diff options
Diffstat (limited to 'gcc-4.8.1/libstdc++-v3/doc/html/manual/using_concurrency.html')
| -rw-r--r-- | gcc-4.8.1/libstdc++-v3/doc/html/manual/using_concurrency.html | 270 |
1 files changed, 0 insertions, 270 deletions
diff --git a/gcc-4.8.1/libstdc++-v3/doc/html/manual/using_concurrency.html b/gcc-4.8.1/libstdc++-v3/doc/html/manual/using_concurrency.html deleted file mode 100644 index 509c48d29..000000000 --- a/gcc-4.8.1/libstdc++-v3/doc/html/manual/using_concurrency.html +++ /dev/null @@ -1,270 +0,0 @@ -<?xml version="1.0" encoding="UTF-8" standalone="no"?> -<!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 V1.77.1" /><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 - of multithreaded applications which use the Standard C++ - library. This information is GCC-specific since the C++ - standard does not address matters of multithreaded applications. - </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 - only supported when libstdc++ and all user code was built with - compilers which report (via <code class="code"> gcc/g++ -v </code>) the same thread - model and that model is not <span class="emphasis"><em>single</em></span>. As long as your - final application is actually single-threaded, then it should be - safe to mix user code built with a thread model of - <span class="emphasis"><em>single</em></span> with a libstdc++ and other C++ libraries built - with another thread model useful on the platform. Other mixes - may or may not work but are not considered supported. (Thus, if - you distribute a shared C++ library in binary form only, it may - be best to compile it with a GCC configured with - --enable-threads for maximal interchangeability and usefulness - with a user population that may have built GCC with either - --enable-threads or --disable-threads.) - </p><p>When you link a multithreaded application, you will probably - need to add a library or flag to g++. This is a very - non-standardized area of GCC across ports. Some ports support a - special flag (the spelling isn't even standardized yet) to add - all required macros to a compilation (if any such flags are - required then you must provide the flag for all compilations not - just linking) and link-library additions and/or replacements at - link time. The documentation is weak. Here is a quick summary - to display how ad hoc this is: On Solaris, both -pthreads and - -threads (with subtly different meanings) are honored. - On GNU/Linux x86, -pthread is honored. On FreeBSD, - -pthread is honored. Some other ports use other switches. - AFAIK, none of this is properly documented anywhere other than - in ``gcc -dumpspecs'' (look at lib and cpp entries). - </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> -In the terms of the 2011 C++ standard a thread-safe program is one which -does not perform any conflicting non-atomic operations on memory locations -and so does not contain any data races. -The standard places requirements on the library to ensure that no data -races are caused by the library itself or by programs which use the -library correctly (as described below). -The C++11 memory model and library requirements are a more formal version -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 -prior to the 2011 standard. -</p><p>The library strives to be thread-safe when all of the following - conditions are met: - </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>The system's libc is itself thread-safe, - </p></li><li class="listitem"><p> - The compiler in use reports a thread model other than - 'single'. This can be tested via output from <code class="code">gcc - -v</code>. Multi-thread capable versions of gcc output - something like this: - </p><pre class="programlisting"> -%gcc -v -Using built-in specs. -... -Thread model: posix -gcc version 4.1.2 20070925 (Red Hat 4.1.2-33) -</pre><p>Look for "Thread model" lines that aren't equal to "single."</p></li><li class="listitem"><p> - Requisite command-line flags are used for atomic operations - and threading. Examples of this include <code class="code">-pthread</code> - and <code class="code">-march=native</code>, although specifics vary - depending on the host environment. See <a class="link" href="http://gcc.gnu.org/onlinedocs/gcc/Option-Summary.html" target="_top">Machine - Dependent Options</a>. - </p></li><li class="listitem"><p> - An implementation of atomicity.h functions - 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>. - </p></li></ul></div><p>The user code must guard against concurrent function calls which - access any particular library object's state when one or more of - those accesses modifies the state. An object will be modified by - invoking a non-const member function on it or passing it as a - non-const argument to a library function. An object will not be - modified by invoking a const member function on it or passing it to - a function as a pointer- or reference-to-const. - Typically, the application - programmer may infer what object locks must be held based on the - objects referenced in a function call and whether the objects are - accessed as const or non-const. Without getting - into great detail, here is an example which requires user-level - locks: - </p><pre class="programlisting"> - library_class_a shared_object_a; - - void thread_main () { - library_class_b *object_b = new library_class_b; - shared_object_a.add_b (object_b); // must hold lock for shared_object_a - shared_object_a.mutate (); // must hold lock for shared_object_a - } - - // 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 - another thread, here is an example that does not require any - user-level locks: - </p><pre class="programlisting"> - void thread_main () { - library_class_a object_a; - library_class_b *object_b = new library_class_b; - object_a.add_b (object_b); - object_a.mutate (); - } </pre><p>All library types are safe to use in a multithreaded program - if objects are not shared between threads or as - long each thread carefully locks out access by any other - thread while it modifies any object visible to another thread. - Unless otherwise documented, the only exceptions to these rules - are atomic operations on the types in - <code class="filename"><atomic></code> - and lock/unlock operations on the standard mutex types in - <code class="filename"><mutex></code>. These - atomic operations allow concurrent accesses to the same object - without introducing data races. - </p><p>The following member functions of standard containers can be - considered to be const for the purposes of avoiding data races: - <code class="code">begin</code>, <code class="code">end</code>, <code class="code">rbegin</code>, <code class="code">rend</code>, - <code class="code">front</code>, <code class="code">back</code>, <code class="code">data</code>, - <code class="code">find</code>, <code class="code">lower_bound</code>, <code class="code">upper_bound</code>, - <code class="code">equal_range</code>, <code class="code">at</code> - and, except in associative or unordered associative containers, - <code class="code">operator[]</code>. In other words, although they are non-const - so that they can return mutable iterators, those member functions - will not modify the container. - Accessing an iterator might cause a non-modifying access to - the container the iterator refers to (for example incrementing a - list iterator must access the pointers between nodes, which are part - of the container and so conflict with other accesses to the container). - </p><p>Programs which follow the rules above will not encounter data - races in library code, even when using library types which share - state between distinct objects. In the example below the - <code class="code">shared_ptr</code> objects share a reference count, but - because the code does not perform any non-const operations on the - globally-visible object, the library ensures that the reference - count updates are atomic and do not introduce data races: - </p><pre class="programlisting"> - std::shared_ptr<int> global_sp; - - void thread_main() { - auto local_sp = global_sp; // OK, copy constructor's parameter is reference-to-const - - int i = *global_sp; // OK, operator* is const - int j = *local_sp; // OK, does not operate on global_sp - - // *global_sp = 2; // NOT OK, modifies int visible to other threads - // *local_sp = 2; // NOT OK, modifies int visible to other threads - - // global_sp.reset(); // NOT OK, reset is non-const - local_sp.reset(); // OK, does not operate on global_sp - } - - int main() { - global_sp.reset(new int(1)); - std::thread t1(thread_main); - std::thread t2(thread_main); - t1.join(); - t2.join(); - } - </pre><p>For further details of the C++11 memory model see Hans-J. Boehm's - <a class="link" href="http://www.hpl.hp.com/personal/Hans_Boehm/c++mm/user-faq.html" target="_top">Threads - 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> - and <a class="link" href="http://www.hpl.hp.com/personal/Hans_Boehm/c++mm/user-faq.html" target="_top">FAQ</a>. - </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> - </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. - </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 - type called <code class="code">__basic_file</code> provides our abstraction layer - for the <code class="code">std::filebuf</code> classes. Nearly all decisions dealing - with actual input and output must be made in <code class="code">__basic_file</code>. - </p><p>A generic locking mechanism is somewhat in place at the filebuf layer, - but is not used in the current code. Providing locking at any higher - level is akin to providing locking within containers, and is not done - for the same reasons (see the links above). - </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 - the C stdio layer (again, see the link under Structure). We do no - locking ourselves, but simply pass through to calls to <code class="code">fopen</code>, - <code class="code">fwrite</code>, and so forth. - </p><p>So, for 3.0, the question of "is multithreading safe for I/O" - must be answered with, "is your platform's C library threadsafe - for I/O?" Some are by default, some are not; many offer multiple - implementations of the C library with varying tradeoffs of threadsafety - and efficiency. You, the programmer, are always required to take care - with multiple threads. - </p><p>(As an example, the POSIX standard requires that C stdio FILE* - operations are atomic. POSIX-conforming C libraries (e.g, on Solaris - and GNU/Linux) have an internal mutex to serialize operations on - FILE*s. However, you still need to not do stupid things like calling - <code class="code">fclose(fs)</code> in one thread followed by an access of - <code class="code">fs</code> in another.) - </p><p>So, if your platform's C library is threadsafe, then your - <code class="code">fstream</code> I/O operations will be threadsafe