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c++config (__USE_MALLOC): Do not define it.

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* include/bits/c++config (__USE_MALLOC): Do not define it.
	Document why not and give pointers to more information.

	* docs/html/23_containers/howto.html: Update documentation
	to reflect recent understanding of problem.
	* docs/html/17_intro/howto.html: Likewise.

From-SVN: r42732
This commit is contained in:
Loren J. Rittle 2001-05-31 02:45:04 +00:00 • committed by Loren J. Rittle
parent fe7cd37f13
commit 85836c0b63
4 changed files with 91 additions and 8 deletions
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9
libstdc++-v3/ChangeLog
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@ -1,3 +1,12 @@
2001-05-30 Loren J. Rittle <ljrittle@acm.org>
* include/bits/c++config (__USE_MALLOC): Do not define it.
Document why not and give pointers to more information.
* docs/html/23_containers/howto.html: Update documentation
to reflect recent understanding of problem.
* docs/html/17_intro/howto.html: Likewise.
2001-05-30 Phil Edwards <pme@sources.redhat.com>
* docs/doxygen/user.cfg.in: Minor addition.

54
libstdc++-v3/docs/html/17_intro/howto.html
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@ -8,7 +8,7 @@
<META NAME="GENERATOR" CONTENT="vi and eight fingers">
<TITLE>libstdc++-v3 HOWTO: Chapter 17</TITLE>
<LINK REL=StyleSheet HREF="../lib3styles.css">
<!-- $Id: howto.html,v 1.3 2001/05/30 08:30:02 ljrittle Exp $ -->
<!-- $Id: howto.html,v 1.4 2001/05/30 21:54:57 pme Exp $ -->
</HEAD>
<BODY>
@ -67,6 +67,48 @@
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.
<EM>Please see the many cautions given in HOWTOs on containers.</EM>
</P>
<P>
Here is another attempt at explaining the dangers of using the
STL with threading support without understanding some important
details. The STL implementation is currently configured to use
the high-speed caching memory allocator. If you absolutely
think you must change this on a global basis for your platform
to support multi-threading, then please consult all commentary
in include/bits/c++config and the HOWTOs on containers. Be
fully aware that you may change the external or internal ABI of
libstdc++-v3 when you provide -D__USE_MALLOC on the command line
or make a change to that configuration file. [Placeholder in
case other patches don't make it before the 3.0 release: That
memory allocator can appear buggy in multithreaded C++ programs
(and has been reported to leak memory), if STL is misconfigured
for your platform. You may need to provide -D_PTHREADS on the
command line in this case to ensure the memory allocator for
containers is really protected by a mutex. Also, be aware that
you just changed the ABI of libstdc++-v3 when you did that thus
your entire application and all libraries must be compiled with
compatible flags. The STL implementation doesn't currently
protect you from changing the mutex locking implementation to
one that doesn't really play together with the implementation
you may have compiled other application code with.]
</P>
<P>
If you don't like caches of objects being retained inside the
STL, then you might be tempted to define __USE_MALLOC either on
the command line or by rebuilding c++config.h. Please note,
once you define __USE_MALLOC, only the malloc allocator is
visible to application code (i.e. the typically higher-speed
allocator is not even available in this configuration). There
is a better way: It is possible to force the malloc-based
allocator on a per-case-basis for some application code even
when the above macro symbol is not defined. The author of this
comment believes that is a better way to tune an application for
high-speed using this implementation of the STL. Here is one
possible example displaying the forcing of the malloc-based
allocator over the typically higher-speed default allocator:
std::list <void*, std::malloc_alloc> my_malloc_based_list;
</P>
<P>A recent journal article has described &quot;atomic integer
operations,&quot; which would allow us to, well, perform updates
@ -82,7 +124,13 @@
&quot;Thread Next&quot; to move down the thread. This farm is in
latest-to-oldest order.
<UL>
<LI>
<LI><A HREF="http://gcc.gnu.org/ml/libstdc++/2001-05/msg00384.html">
inspired this most recent updating of issues with threading
and the SGI STL library. It also contains some example
POSIX-multithreaded STL code.</A>
<LI> <A HREF="http://gcc.gnu.org/ml/libstdc++/2001-05/msg00136.html">
an early analysis of why __USE_MALLOC should be disable for
the 3.0 release of libstdc++.</A>
</UL>
<BR>
Here are discussions that took place before the current snapshot;
@ -144,7 +192,7 @@
<P CLASS="fineprint"><EM>
Comments and suggestions are welcome, and may be sent to
<A HREF="mailto:libstdc++@gcc.gnu.org">the mailing list</A>.
<BR> $Id: howto.html,v 1.3 2001/05/30 08:30:02 ljrittle Exp $
<BR> $Id: howto.html,v 1.4 2001/05/30 21:54:57 pme Exp $
</EM></P>

