mirror of
https://github.com/autc04/Retro68.git
synced 2024-11-28 05:51:04 +00:00
276 lines
20 KiB
HTML
276 lines
20 KiB
HTML
<?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 V1.78.1" /><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
|
||
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. On several targets (including
|
||
GNU/Linux, Solaris and various BSDs) -pthread is honored.
|
||
Some other ports use other switches.
|
||
This is not well documented anywhere other than
|
||
in "gcc -dumpspecs" (look at the 'lib' and 'cpp' entries).
|
||
</p><p>
|
||
Some uses of <code class="classname">std::atomic</code> also require linking
|
||
to <code class="filename">libatomic</code>.
|
||
</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="using.html#manual.intro.using.flags" title="Command Options">Command Options</a> and
|
||
<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 the
|
||
<code class="filename">atomicity.h</code> functions
|
||
exists for the architecture in question. See the
|
||
<a class="link" href="internals.html#internals.thread_safety" title="Thread Safety">internals
|
||
documentation</a> for more details.
|
||
</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> |