+=head3 Threads
+
+On some platforms, perl can be compiled with support for threads. To
+enable this, run
+
+ sh Configure -Dusethreads
+
+Currently, you need to specify -Dusethreads on the Configure command
+line so that the hint files can make appropriate adjustments.
+
+The default is to compile without thread support.
+
+Perl has two different internal threads implementations. The current
+model (available internally since 5.6, and as a user-level module since
+5.8) is called interpreter-based implementation (ithreads), with one
+interpreter per thread, and explicit sharing of data. The 5.005
+version (5005threads) is considered obsolete, buggy, and unmaintained.
+
+By default, Configure selects ithreads if -Dusethreads is specified.
+
+However, if you insist, you can select the unsupported old 5005threads behavior
+
+ sh Configure -Dusethreads -Duse5005threads
+
+The 'threads' module is for use with the ithreads implementation. The
+'Thread' module offers an interface to either 5005threads or ithreads
+(whichever has been configured).
+
+When using threads, perl uses a dynamically-sized buffer for some of
+the thread-safe library calls, such as those in the getpw*() family.
+This buffer starts small, but it will keep growing until the result
+fits. To get a fixed upper limit, you should compile Perl with
+PERL_REENTRANT_MAXSIZE defined to be the number of bytes you want. One
+way to do this is to run Configure with
+C<-Accflags=-DPERL_REENTRANT_MAXSIZE=65536>
+
+=head3 Large file support.
+
+Since Perl 5.6.0, Perl has supported large files (files larger than
+2 gigabytes), and in many common platforms like Linux or Solaris this
+support is on by default.
+
+This is both good and bad. It is good in that you can use large files,
+seek(), stat(), and -s them. It is bad in that if you are interfacing Perl
+using some extension, the components you are connecting to must also
+be large file aware: if Perl thinks files can be large but the other
+parts of the software puzzle do not understand the concept, bad things
+will happen. One popular extension suffering from this ailment is the
+Apache extension mod_perl.
+
+There's also one known limitation with the current large files
+implementation: unless you also have 64-bit integers (see the next
+section), you cannot use the printf/sprintf non-decimal integer formats
+like C<%x> to print filesizes. You can use C<%d>, though.
+
+=head3 64 bit support.
+
+If your platform does not have run natively at 64 bits, but can
+simulate them with compiler flags and/or C<long long> or C<int64_t>,
+you can build a perl that uses 64 bits.
+
+There are actually two modes of 64-bitness: the first one is achieved
+using Configure -Duse64bitint and the second one using Configure
+-Duse64bitall. The difference is that the first one is minimal and
+the second one maximal. The first works in more places than the second.
+
+The C<use64bitint> option does only as much as is required to get
+64-bit integers into Perl (this may mean, for example, using "long
+longs") while your memory may still be limited to 2 gigabytes (because
+your pointers could still be 32-bit). Note that the name C<64bitint>
+does not imply that your C compiler will be using 64-bit C<int>s (it
+might, but it doesn't have to). The C<use64bitint> simply means that
+you will be able to have 64 bit-wide scalar values.
+
+The C<use64bitall> option goes all the way by attempting to switch
+integers (if it can), longs (and pointers) to being 64-bit. This may
+create an even more binary incompatible Perl than -Duse64bitint: the
+resulting executable may not run at all in a 32-bit box, or you may
+have to reboot/reconfigure/rebuild your operating system to be 64-bit
+aware.
+
+Natively 64-bit systems like Alpha and Cray need neither -Duse64bitint
+nor -Duse64bitall.
+
+ NOTE: 64-bit support is still experimental on most platforms.
+ Existing support only covers the LP64 data model. In particular, the
+ LLP64 data model is not yet supported. 64-bit libraries and system
+ APIs on many platforms have not stabilized--your mileage may vary.
+
+=head3 Long doubles
+
+In some systems you may be able to use long doubles to enhance the
+range and precision of your double precision floating point numbers
+(that is, Perl's numbers). Use Configure -Duselongdouble to enable
+this support (if it is available).
+
+=head3 "more bits"
+
+You can "Configure -Dusemorebits" to turn on both the 64-bit support
+and the long double support.
+
+=head3 Selecting File IO mechanisms
+
+Executive summary: as of Perl 5.8, you should use the default "PerlIO"
+as the IO mechanism unless you have a good reason not to.
