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<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
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<meta name="AUTHOR" content="pme@gcc.gnu.org (Phil Edwards)">
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<meta name="KEYWORDS" content="HOWTO, libstdc++, gcc, g++, libg++, STL">
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<meta name="DESCRIPTION" content="HOWTO for libstdc++ chapter 17.">
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<meta name="GENERATOR" content="vi and eight fingers">
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<title>libstdc++-v3 HOWTO: Chapter 17</title>
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2001-10-12 02:41:47 +08:00
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<link rel="StyleSheet" href="../lib3styles.css">
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2001-10-10 04:18:14 +08:00
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<h1 class="centered"><a name="top">Chapter 17: Library Introduction</a></h1>
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<p>Chapter 17 is actually a list of definitions and descriptions used
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in the following chapters of the Standard when describing the actual
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library. Here, we use "Introduction" as an introduction
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to the <em>GNU implementation of</em> the ISO Standard C++ Library.
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</p>
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<!-- ####################################################### -->
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<hr>
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<h1>Contents</h1>
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<ul>
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<li><a href="#2">The Standard C++ header files</a>
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<li><a href="#3">Thread-safety</a>
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<li><a href="#4"><code><foo></code> vs <code><foo.h></code></a>
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<li><a href="porting-howto.html">Porting HOWTO</a>
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</ul>
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<hr>
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<!-- ####################################################### -->
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<h2><a name="2">The Standard C++ header files</a></h2>
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<p>The Standard C++ Library specifies 50 header files that must be
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available to all hosted implementations. Actually, the word
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"files" is a misnomer, since the contents of the headers
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don't necessarily have to be in any kind of external file. The
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only rule is that when you <code>#include</code> a certain header, the
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contents of that header, as defined by the Standard, become
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available to you, no matter how.
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</p>
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<p>The names of the headers can be easily seen in
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<a href="headers_cc.txt"><code>testsuite/17_intro/headers.cc</code></a>,
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which is a small testbed we use to make certain that the headers
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all compile and run.
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</p>
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<hr>
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<h2><a name="3">Thread-safety</a></h2>
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<p>This is a thorny issue that gets brought up on the libstdc++-v3
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and gcc mailing lists on a regular basis (probably by a cron job).
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This entry will mention a very little bit about the general MT
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issues with libstdc++. The latest status and quick notes will be
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in FAQ 5.6. Some discussion about thread-safe containers will be
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in section 6.8 (the HOWTOs on containers). This section only applies
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when gcc and libstdc++-v3 were configured with --enable-threads.
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</p>
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<p>The libstdc++ code (all of it, not just the containers) has been
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designed so that thread-safety will be easily possible. The first
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(!) problem is finding a <em>fast</em> method of implementation
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portable to all platforms. A minor problem that pops up every so
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often is different interpretations of what "thread-safe"
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means for a library (not a general program). We currently use the
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<a href="http://www.sgi.com/tech/stl/thread_safety.html">same
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definition that SGI</a> uses for their STL subset.
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<em>Please see the many cautions given in HOWTOs on containers.</em>
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</p>
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<p>Here is another attempt at explaining the dangers of using the
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STL with threading support without understanding some important
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details. The STL implementation is currently configured to use
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the high-speed caching memory allocator. If you absolutely
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think you must change this on a global basis for your platform
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to support multi-threading, then please consult all commentary
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in include/bits/c++config and the HOWTOs on containers. Be
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fully aware that you may change the external or internal ABI of
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libstdc++-v3 when you provide -D__USE_MALLOC on the command line
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or make a change to that configuration file.
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</p>
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<p>If you don't like caches of objects being retained inside the STL, then
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you might be tempted to define __USE_MALLOC either on the command
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line or by rebuilding c++config.h. Please note, once you define
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__USE_MALLOC, only the malloc allocator is visible to application code
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(i.e. the typically higher-speed allocator is not even available
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in this configuration). There is a better way: It is possible
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to force the malloc-based allocator on a per-case-basis for some
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application code even when the above macro symbol is not defined.
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The library team generally believes that this is a better way to tune
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an application for high-speed using this implementation of the STL.
