
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exists for the architecture in question. See the internals documentation for more <a class="link" href="internals.html#internals.thread_safety" title="Thread Safety">details</a>.
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</p></li></ul></div><p>The user-code must guard against concurrent method calls which may
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access any particular library object's state. Typically, the
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application programmer may infer what object locks must be held
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based on the objects referenced in a method call. Without getting
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into great detail, here is an example which requires user-level
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locks:
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</p><pre class="programlisting">
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library_class_a shared_object_a;
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thread_main () {
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library_class_b *object_b = new library_class_b;
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shared_object_a.add_b (object_b); // must hold lock for shared_object_a
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shared_object_a.mutate (); // must hold lock for shared_object_a
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}
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// Multiple copies of thread_main() are started in independent threads.</pre><p>Under the assumption that object_a and object_b are never exposed to
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another thread, here is an example that should not require any
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user-level locks:
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</p><pre class="programlisting">
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thread_main () {
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library_class_a object_a;
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library_class_b *object_b = new library_class_b;
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object_a.add_b (object_b);
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object_a.mutate ();
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} </pre><p>All library objects are safe to use in a multithreaded program as
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long as each thread carefully locks out access by any other
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thread while it uses any object visible to another thread, i.e.,
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treat library objects like any other shared resource. In general,
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this requirement includes both read and write access to objects;
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unless otherwise documented as safe, do not assume that two threads
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may access a shared standard library object at the same time.
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</p></div><div class="sect2" title="Atomics"><div class="titlepage"><div><div><h3 class="title"><a id="manual.intro.using.concurrency.atomics"></a>Atomics</h3></div></div></div><p>
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</p></div><div class="sect2" title="IO"><div class="titlepage"><div><div><h3 class="title"><a id="manual.intro.using.concurrency.io"></a>IO</h3></div></div></div><p>This gets a bit tricky. Please read carefully, and bear with me.
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</p><div class="sect3" title="Structure"><div class="titlepage"><div><div><h4 class="title"><a id="concurrency.io.structure"></a>Structure</h4></div></div></div><p>A wrapper
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type called <code class="code">__basic_file</code> provides our abstraction layer
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for the <code class="code">std::filebuf</code> classes. Nearly all decisions dealing
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with actual input and output must be made in <code class="code">__basic_file</code>.
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</p><p>A generic locking mechanism is somewhat in place at the filebuf layer,
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but is not used in the current code. Providing locking at any higher
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level is akin to providing locking within containers, and is not done
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for the same reasons (see the links above).
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</p></div><div class="sect3" title="Defaults"><div class="titlepage"><div><div><h4 class="title"><a id="concurrency.io.defaults"></a>Defaults</h4></div></div></div><p>The __basic_file type is simply a collection of small wrappers around
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the C stdio layer (again, see the link under Structure). We do no
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locking ourselves, but simply pass through to calls to <code class="code">fopen</code>,
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<code class="code">fwrite</code>, and so forth.
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</p><p>So, for 3.0, the question of "is multithreading safe for I/O"
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must be answered with, "is your platform's C library threadsafe
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for I/O?" Some are by default, some are not; many offer multiple
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implementations of the C library with varying tradeoffs of threadsafety
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and efficiency. You, the programmer, are always required to take care
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with multiple threads.
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</p><p>(As an example, the POSIX standard requires that C stdio FILE*
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operations are atomic. POSIX-conforming C libraries (e.g, on Solaris
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and GNU/Linux) have an internal mutex to serialize operations on
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FILE*s. However, you still need to not do stupid things like calling
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<code class="code">fclose(fs)</code> in one thread followed by an access of
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<code class="code">fs</code> in another.)
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</p><p>So, if your platform's C library is threadsafe, then your
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<code class="code">fstream</code> I/O operations will be threadsafe at the lowest
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level. For higher-level operations, such as manipulating the data
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contained in the stream formatting classes (e.g., setting up callbacks
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inside an <code class="code">std::ofstream</code>), you need to guard such accesses
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like any other critical shared resource.
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</p></div><div class="sect3" title="Future"><div class="titlepage"><div><div><h4 class="title"><a id="concurrency.io.future"></a>Future</h4></div></div></div><p> A
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second choice may be available for I/O implementations: libio. This is
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disabled by default, and in fact will not currently work due to other
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issues. It will be revisited, however.
