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- <meta name="AUTHOR" content="pme@gcc.gnu.org (Phil Edwards)" />
- <meta name="KEYWORDS" content="HOWTO, libstdc++, gcc, g++, libg++, STL" />
- <meta name="DESCRIPTION" content="HOWTO for libstdc++ chapter 17." />
- <meta name="GENERATOR" content="vi and eight fingers" />
- <title>libstdc++-v3 HOWTO: Chapter 17</title>
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-</head>
-<body>
-
-<h1 class="centered"><a name="top">Chapter 17: Library Introduction</a></h1>
-
-<p>Chapter 17 is actually a list of definitions and descriptions used
- in the following chapters of the Standard when describing the actual
- library. Here, we use "Introduction" as an introduction
- to the <em>GNU implementation of</em> the ISO Standard C++ Library.
-</p>
-
-
-<!-- ####################################################### -->
-<hr />
-<h1>Contents</h1>
-<ul>
- <li><a href="#2">The Standard C++ header files</a></li>
- <li><a href="#3">The Standard C++ library and multithreading</a></li>
- <li><a href="#4"><code><foo></code> vs <code><foo.h></code></a></li>
- <li><a href="porting-howto.html">Porting HOWTO</a></li>
- <li><a href="#5">Behavior specific to libstdc++-v3</a></li>
- <li><a href="#6">Preprocessor macros controlling the library</a></li>
-</ul>
-
-<hr />
-
-<!-- ####################################################### -->
-
-<h2><a name="2">The Standard C++ header files</a></h2>
- <p>The Standard C++ Library specifies 50 header files that must be
- available to all hosted implementations. Actually, the word
- "files" is a misnomer, since the contents of the headers
- don't necessarily have to be in any kind of external file. The
- only rule is that when you <code>#include</code> a certain header, the
- contents of that header, as defined by the Standard, become
- available to you, no matter how.
- </p>
- <p>The names of the headers can be easily seen in
- <a href="headers_cc.txt"><code>testsuite/17_intro/headers.cc</code></a>,
- which is a small testbed we use to make certain that the headers
- all compile and run.
- </p>
-
-<hr />
-<h2><a name="3">The Standard C++ library and multithreading</a></h2>
- <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.
- Unless explicitly prefaced, all information in this section is
- current as of the GCC 3.0 release and all later point releases.
- </p>
- <p>Earlier GCC releases had a somewhat different approach to
- threading configuration and proper compilation. Before GCC 3.0,
- configuration of the threading model was dictated by compiler
- command-line options and macros (both of which were somewhat
- thread-implementation and port-specific). There were no
- guarantees related to being able to link code compiled with one
- set of options and macro setting with another set. For GCC 3.0,
- configuration of the threading model used with libraries and
- user-code is performed when GCC is configured and built using
- the --enable-threads and --disable-threads options. The ABI is
- stable for symbol name-mangling and limited functional
- compatibility exists between code compiled under different
- threading models.
- </p>
- <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> gcc/g++ -v </code>) the same thread
- model and that model is not <em>single</em>. 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
- <em>single</em> 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 Linux/i386, -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>
- <p>See <a href="../faq/index.html#3">FAQ</a> (general overview), <a
- href="../23_containers/howto.html#3">23</a> (containers), and <a
- href="../27_io/howto.html#9">27</a> (I/O) for more information.
- </p>
- <p>The libstdc++-v3 library (unlike libstdc++-v2, all of it, not
- just the STL) has been designed so that multithreaded
- applications using it may be written. The first problem is
- finding a <em>fast</em> method of implementation portable to all
- platforms. Due to historical reasons, some of the library is
- written against per-CPU-architecture spinlocks and other parts
- against the gthr.h abstraction layer which is provided by gcc.
- A minor problem that pops up every so often is different
- interpretations of what "thread-safe" means for a
- library (not a general program). We currently use the <a
- href="http://www.sgi.com/tech/stl/thread_safety.html">same
- definition that SGI</a> uses for their STL subset. However, the
- exception for read-only containers only applies to the STL
- components.
- </p>
- <p>Here is a small link farm to threads (no pun) in the mail archives
- that discuss the threading problem. Each link is to the first
- relevant message in the thread; from there you can use
- "Thread Next" to move down the thread. This farm is in
- latest-to-oldest order.
- </p>
- <ul>
- <li>Our threading expert Loren gives a breakdown of
- <a href="http://gcc.gnu.org/ml/libstdc++/2001-10/msg00024.html">the
- six situations involving threads</a> for the 3.0 release series.</li>
- <li><a href="http://gcc.gnu.org/ml/libstdc++/2001-05/msg00384.html">
- This message</a> inspired a recent updating of issues with threading
- and the SGI STL library. It also contains some example
- POSIX-multithreaded STL code.</li>
- </ul>
- <p> (A large selection of links to older messages has been removed; many
- of the messages from 1999 were lost in a disk crash, and the few
- people with access to the backup tapes have been too swamped with work
- to restore them. Many of the points have been superseded anyhow.)