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Here is one possible example displaying the forcing of the malloc-based
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allocator over the typically higher-speed default allocator:
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<pre>
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std::list <my_type, std::malloc_alloc> my_malloc_based_list;</pre>
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</p>
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<p>A recent journal article has described "atomic integer
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operations," which would allow us to, well, perform updates
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on integers atomically, and without requiring an explicit mutex
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lock. This appears promising, but the major difficulty is that
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these operations "may not be available on all systems, and
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if they are, may have different interfaces." [quoting from
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mailing list messages]
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</p>
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<p>Here is a small link farm to threads (no pun) in the mail archives
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that discuss the threading problem. Each link is to the first
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relevent message in the thread; from there you can use
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"Thread Next" to move down the thread. This farm is in
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latest-to-oldest order.
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<ul>
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<li><a href="http://gcc.gnu.org/ml/libstdc++/2001-05/msg00384.html">
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inspired this most recent updating of issues with threading
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and the SGI STL library. It also contains some example
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POSIX-multithreaded STL code.</a>
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<li> <a href="http://gcc.gnu.org/ml/libstdc++/2001-05/msg00136.html">
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an early analysis of why __USE_MALLOC should be disabled for
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the 3.0 release of libstdc++.</a>
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</ul>
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<br>
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Here are discussions that took place before the current snapshot;
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they are still relevant and instructive. (Some of them may not work;
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as the drive containing some of the 1999 archives crashed, and nobody
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has had time to recover the backups.)
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<br>
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<ul>
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<li>One way of preventing memory leaks by the old default memory
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allocator in multithreaded code is
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<a href="http://gcc.gnu.org/ml/gcc/1999-11n/msg00431.html">discussed here</a>.
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<li><a href="http://gcc.gnu.org/ml/libstdc++/1999-q3/msg00167.html">This thread
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concerns strings</a>.
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<li><a href="http://gcc.gnu.org/ml/libstdc++/1999-q2/msg00339.html">So does this
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one</a>. This initial message also refers to another
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thread in the GCC mailing list...
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<li><a href="http://gcc.gnu.org/ml/gcc/1999-06n/msg00680.html">which is here</a>,
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and goes on for some time. Ironically, the initial message
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in this thread also mentions another threading thread...
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<li><a href="http://gcc.gnu.org/ml/gcc-bugs/1999-04n/msg00777.html">beginning here</a>,
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and talking about pthreads. (Note that a much more recent
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message from the first thread in this list notes that
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<a href="http://gcc.gnu.org/ml/libstdc++/1999-q3/msg00176.html">pthreads
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should not be used as a starting point</a> for making
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libstdc++ threadsafe.)
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<li><a href="http://gcc.gnu.org/ml/libstdc++/1999-q2/msg00168.html">This
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message</a>,
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<a href="http://gcc.gnu.org/ml/libstdc++/1999-q2/msg00159.html">this one</a>,
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and <a href="http://gcc.gnu.org/ml/libstdc++/1999-q2/msg00156.html">this one</a>
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are the tops of related threads (all within the same time
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period) discussing threading and the IO library. Much of it
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is dealing with the C library, but C++ is included as well.
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</ul>
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</p>
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<p>This section will be updated as new and interesting issues come
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to light.
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</p>
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<p>Return <a href="#top">to top of page</a> or
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<a href="../faq/index.html">to the FAQ</a>.
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</p>
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<hr>
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<h2><a name="4"><code><foo></code> vs <code><foo.h></code></a></h2>
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<p>The new-style headers are fully supported in libstdc++-v3. The compiler
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itself fully supports namespaces, including <code>std::</code>.
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</p>
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<p>For those of you new to ISO C++98, no, that isn't a typo, the headers
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really have new names. Marshall Cline's C++ FAQ Lite has a good
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explanation in
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<a href="http://www.cerfnet.com/~mpcline/On-Line-C++-FAQ/coding-standards.html#[25.4]">item [25.4]</a>.
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</p>
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<p>Return <a href="#top">to top of page</a> or
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<a href="../faq/index.html">to the FAQ</a>.
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</p>
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<!-- ####################################################### -->
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<hr>
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<p class="fineprint"><em>
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See <a href="license.html">license.html</a> for copying conditions.
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Comments and suggestions are welcome, and may be sent to
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<a href="mailto:libstdc++@gcc.gnu.org">the libstdc++ mailing list</a>.
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</em></p>
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2001-09-18 07:24:40 +08:00
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</body>
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</html>
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