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3 <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 V1.74.3" /><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_macros.html" title="Macros" /><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_macros.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" lang="en" xml:lang="en"><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
4 of multithreaded applications which use the Standard C++
5 library. This information is GCC-specific since the C++
6 standard does not address matters of multithreaded applications.
7 </p><div class="sect2" lang="en" xml:lang="en"><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
8 only supported when libstdc++ and all user code was built with
9 compilers which report (via <code class="code"> gcc/g++ -v </code>) the same thread
10 model and that model is not <span class="emphasis"><em>single</em></span>. As long as your
11 final application is actually single-threaded, then it should be
12 safe to mix user code built with a thread model of
13 <span class="emphasis"><em>single</em></span> with a libstdc++ and other C++ libraries built
14 with another thread model useful on the platform. Other mixes
15 may or may not work but are not considered supported. (Thus, if
16 you distribute a shared C++ library in binary form only, it may
17 be best to compile it with a GCC configured with
18 --enable-threads for maximal interchangeability and usefulness
19 with a user population that may have built GCC with either
20 --enable-threads or --disable-threads.)
21 </p><p>When you link a multithreaded application, you will probably
22 need to add a library or flag to g++. This is a very
23 non-standardized area of GCC across ports. Some ports support a
24 special flag (the spelling isn't even standardized yet) to add
25 all required macros to a compilation (if any such flags are
26 required then you must provide the flag for all compilations not
27 just linking) and link-library additions and/or replacements at
28 link time. The documentation is weak. Here is a quick summary
29 to display how ad hoc this is: On Solaris, both -pthreads and
30 -threads (with subtly different meanings) are honored. On OSF,
31 -pthread and -threads (with subtly different meanings) are
32 honored. On Linux/i386, -pthread is honored. On FreeBSD,
33 -pthread is honored. Some other ports use other switches.
34 AFAIK, none of this is properly documented anywhere other than
35 in ``gcc -dumpspecs'' (look at lib and cpp entries).
36 </p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="manual.intro.using.concurrency.thread_safety"></a>Thread Safety</h3></div></div></div><p>
37 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.
38 </p><p>The library strives to be thread-safe when all of the following
40 </p><div class="itemizedlist"><ul type="disc"><li><p>The system's libc is itself thread-safe,
42 The compiler in use reports a thread model other than
43 'single'. This can be tested via output from <code class="code">gcc
44 -v</code>. Multi-thread capable versions of gcc output
46 </p><pre class="programlisting">
51 gcc version 4.1.2 20070925 (Red Hat 4.1.2-33)
52 </pre><p>Look for "Thread model" lines that aren't equal to "single."</p></li><li><p>
53 Requisite command-line flags are used for atomic operations
54 and threading. Examples of this include <code class="code">-pthread</code>
55 and <code class="code">-march=native</code>, although specifics vary
56 depending on the host environment. See <a class="ulink" href="http://gcc.gnu.org/onlinedocs/gcc/Option-Summary.html" target="_top">Machine
57 Dependent Options</a>.
59 An implementation of atomicity.h functions
60 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>.
61 </p></li></ul></div><p>The user-code must guard against concurrent method calls which may
62 access any particular library object's state. Typically, the
63 application programmer may infer what object locks must be held
64 based on the objects referenced in a method call. Without getting
65 into great detail, here is an example which requires user-level
67 </p><pre class="programlisting">
68 library_class_a shared_object_a;
71 library_class_b *object_b = new library_class_b;
72 shared_object_a.add_b (object_b); // must hold lock for shared_object_a
73 shared_object_a.mutate (); // must hold lock for shared_object_a
76 // 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
77 another thread, here is an example that should not require any
79 </p><pre class="programlisting">
81 library_class_a object_a;
82 library_class_b *object_b = new library_class_b;
83 object_a.add_b (object_b);
85 } </pre><p>All library objects are safe to use in a multithreaded program as
86 long as each thread carefully locks out access by any other
87 thread while it uses any object visible to another thread, i.e.,
88 treat library objects like any other shared resource. In general,
89 this requirement includes both read and write access to objects;
90 unless otherwise documented as safe, do not assume that two threads
91 may access a shared standard library object at the same time.
92 </p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="manual.intro.using.concurrency.atomics"></a>Atomics</h3></div></div></div><p>
93 </p></div><div class="sect2" lang="en" xml:lang="en"><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.
94 </p><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="concurrency.io.structure"></a>Structure</h4></div></div></div><p>A wrapper
95 type called <code class="code">__basic_file</code> provides our abstraction layer
96 for the <code class="code">std::filebuf</code> classes. Nearly all decisions dealing
97 with actual input and output must be made in <code class="code">__basic_file</code>.
98 </p><p>A generic locking mechanism is somewhat in place at the filebuf layer,
99 but is not used in the current code. Providing locking at any higher
100 level is akin to providing locking within containers, and is not done
101 for the same reasons (see the links above).
102 </p></div><div class="sect3" lang="en" xml:lang="en"><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
103 the C stdio layer (again, see the link under Structure). We do no
104 locking ourselves, but simply pass through to calls to <code class="code">fopen</code>,
105 <code class="code">fwrite</code>, and so forth.
106 </p><p>So, for 3.0, the question of "is multithreading safe for I/O"
107 must be answered with, "is your platform's C library threadsafe
108 for I/O?" Some are by default, some are not; many offer multiple
109 implementations of the C library with varying tradeoffs of threadsafety
110 and efficiency. You, the programmer, are always required to take care
111 with multiple threads.
