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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><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-NS Stylesheets V1.78.1" /><meta name="keywords" content="ISO C++, library" /><meta name="keywords" content="ISO C++, runtime, library" /><link rel="home" href="../index.html" title="The GNU C++ Library" /><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="section"><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="section"><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.
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On GNU/Linux x86, -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="section"><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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In the terms of the 2011 C++ standard a thread-safe program is one which
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does not perform any conflicting non-atomic operations on memory locations
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and so does not contain any data races.
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The standard places requirements on the library to ensure that no data
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races are caused by the library itself or by programs which use the
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library correctly (as described below).
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The C++11 memory model and library requirements are a more formal version
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of the <a class="link" href="http://www.sgi.com/tech/stl/thread_safety.html" target="_top">SGI STL</a> definition of thread safety, which the library used
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prior to the 2011 standard.
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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" style="list-style-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="link" 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 function calls which
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access any particular library object's state when one or more of
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those accesses modifies the state. An object will be modified by
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invoking a non-const member function on it or passing it as a
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non-const argument to a library function. An object will not be
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modified by invoking a const member function on it or passing it to
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a function as a pointer- or reference-to-const.
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Typically, the application
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programmer may infer what object locks must be held based on the
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objects referenced in a function call and whether the objects are
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accessed as const or non-const. 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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void 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 does not require any
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user-level locks:
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</p><pre class="programlisting">
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void 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 types are safe to use in a multithreaded program
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if objects are not shared between threads or as
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long each thread carefully locks out access by any other
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thread while it modifies any object visible to another thread.
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Unless otherwise documented, the only exceptions to these rules
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are atomic operations on the types in
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<code class="filename"><atomic></code>
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and lock/unlock operations on the standard mutex types in
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<code class="filename"><mutex></code>. These
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atomic operations allow concurrent accesses to the same object
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without introducing data races.
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</p><p>The following member functions of standard containers can be
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considered to be const for the purposes of avoiding data races:
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<code class="code">begin</code>, <code class="code">end</code>, <code class="code">rbegin</code>, <code class="code">rend</code>,
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<code class="code">front</code>, <code class="code">back</code>, <code class="code">data</code>,
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<code class="code">find</code>, <code class="code">lower_bound</code>, <code class="code">upper_bound</code>,
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<code class="code">equal_range</code>, <code class="code">at</code>
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and, except in associative or unordered associative containers,
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<code class="code">operator[]</code>. In other words, although they are non-const
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so that they can return mutable iterators, those member functions
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will not modify the container.
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Accessing an iterator might cause a non-modifying access to
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the container the iterator refers to (for example incrementing a
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list iterator must access the pointers between nodes, which are part
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of the container and so conflict with other accesses to the container).
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</p><p>Programs which follow the rules above will not encounter data
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races in library code, even when using library types which share
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state between distinct objects. In the example below the
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<code class="code">shared_ptr</code> objects share a reference count, but
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because the code does not perform any non-const operations on the
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globally-visible object, the library ensures that the reference
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count updates are atomic and do not introduce data races:
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</p><pre class="programlisting">
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std::shared_ptr<int> global_sp;
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void thread_main() {
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auto local_sp = global_sp; // OK, copy constructor's parameter is reference-to-const
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int i = *global_sp; // OK, operator* is const
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int j = *local_sp; // OK, does not operate on global_sp
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// *global_sp = 2; // NOT OK, modifies int visible to other threads
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// *local_sp = 2; // NOT OK, modifies int visible to other threads
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// global_sp.reset(); // NOT OK, reset is non-const
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local_sp.reset(); // OK, does not operate on global_sp
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}
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int main() {
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global_sp.reset(new int(1));
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std::thread t1(thread_main);
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std::thread t2(thread_main);
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t1.join();
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t2.join();
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}
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</pre><p>For further details of the C++11 memory model see Hans-J. Boehm's
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<a class="link" href="http://www.hpl.hp.com/personal/Hans_Boehm/c++mm/user-faq.html" target="_top">Threads
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and memory model for C++</a> pages, particularly the <a class="link" href="http://www.hpl.hp.com/personal/Hans_Boehm/c++mm/threadsintro.html" target="_top">introduction</a>
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and <a class="link" href="http://www.hpl.hp.com/personal/Hans_Boehm/c++mm/user-faq.html" target="_top">FAQ</a>.
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</p></div><div class="section"><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="section"><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="section"><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="section"><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="section"><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="section"><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="section"><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="link" 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="link" 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="../index.html">Home</a></td><td width="40%" align="right" valign="top"> Exceptions</td></tr></table></div></body></html> |