howto.html: Remove 1999 links (and explain why).
2001-10-17 Phil Edwards <pme@gcc.gnu.org> * docs/html/17_intro/howto.html: Remove 1999 links (and explain why). Add link to recent message. * docs/html/faq/index.html (5.6): Reformat text only; fixup <pre> markup. * docs/html/faq/index.txt: Regenerate. From-SVN: r46321
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Phil Edwards
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2001-10-17 Phil Edwards <pme@gcc.gnu.org>
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* docs/html/17_intro/howto.html: Remove 1999 links (and explain
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why). Add link to recent message.
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* docs/html/faq/index.html (5.6): Reformat text only; fixup <pre>
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markup.
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* docs/html/faq/index.txt: Regenerate.
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2001-10-12 Loren J. Rittle <ljrittle@acm.org>
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* docs/html/faq/index.html (Is libstdc++-v3 thread-safe?): Update
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@ -67,7 +67,8 @@
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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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<em>Please see the many cautions given in
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<a href="../23_containers/howto.html">HOWTOs on containers</a>.</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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@ -109,46 +110,21 @@
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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>Our threading expert Loren gives a breakdown of
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<a href="http://gcc.gnu.org/ml/libstdc++/2001-10/msg00024.html">the
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six situations involving threads</a> for the 3.0 release series.
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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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This message</a> inspired a 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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POSIX-multithreaded STL code.
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<li><a href="http://gcc.gnu.org/ml/libstdc++/2001-05/msg00136.html">
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Here</a> is an early analysis of why __USE_MALLOC should be disabled
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for the 3.0 release of libstdc++.</a>
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</ul>
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(A large selection of links to older messages has been removed; many
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of the messages from 1999 were lost in a disk crash, and the few
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people with access to the backup tapes have been too swamped with work
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to restore them. Many of the points have been superceded anyhow.)
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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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@ -687,49 +687,46 @@ http://clisp.cons.org/~haible/gccinclude-glibc-2.2-compat.diff
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<hr>
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<h2><a name="5_6">5.6 Is libstdc++-v3 thread-safe?</a></h2>
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<p>When the system's libc is itself thread-safe, a non-generic
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implementation of atomicity.h exists for the architecture, and
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gcc itself reports a thread model other than single; libstdc++-v3
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implementation of atomicity.h exists for the architecture, and gcc
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itself reports a thread model other than single; libstdc++-v3
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strives to be thread-safe. The user-code must guard against
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concurrent method calls which may access any particular
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library object's state. Typically, the application
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programmer may infer what object locks must be held based on
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the objects referenced in a method call. Without getting
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into great detail, here is an example which requires
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concurrent method calls which may access any particular library
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object's state. Typically, the application programmer may infer
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what object locks must be held based on the objects referenced in
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a method call. Without getting into great detail, here is an
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example which requires user-level locks:
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<pre>
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library_class_a shared_object_a;
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thread_main () {
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library_class_b *object_b = new library_class_b;
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shared_object_a.add_b (object_b); // must hold lock for shared_object_a
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shared_object_a.mutate (); // must hold lock for shared_object_a
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}
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// Multiple copies of thread_main() are started in independent threads.</pre>
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</p>
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<p>Under the assumption that object_a and object_b are never exposed to
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another thread, here is an example that should not require any
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user-level locks:
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</p>
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<pre>
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library_class_a shared_object_a;
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thread_main () {
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library_class_b *object_b = new library_class_b;
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shared_object_a.add_b (object_b); // must hold lock for shared_object_a
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shared_object_a.mutate (); // must hold lock for shared_object_a
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}
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// Multiple copies of thread_main() are started in independent threads.