- </p>
- <p>This section will be updated as new and interesting issues come
- to light.
- </p>
- <p>Return <a href="#top">to top of page</a> or
- <a href="../faq/index.html">to the FAQ</a>.
- </p>
-
-<hr />
-<h2><a name="4"><code><foo></code> vs <code><foo.h></code></a></h2>
- <p>The new-style headers are fully supported in libstdc++-v3. The compiler
- itself fully supports namespaces, including <code>std::</code>.
- </p>
- <p>For those of you new to ISO C++98, no, that isn't a typo, the headers
- really have new names. Marshall Cline's C++ FAQ Lite has a good
- explanation in
-<a href="http://www.parashift.com/c++-faq-lite/coding-standards.html#faq-26.4">item [26.4]</a>.
- </p>
- <p>Return <a href="#top">to top of page</a> or
- <a href="../faq/index.html">to the FAQ</a>.
- </p>
-
-<hr />
-<h2><a name="5">Behavior specific to libstdc++-v3</a></h2>
- <p>The ISO standard defines the following phrase:
- </p>
- <blockquote><dl>
- <dt><code>[1.3.5] implementation-defined behavior</code></dt>
- <dd>behavior, for a well-formed program construct and correct data, that
- depends on the implementation <strong>and that each implementation
- shall document</strong>.
- </dd>
- </dl></blockquote>
- <p>We do so here, for the C++ library only. Behavior of the compiler,
- linker, runtime loader, and other elements of "the
- implementation" are documented elsewhere. Everything listed in
- Annex B, Implemenation Qualities, are also part of the compiler, not
- the library.
- </p>
- <p>For each entry, we give the section number of the standard, when
- applicable. This list is probably incomplet and inkorrekt.
- </p>
- <p><strong>[1.9]/11 #3</strong> If <code>isatty(3)</code> is true, then
- interactive stream support is implied.
- </p>
- <p><strong>[17.4.4.5]</strong> Non-reentrant functions are probably best
- discussed in the various sections on multithreading (see above).
- </p>
- <!-- [17.4.4.8]/3 says any function that doesn't have an exception-spec
- can throw whatever we want; see also its footnote. Let's list those
- in the sections where the function itself occurs.
- -->
- <p><strong>[18.1]/4</strong> The type of <code>NULL</code> is described
- <a href="../18_support/howto.html#1">here</a>.
- </p>
- <p><strong>[18.3]/8</strong> Even though it's listed in the library
- sections, libstdc++-v3 has zero control over what the cleanup code hands
- back to the runtime loader. Talk to the compiler people. :-)
- </p>
- <p><strong>[18.4.2.1]/5</strong> (bad_alloc),<br />
- <strong>[18.5.2]/5</strong> (bad_cast),<br />
- <strong>[18.5.3]/5</strong> (bad_typeid),<br />
- <strong>[18.6.1]/8</strong> (exception),<br />
- <strong>[18.6.2.1]/5</strong> (bad_exception): The <code>what()</code>
- member function of class <code>std::exception</code>, and these other
- classes publicly derived from it, simply returns the name of the
- class. But they are the <em>mangled</em> names; you will need to call
- <code>c++filt</code> and pass the names as command-line parameters to
- demangle them, or call a
- <a href="../18_support/howto.html#5">runtime demangler function</a>.
- (The classes in <code><stdexcept></code> have constructors which
- require an argument to use later for <code>what()</code> calls, so the
- question does not arise in most user-defined exceptions.)
- </p>
- <p><strong>[18.5.1]/7</strong> The return value of
- <code>std::type_info::name()</code> is the mangled type name (see the
- previous entry for more).
- </p>
- <p><strong>[20.1.5]/5</strong> <em>"Implementors are encouraged to
- supply libraries that can accept allocators that encapsulate more
- general memory models and that support non-equal instances. In such
- implementations, any requirements imposed on allocators by containers
- beyond those requirements that appear in Table 32, and the semantics
- of containers and algorithms when allocator instances compare
- non-equal, are implementation-defined."</em> As yet we don't
- have any allocators which compare non-equal, so we can't describe how
- they behave.
- </p>
- <p><strong>[21.1.3.1]/3,4</strong>,<br />
- <strong>[21.1.3.2]/2</strong>,<br />
- <strong>[23.*]'s foo::iterator</strong>,<br />
- <strong>[27.*]'s foo::*_type</strong>,<br />
- <strong>others...</strong>
- Nope, these types are called implementation-defined because you
- shouldn't be taking advantage of their underlying types. Listing them
- here would defeat the purpose. :-)
- </p>
- <p><strong>[21.1.3.1]/5</strong> I don't really know about the mbstate_t
- stuff... see the <a href="../22_locale/howto.html">chapter 22 notes</a>
- for what does exist.