at the lowest - level. For higher-level operations, such as manipulating the data - contained in the stream formatting classes (e.g., setting up callbacks - inside an <code class="code">std::ofstream</code>), you need to guard such accesses - like any other critical shared resource. - </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 - second choice may be available for I/O implementations: libio. This is - disabled by default, and in fact will not currently work due to other - issues. It will be revisited, however. - </p><p>The libio code is a subset of the guts of the GNU libc (glibc) I/O - implementation. When libio is in use, the <code class="code">__basic_file</code> - type is basically derived from FILE. (The real situation is more - complex than that... it's derived from an internal type used to - implement FILE. See libio/libioP.h to see scary things done with - vtbls.) The result is that there is no "layer" of C stdio - to go through; the filebuf makes calls directly into the same - functions used to implement <code class="code">fread</code>, <code class="code">fwrite</code>, - and so forth, using internal data structures. (And when I say - "makes calls directly," I mean the function is literally - replaced by a jump into an internal function. Fast but frightening. - *grin*) - </p><p>Also, the libio internal locks are used. This requires pulling in - large chunks of glibc, such as a pthreads implementation, and is one - of the issues preventing widespread use of libio as the libstdc++ - cstdio implementation. - </p><p>But we plan to make this work, at least as an option if not a future - default. Platforms running a copy of glibc with a recent-enough - version will see calls from libstdc++ directly into the glibc already - installed. For other platforms, a copy of the libio subsection will - be built and included in libstdc++. - </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 - easily write one to perform your own forms of locking, to solve your - "interesting" problems. - </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 - multithreaded applications which use Standard C++ containers. - All information in this section is current as of the gcc 3.0 - release and all later point releases. Although earlier gcc - releases had a different approach to threading configuration and - proper compilation, the basic code design rules presented here - were similar. For information on all other aspects of - multithreading as it relates to libstdc++, including details on - the proper compilation of threaded code (and compatibility between - threaded and non-threaded code), see Chapter 17. - </p><p>Two excellent pages to read when working with the Standard C++ - containers and threads are - <a class="link" href="http://www.sgi.com/tech/stl/thread_safety.html" target="_top">SGI's - http://www.sgi.com/tech/stl/thread_safety.html</a> and - <a class="link" href="http://www.sgi.com/tech/stl/Allocators.html" target="_top">SGI's - http://www.sgi.com/tech/stl/Allocators.html</a>. - </p><p><span class="emphasis"><em>However, please ignore all discussions about the user-level - configuration of the lock implementation inside the STL - container-memory allocator on those pages. For the sake of this - discussion, libstdc++ configures the SGI STL implementation, - not you. This is quite different from how gcc pre-3.0 worked. - In particular, past advice was for people using g++ to - explicitly define _PTHREADS or other macros or port-specific - compilation options on the command line to get a thread-safe - STL. This is no longer required for any port and should no - longer be done unless you really know what you are doing and - assume all responsibility.</em></span> - </p><p>Since the container implementation of libstdc++ uses the SGI - code, we use the same definition of thread safety as SGI when - discussing design. A key point that beginners may miss is the - fourth major paragraph of the first page mentioned above - (<span class="emphasis"><em>For most clients...</em></span>), which points out that - locking must nearly always be done outside the container, by - client code (that'd be you, not us). There is a notable - exceptions to this rule. Allocators called while a container or - element is constructed uses an internal lock obtained and - released solely within libstdc++ code (in fact, this is the - reason STL requires any knowledge of the thread configuration). - </p><p>For implementing a container which does its own locking, it is - trivial to provide a wrapper class which obtains the lock (as - SGI suggests), performs the container operation, and then - releases the lock. This could be templatized <span class="emphasis"><em>to a certain - extent</em></span>, on the underlying container and/or a locking - mechanism. Trying to provide a catch-all general template - solution would probably be more trouble than it's worth. - </p><p>The library implementation may be configured to use the - high-speed caching memory allocator, which complicates thread - safety issues. For all details about how to globally override - this at application run-time - see <a class="link" href="using_macros.html" title="Macros">here</a>. Also - useful are details - on <a class="link" href="memory.html#std.util.memory.allocator" title="Allocators">allocator</a> - options and capabilities. - </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>
\ No newline at end of file |