+
+In more detail: previous versions of perl used the standard IO
+mechanisms as defined in stdio.h. Versions 5.003_02 and later of perl
+introduced alternate IO mechanisms via a "PerlIO" abstraction, but up
+until and including Perl 5.6, the stdio mechanism was still the default
+and the only supported mechanism.
+
+Starting from Perl 5.8, the default mechanism is to use the PerlIO
+abstraction, because it allows better control of I/O mechanisms,
+instead of having to work with (often, work around) vendors' I/O
+implementations.
+
+This PerlIO abstraction can be (but again, unless you know what you
+are doing, should not be) disabled either on the Configure command
+line with
+
+ sh Configure -Uuseperlio
+
+or interactively at the appropriate Configure prompt.
+
+With the PerlIO abstraction layer, there is another possibility for
+the underlying IO calls, AT&T's "sfio". This has superior performance
+to stdio.h in many cases, and is extensible by the use of "discipline"
+modules ("Native" PerlIO has them too). Sfio currently only builds on
+a subset of the UNIX platforms perl supports. Because the data
+structures are completely different from stdio, perl extension modules
+or external libraries may not work. This configuration exists to
+allow these issues to be worked on.
+
+This option requires the 'sfio' package to have been built and installed.
+The latest sfio is available from http://www.research.att.com/sw/tools/sfio/
+
+You select this option by
+
+ sh Configure -Duseperlio -Dusesfio
+
+If you have already selected -Duseperlio, and if Configure detects
+that you have sfio, then sfio will be the default suggested by
+Configure.
+
+Note: On some systems, sfio's iffe configuration script fails to
+detect that you have an atexit function (or equivalent). Apparently,
+this is a problem at least for some versions of Linux and SunOS 4.
+Configure should detect this problem and warn you about problems with
+_exit vs. exit. If you have this problem, the fix is to go back to
+your sfio sources and correct iffe's guess about atexit.
+
+=head3 Algorithmic Complexity Attacks on Hashes
+
+In Perls 5.8.0 and earlier it was easy to create degenerate hashes.
+Processing such hashes would consume large amounts of CPU time,
+enabling a "Denial of Service" attack against Perl. Such hashes may be
+a problem for example for mod_perl sites, sites with Perl CGI scripts
+and web services, that process data originating from external sources.
+
+In Perl 5.8.1 a security feature was introduced to make it harder to
+create such degenerate hashes. A visible side effect of this was that
+the keys(), values(), and each() functions may return the hash elements
+in different order between different runs of Perl even with the same
+data. It also had unintended binary incompatibility issues with
+certain modules compiled against Perl 5.8.0.
+
+In Perl 5.8.2 an improved scheme was introduced. Hashes will return
+elements in the same order as Perl 5.8.0 by default. On a hash by hash
+basis, if pathological data is detected during a hash key insertion,
+then that hash will switch to an alternative random hash seed. As
+adding keys can always dramatically change returned hash element order,
+existing programs will not be affected by this, unless they
+specifically test for pre-recorded hash return order for contrived
+data. (eg the list of keys generated by C<map {"\0"x$_} 0..15> trigger
+randomisation) In effect the new implementation means that 5.8.1 scheme
+is only being used on hashes which are under attack.
+
+One can still revert to the old guaranteed repeatable order (and be
+vulnerable to attack by wily crackers) by setting the environment
+variable PERL_HASH_SEED, see L<perlrun/PERL_HASH_SEED>. Another option
+is to add -DUSE_HASH_SEED_EXPLICIT to the compilation flags (for
+example by using C<Configure -Accflags=-DUSE_HASH_SEED_EXPLICIT>), in
+which case one has to explicitly set the PERL_HASH_SEED environment
+variable to enable the security feature, or by adding -DNO_HASH_SEED to
+the compilation flags to completely disable the randomisation feature.
+
+B<Perl has never guaranteed any ordering of the hash keys>, and the
+ordering has already changed several times during the lifetime of Perl
+5. Also, the ordering of hash keys has always been, and continues to
+be, affected by the insertion order. It is likely that Perl 5.10 and
+Perl 6 will randomise all hashes. Note that because of this
+randomisation for example the Data::Dumper results will be different
+between different runs of Perl since Data::Dumper by default dumps
+hashes "unordered". The use of the Data::Dumper C<Sortkeys> option is
+recommended.
