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<html xmlns="http://www.w3.org/1999/xhtml"><head><title>Concurrency</title><meta name="generator" content="DocBook XSL-NS Stylesheets V1.76.1"/><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 align="left"><a accesskey="p" href="using_dynamic_or_shared.html">Prev</a> </td><th width="60%" align="center">Chapter 3. Using</th><td align="right"> <a accesskey="n" href="using_exceptions.html">Next</a></td></tr></table><hr/></div><div class="section" title="Concurrency"><div class="titlepage"><div><div><h2 class="title"><a id="manual.intro.using.concurrency"/>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" title="Prerequisites"><div class="titlepage"><div><div><h3 class="title"><a id="manual.intro.using.concurrency.prereq"/>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 OSF,
-pthread 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" title="Thread Safety"><div class="titlepage"><div><div><h3 class="title"><a id="manual.intro.using.concurrency.thread_safety"/>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">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"><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">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">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">introduction</a>
and <a class="link" href="http://www.hpl.hp.com/personal/Hans_Boehm/c++mm/user-faq.html">FAQ</a>.
</p></div><div class="section" title="Atomics"><div class="titlepage"><div><div><h3 class="title"><a id="manual.intro.using.concurrency.atomics"/>Atomics</h3></div></div></div><p>
</p></div><div class="section" title="IO"><div class="titlepage"><div><div><h3 class="title"><a id="manual.intro.using.concurrency.io"/>IO</h3></div></div></div><p>This gets a bit tricky. Please read carefully, and bear with me.
</p><div class="section" title="Structure"><div class="titlepage"><div><div><h4 class="title"><a id="concurrency.io.structure"/>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" title="Defaults"><div class="titlepage"><div><div><h4 class="title"><a id="concurrency.io.defaults"/>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" title="Future"><div class="titlepage"><div><div><h4 class="title"><a id="concurrency.io.future"/>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" title="Alternatives"><div class="titlepage"><div><div><h4 class="title"><a id="concurrency.io.alt"/>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" title="Containers"><div class="titlepage"><div><div><h3 class="title"><a id="manual.intro.using.concurrency.containers"/>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">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">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.
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