112 </p><p>(As an example, the POSIX standard requires that C stdio FILE*
113 operations are atomic. POSIX-conforming C libraries (e.g, on Solaris
114 and GNU/Linux) have an internal mutex to serialize operations on
115 FILE*s. However, you still need to not do stupid things like calling
116 <code class="code">fclose(fs)</code> in one thread followed by an access of
117 <code class="code">fs</code> in another.)
118 </p><p>So, if your platform's C library is threadsafe, then your
119 <code class="code">fstream</code> I/O operations will be threadsafe at the lowest
120 level. For higher-level operations, such as manipulating the data
121 contained in the stream formatting classes (e.g., setting up callbacks
122 inside an <code class="code">std::ofstream</code>), you need to guard such accesses
123 like any other critical shared resource.
124 </p></div><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="concurrency.io.future"></a>Future</h4></div></div></div><p> A
125 second choice may be available for I/O implementations: libio. This is
126 disabled by default, and in fact will not currently work due to other
127 issues. It will be revisited, however.
128 </p><p>The libio code is a subset of the guts of the GNU libc (glibc) I/O
129 implementation. When libio is in use, the <code class="code">__basic_file</code>
130 type is basically derived from FILE. (The real situation is more
131 complex than that... it's derived from an internal type used to
132 implement FILE. See libio/libioP.h to see scary things done with
133 vtbls.) The result is that there is no "layer" of C stdio
134 to go through; the filebuf makes calls directly into the same
135 functions used to implement <code class="code">fread</code>, <code class="code">fwrite</code>,
136 and so forth, using internal data structures. (And when I say
137 "makes calls directly," I mean the function is literally
138 replaced by a jump into an internal function. Fast but frightening.
140 </p><p>Also, the libio internal locks are used. This requires pulling in
141 large chunks of glibc, such as a pthreads implementation, and is one
142 of the issues preventing widespread use of libio as the libstdc++
143 cstdio implementation.
144 </p><p>But we plan to make this work, at least as an option if not a future
145 default. Platforms running a copy of glibc with a recent-enough
146 version will see calls from libstdc++ directly into the glibc already
147 installed. For other platforms, a copy of the libio subsection will
148 be built and included in libstdc++.
149 </p></div><div class="sect3" lang="en" xml:lang="en"><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
150 easily write one to perform your own forms of locking, to solve your
151 "interesting" problems.
152 </p></div></div><div class="sect2" lang="en" xml:lang="en"><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
153 multithreaded applications which use Standard C++ containers.
154 All information in this section is current as of the gcc 3.0
155 release and all later point releases. Although earlier gcc
156 releases had a different approach to threading configuration and
157 proper compilation, the basic code design rules presented here
158 were similar. For information on all other aspects of
159 multithreading as it relates to libstdc++, including details on
160 the proper compilation of threaded code (and compatibility between
161 threaded and non-threaded code), see Chapter 17.
162 </p><p>Two excellent pages to read when working with the Standard C++
163 containers and threads are
164 <a class="ulink" href="http://www.sgi.com/tech/stl/thread_safety.html" target="_top">SGI's
165 http://www.sgi.com/tech/stl/thread_safety.html</a> and
166 <a class="ulink" href="http://www.sgi.com/tech/stl/Allocators.html" target="_top">SGI's
167 http://www.sgi.com/tech/stl/Allocators.html</a>.
168 </p><p><span class="emphasis"><em>However, please ignore all discussions about the user-level
169 configuration of the lock implementation inside the STL
170 container-memory allocator on those pages. For the sake of this
171 discussion, libstdc++ configures the SGI STL implementation,
172 not you. This is quite different from how gcc pre-3.0 worked.
173 In particular, past advice was for people using g++ to
174 explicitly define _PTHREADS or other macros or port-specific
175 compilation options on the command line to get a thread-safe
176 STL. This is no longer required for any port and should no
177 longer be done unless you really know what you are doing and
178 assume all responsibility.</em></span>
179 </p><p>Since the container implementation of libstdc++ uses the SGI
180 code, we use the same definition of thread safety as SGI when
181 discussing design. A key point that beginners may miss is the
182 fourth major paragraph of the first page mentioned above
183 ("For most clients,"...), which points out that
184 locking must nearly always be done outside the container, by
185 client code (that'd be you, not us). There is a notable
186 exceptions to this rule. Allocators called while a container or
187 element is constructed uses an internal lock obtained and
188 released solely within libstdc++ code (in fact, this is the
189 reason STL requires any knowledge of the thread configuration).
190 </p><p>For implementing a container which does its own locking, it is
191 trivial to provide a wrapper class which obtains the lock (as
192 SGI suggests), performs the container operation, and then
193 releases the lock. This could be templatized <span class="emphasis"><em>to a certain
194 extent</em></span>, on the underlying container and/or a locking
195 mechanism. Trying to provide a catch-all general template
196 solution would probably be more trouble than it's worth.
197 </p><p>The library implementation may be configured to use the
198 high-speed caching memory allocator, which complicates thread
199 safety issues. For all details about how to globally override
200 this at application run-time
201 see <a class="link" href="using_macros.html" title="Macros">here</a>. Also
203 on <a class="link" href="memory.html#manual.util.memory.allocator" title="Allocators">allocator</a>
204 options and capabilities.
205 </p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="using_macros.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">Macros </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>