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</pre>
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<p>Under the assumption that object_a and object_b are never
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exposed to another thread, here is an example that should not
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require any user-level locks:
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thread_main () {
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library_class_a object_a;
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library_class_b *object_b = new library_class_b;
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object_a.add_b (object_b);
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object_a.mutate ();
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} </pre>
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</p>
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<pre>
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thread_main () {
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library_class_a object_a;
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library_class_b *object_b = new library_class_b;
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object_a.add_b (object_b);
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object_a.mutate ();
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}
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</pre>
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<p>All library objects are safe to use in a multithreaded
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program as long as each thread carefully locks out access by
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any other thread while it uses any object visible to another
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thread. This requirement includes both read and write access
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to objects; do not assume that two threads may read a shared
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standard container at the same time.
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<p>All library objects are safe to use in a multithreaded program as
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long as each thread carefully locks out access by any other thread
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while it uses any object visible to another thread. This requirement
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includes both read and write access to objects; do not assume that
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two threads may read a shared standard container at the same time.
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</p>
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<p>See chapters <a href="../17_intro/howto.html#3">17</a>,
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<a href="../23_containers/howto.html#3">23</a> and
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<a href="../27_io/howto.html#9">27</a> for more information.
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<p>See chapters <a href="../17_intro/howto.html#3">17</a> (library
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introduction), <a href="../23_containers/howto.html#3">23</a>
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(containers), and <a href="../27_io/howto.html#9">27</a> (I/O) for
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more information.
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</p>
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<hr>
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@ -548,15 +548,45 @@ http://clisp.cons.org/~haible/gccinclude-glibc-2.2-compat.diff
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5.6 Is libstdc++-v3 thread-safe?
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Quick answer: no, as of 3.0, most of the library is not safe for
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multithreaded access. The string class is MT-safe.
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When the system's libc is itself thread-safe, a non-generic
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implementation of atomicity.h exists for the architecture, and gcc
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itself reports a thread model other than single; libstdc++-v3 strives
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to be thread-safe. The user-code must guard against concurrent method
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calls which may access any particular library object's state.
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Typically, the application programmer may infer what object locks must
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be held based on the objects referenced in a method call. Without
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getting into great detail, here is an example which requires
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user-level locks:
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library_class_a shared_object_a;
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This is assuming that your idea of "multithreaded" is the same as
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ours... The general question of multithreading and libstdc++-v3 is
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addressed in the chapter-specific advice for [80]Library Introduction.
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Threadsafe containers are covered in more detail in [81]the Received
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Wisdom section on containers. Threading and I/O are discussed in
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[82]the I/O chapter.
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thread_main () {
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library_class_b *object_b = new library_class_b;
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shared_object_a.add_b (object_b); // must hold lock for shared_object_
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a
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shared_object_a.mutate (); // must hold lock for shared_object_
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a
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}
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// Multiple copies of thread_main() are started in independent threads.
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Under the assumption that object_a and object_b are never exposed to
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another thread, here is an example that should not require any
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user-level locks:
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thread_main () {
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library_class_a object_a;
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library_class_b *object_b = new library_class_b;
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object_a.add_b (object_b);
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object_a.mutate ();
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}
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All library objects are safe to use in a multithreaded program as long
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as each thread carefully locks out access by any other thread while it
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uses any object visible to another thread. This requirement includes
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both read and write access to objects; do not assume that two threads
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may read a shared standard container at the same time.
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See chapters [80]17 (library introduction), [81]23 (containers), and
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[82]27 (I/O) for more information.
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_________________________________________________________________
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5.7 How do I get a copy of the ISO C++ Standard?
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@ -660,8 +690,8 @@ References
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78. http://www.sgi.com/Technology/STL/
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79. ../ext/howto.html
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80. ../17_intro/howto.html#3
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81. ../23_containers/howto.html
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82. ../27_io/howto.html
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81. ../23_containers/howto.html#3
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82. ../27_io/howto.html#9
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83. http://www.ansi.org/
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84. http://webstore.ansi.org/ansidocstore/product.asp?sku=ISO%2FIEC+14882%2D1998
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85. http://www.iso.ch/
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