- </p>
- <p><strong>[22.*]</strong> Anything and everything we have on locale
- implemenation will be described
- <a href="../22_locale/howto.html">over here</a>.
- </p>
- <p><strong>[26.2.8]/9</strong> I have no idea what
- <code>complex<T></code>'s pow(0,0) returns.
- </p>
- <p><strong>[27.4.2.4]/2</strong> Calling
- <code>std::ios_base::sync_with_stdio</code> after I/O has already been
- performed on the standard stream objects will
- flush the buffers, and <!-- this line might go away -->
- destroy and recreate the underlying buffer instances. Whether or not
- the previously-written I/O is destroyed in this process depends mostly
- on the --enable-libio choice: for stdio, if the written data is
- already in the stdio buffer, the data may be completely safe!
- </p>
- <p><strong>[27.6.1.1.2]</strong>,<br />
- <strong>[27.6.2.3]</strong> The I/O sentry ctor and dtor can perform
- additional work than the minimum required. We are not currently taking
- advantage of this yet.
- </p>
- <p><strong>[27.7.1.3]/16</strong>,<br />
- <strong>[27.8.1.4]/10</strong>
- The effects of <code>pubsetbuf/setbuf</code> are described
- <a href="../27_io/howto.html#2">in this chapter</a>.
- </p>
- <p><strong>[27.8.1.4]/16</strong> Calling <code>fstream::sync</code> when
- a get area exists will... whatever <code>fflush()</code> does, I think.
- </p>
- <p>Return <a href="#top">to top of page</a> or
- <a href="../faq/index.html">to the FAQ</a>.
- </p>
-
-<hr />
-<h2><a name="6">Preprocessor macros controlling the library</a></h2>
- <p>Some of the semantics of the libstdc++-v3 implementation are
- controlled by preprocessor macros, both during build/installation and
- during compilation of user code. Many of these choices are made when
- the library is built and installed (actually, during
- <a href="../configopts.html">the configuration step</a>, with the
- various --enable/--disable choices being translated to #define/#undef).
- </p>
- <p>All library macros begin with <code>_GLIBCPP_</code>. The fact that
- these symbols start with a leading underscore should give you a clue
- that (by default) they aren't meant to be changed by the user. :-)
- </p>
- <p>These macros are all gathered in the file <code>c++config.h</code>,
- which is generated during installation. <strong>You must assume that
- these macros cannot be redefined by your own code</strong>, unless we
- document otherwise here. Some of the choices control code which has
- already been compiled (i.e., libstdc++.a/.so). If you explicitly
- #define or #undef these macros, the <em>headers</em> may see different
- code paths, but the <em>libraries</em> which you link against will not.
- If you want to experiment with different values, you must change the
- config headers before building/installing the library.
- </p>
- <p>Below are macros which, for 3.1 and later, you may change yourself,
- in your own code with #define/#undef or with -D/-U compiler flags.
- The default state of the symbol is listed. "Configurable"
- (or "Not configurable") means that the symbol is initially
- chosen (or not) based on --enable/--disable options at configure time.
- </p>
- <dl>
- <dt><code>_GLIBCPP_DEPRECATED</code></dt>
- <dd>Undefined by default. Not configurable. Turning this on enables
- older ARM-style iostreams code, and other anachronisms. This may be
- useful in updating old C++ programs which no longer meet the
- requirements of the language.
- </dd>
- <!--
- Can this actually be turned off and still produce a working lib? Must
- check. -pme
- No, it can't. Hmmm. -pme
- <dt><code>_GLIBCPP_RESOLVE_LIB_DEFECTS</code></dt>
- <dd>Defined by default. Not configurable. The library follows
- corrections and updates from the ISO committee, see
- <a href="../faq/index.html#5_2">here</a> and
- <a href="../ext/howto.html#5">here</a> for more on this feature.
- If you have code which depends on the first version of the standard,
- you might try undefining this macro.
- </dd>
- -->
- <dt><code>_GLIBCPP_CONCEPT_CHECKS</code></dt>
- <dd>Undefined by default. Configurable. When defined, performs
- compile-time checking on certain template instantiations to detect
- violations of the requirements of the standard. This is described
- in more detail <a href="../19_diagnostics/howto.html#3">here</a>.
- </dd>
- <!--
- <dt><code></code></dt>
- <dd>
- </dd>
- -->
- </dl>
- <p>Return <a href="#top">to top of page</a> or
- <a href="../faq/index.html">to the FAQ</a>.
- </p>
-
-
-
-<!-- ####################################################### -->
-
-<hr />
-<p class="fineprint"><em>
-See <a href="license.html">license.html</a> for copying conditions.
-Comments and suggestions are welcome, and may be sent to
-<a href="mailto:libstdc++@gcc.gnu.org">the libstdc++ mailing list</a>.
-</em></p>
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