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</p><p>The libio code is a subset of the guts of the GNU libc (glibc) I/O
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implementation. When libio is in use, the <code class="code">__basic_file</code>
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type is basically derived from FILE. (The real situation is more
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complex than that... it's derived from an internal type used to
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implement FILE. See libio/libioP.h to see scary things done with
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vtbls.) The result is that there is no "layer" of C stdio
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to go through; the filebuf makes calls directly into the same
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functions used to implement <code class="code">fread</code>, <code class="code">fwrite</code>,
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and so forth, using internal data structures. (And when I say
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"makes calls directly," I mean the function is literally
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replaced by a jump into an internal function. Fast but frightening.
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*grin*)
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</p><p>Also, the libio internal locks are used. This requires pulling in
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large chunks of glibc, such as a pthreads implementation, and is one
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of the issues preventing widespread use of libio as the libstdc++
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cstdio implementation.
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</p><p>But we plan to make this work, at least as an option if not a future
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default. Platforms running a copy of glibc with a recent-enough
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version will see calls from libstdc++ directly into the glibc already
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installed. For other platforms, a copy of the libio subsection will
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be built and included in libstdc++.
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</p></div><div class="sect3" title="Alternatives"><div class="titlepage"><div><div><h4 class="title"><a id="concurrency.io.alt"></a>Alternatives</h4></div></div></div><p>Don't forget that other cstdio implementations are possible. You could
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easily write one to perform your own forms of locking, to solve your
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"interesting" problems.
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</p></div></div><div class="sect2" title="Containers"><div class="titlepage"><div><div><h3 class="title"><a id="manual.intro.using.concurrency.containers"></a>Containers</h3></div></div></div><p>This section discusses issues surrounding the design of
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multithreaded applications which use Standard C++ containers.
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All information in this section is current as of the gcc 3.0
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release and all later point releases. Although earlier gcc
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releases had a different approach to threading configuration and
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proper compilation, the basic code design rules presented here
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were similar. For information on all other aspects of
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multithreading as it relates to libstdc++, including details on
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the proper compilation of threaded code (and compatibility between
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threaded and non-threaded code), see Chapter 17.
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</p><p>Two excellent pages to read when working with the Standard C++
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containers and threads are
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<a class="ulink" href="http://www.sgi.com/tech/stl/thread_safety.html" target="_top">SGI's
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http://www.sgi.com/tech/stl/thread_safety.html</a> and
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<a class="ulink" href="http://www.sgi.com/tech/stl/Allocators.html" target="_top">SGI's
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http://www.sgi.com/tech/stl/Allocators.html</a>.
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</p><p><span class="emphasis"><em>However, please ignore all discussions about the user-level
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configuration of the lock implementation inside the STL
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container-memory allocator on those pages. For the sake of this
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discussion, libstdc++ configures the SGI STL implementation,
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not you. This is quite different from how gcc pre-3.0 worked.
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In particular, past advice was for people using g++ to
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explicitly define _PTHREADS or other macros or port-specific
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compilation options on the command line to get a thread-safe
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STL. This is no longer required for any port and should no
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longer be done unless you really know what you are doing and
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assume all responsibility.</em></span>
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</p><p>Since the container implementation of libstdc++ uses the SGI
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code, we use the same definition of thread safety as SGI when
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discussing design. A key point that beginners may miss is the
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fourth major paragraph of the first page mentioned above
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(<span class="emphasis"><em>For most clients...</em></span>), which points out that
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locking must nearly always be done outside the container, by
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client code (that'd be you, not us). There is a notable
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exceptions to this rule. Allocators called while a container or
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element is constructed uses an internal lock obtained and
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released solely within libstdc++ code (in fact, this is the
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reason STL requires any knowledge of the thread configuration).
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</p><p>For implementing a container which does its own locking, it is
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trivial to provide a wrapper class which obtains the lock (as
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SGI suggests), performs the container operation, and then
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releases the lock. This could be templatized <span class="emphasis"><em>to a certain
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extent</em></span>, on the underlying container and/or a locking
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mechanism. Trying to provide a catch-all general template
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solution would probably be more trouble than it's worth.
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</p><p>The library implementation may be configured to use the
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high-speed caching memory allocator, which complicates thread
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safety issues. For all details about how to globally override
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this at application run-time
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see <a class="link" href="using_macros.html" title="Macros">here</a>. Also
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useful are details
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on <a class="link" href="memory.html#std.util.memory.allocator" title="Allocators">allocator</a>
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options and capabilities.
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</p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="using_dynamic_or_shared.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="using.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="using_exceptions.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Linking </td><td width="20%" align="center"><a accesskey="h" href="../spine.html">Home</a></td><td width="40%" align="right" valign="top"> Exceptions</td></tr></table></div></body></html>
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