25
libstdc++-v3/docs/html/23_containers/howto.html
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@ -8,7 +8,7 @@
<META NAME="GENERATOR" CONTENT="vi and eight fingers">
<TITLE>libstdc++-v3 HOWTO: Chapter 23</TITLE>
<LINK REL=StyleSheet HREF="../lib3styles.css">
<!-- $Id: howto.html,v 1.3 2001/05/30 08:30:01 ljrittle Exp $ -->
<!-- $Id: howto.html,v 1.4 2001/05/30 21:55:01 pme Exp $ -->
</HEAD>
<BODY>
@ -199,6 +199,27 @@
(&quot;For most clients,&quot;...), pointing
out that locking must nearly always be done outside the container,
by client code (that'd be you, not us *grin*).
<EM>However, please take caution when considering the discussion
about the user-level configuration of the mutex lock
implementation inside the STL container-memory allocator on that
page. For the sake of this discussion, libstdc++-v3 configures
the SGI STL implementation, not you. We attempt to configure
the mutex lock as is best for your platform. In particular,
past advice was for people using g++ to explicitly define
_PTHREADS on the command line to get a thread-safe STL. This
may or may not be required for your port. It may or may not be
a good idea for your port. Extremely big caution: if you
compile some of your application code against the STL with one
set of threading flags and macros and another portion of the
code with different flags and macros that influence the
selection of the mutex lock, you may well end up with multiple
locking mechanisms in use which don't impact each other in the
manner that they should. Everything might link and all code
might have been built with a perfectly reasonable thread model
but you may have two internal ABIs in play within the
application. This might produce races, memory leaks and fatal
crashes. In any multithreaded application using STL, this
is the first thing to study well before blaming the allocator.</EM>
</P>
<P>You didn't read it, did you? *sigh* I'm serious, go read the
SGI page. It's really good and doesn't take long, and makes most
@ -237,7 +258,7 @@
<P CLASS="fineprint"><EM>
Comments and suggestions are welcome, and may be sent to
<A HREF="mailto:libstdc++@gcc.gnu.org">the mailing list</A>.
<BR> $Id: howto.html,v 1.3 2001/05/30 08:30:01 ljrittle Exp $
<BR> $Id: howto.html,v 1.4 2001/05/30 21:55:01 pme Exp $
</EM></P>

11
libstdc++-v3/include/bits/c++config
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@ -101,9 +101,14 @@
# define __STL_UNWIND(action)
#endif
// This is the "underlying allocator" for STL. The alternatives are
// homegrown schemes involving a kind of mutex and free list; see stl_alloc.h.
#define __USE_MALLOC
// Default to the typically high-speed, pool-based allocator (as
// libstdc++-v2) instead of the malloc-based allocator (libstdc++-v3
// snapshots). See libstdc++-v3/docs/html/17_intro/howto.html for
// details on why you don't want to override this setting. Ensure
// that threads are properly configured on your platform before
// assigning blame to the STL container-memory allocator. After doing
// so, please report any possible issues to libstdc++@gcc.gnu.org .
// Do not blindly #define __USE_MALLOC here or on the command line.
// The remainder of the prewritten config is mostly automatic; all the
// user hooks are listed above.