+
+=head3 SOCKS
+
+Perl can be configured to be 'socksified', that is, to use the SOCKS
+TCP/IP proxy protocol library. SOCKS is used to give applications
+access to transport layer network proxies. Perl supports only SOCKS
+Version 5. You can find more about SOCKS from http://www.socks.nec.com/
+
+=head3 Dynamic Loading
+
+By default, Configure will compile perl to use dynamic loading if
+your system supports it. If you want to force perl to be compiled
+statically, you can either choose this when Configure prompts you or
+you can use the Configure command line option -Uusedl.
+
+=head3 Building a shared Perl library
+
+Currently, for most systems, the main perl executable is built by
+linking the "perl library" libperl.a with perlmain.o, your static
+extensions (usually just DynaLoader.a) and various extra libraries,
+such as -lm.
+
+On some systems that support dynamic loading, it may be possible to
+replace libperl.a with a shared libperl.so. If you anticipate building
+several different perl binaries (e.g. by embedding libperl into
+different programs, or by using the optional compiler extension), then
+you might wish to build a shared libperl.so so that all your binaries
+can share the same library.
+
+The disadvantages are that there may be a significant performance
+penalty associated with the shared libperl.so, and that the overall
+mechanism is still rather fragile with respect to different versions
+and upgrades.
+
+In terms of performance, on my test system (Solaris 2.5_x86) the perl
+test suite took roughly 15% longer to run with the shared libperl.so.
+Your system and typical applications may well give quite different
+results.
+
+The default name for the shared library is typically something like
+libperl.so.6.2 (for Perl 5.6.2), or libperl.so.602, or simply
+libperl.so. Configure tries to guess a sensible naming convention
+based on your C library name. Since the library gets installed in a
+version-specific architecture-dependent directory, the exact name
+isn't very important anyway, as long as your linker is happy.
+
+For some systems (mostly SVR4), building a shared libperl is required
+for dynamic loading to work, and hence is already the default.
+
+You can elect to build a shared libperl by
+
+ sh Configure -Duseshrplib
+
+To build a shared libperl, the environment variable controlling shared
+library search (LD_LIBRARY_PATH in most systems, DYLD_LIBRARY_PATH for
+NeXTSTEP/OPENSTEP/Darwin, LIBRARY_PATH for BeOS, LD_LIBRARY_PATH/SHLIB_PATH
+for HP-UX, LIBPATH for AIX, PATH for Cygwin) must be set up to include
+the Perl build directory because that's where the shared libperl will
+be created. Configure arranges makefile to have the correct shared
+library search settings. You can find the name of the environment
+variable Perl thinks works in your your system by
+
+ grep ldlibpthname config.sh
+
+However, there are some special cases where manually setting the
+shared library path might be required. For example, if you want to run
+something like the following with the newly-built but not-yet-installed
+./perl:
+
+ cd t; ./perl misc/failing_test.t
+or
+ ./perl -Ilib ~/my_mission_critical_test
+
+then you need to set up the shared library path explicitly.
+You can do this with
+
+ LD_LIBRARY_PATH=`pwd`:$LD_LIBRARY_PATH; export LD_LIBRARY_PATH
+
+for Bourne-style shells, or
+
+ setenv LD_LIBRARY_PATH `pwd`
+
+for Csh-style shells. (This procedure may also be needed if for some
+unexpected reason Configure fails to set up makefile correctly.) (And
+again, it may be something other than LD_LIBRARY_PATH for you, see above.)
+
+You can often recognize failures to build/use a shared libperl from error
+messages complaining about a missing libperl.so (or libperl.sl in HP-UX),
+for example:
+18126:./miniperl: /sbin/loader: Fatal Error: cannot map libperl.so
+
+There is also an potential problem with the shared perl library if you
+want to have more than one "flavor" of the same version of perl (e.g.
+with and without -DDEBUGGING). For example, suppose you build and
+install a standard Perl 5.8.0 with a shared library. Then, suppose you
+try to build Perl 5.8.0 with -DDEBUGGING enabled, but everything else
+the same, including all the installation directories. How can you
+ensure that your newly built perl will link with your newly built
+libperl.so.8 rather with the installed libperl.so.8? The answer is
+that you might not be able to. The installation directory is encoded
+in the perl binary with the LD_RUN_PATH environment variable (or
+equivalent ld command-line option). On Solaris, you can override that
+with LD_LIBRARY_PATH; on Linux, you can only override at runtime via
+LD_PRELOAD, specifying the exact filename you wish to be used; and on
+Digital Unix, you can override LD_LIBRARY_PATH by setting the
+_RLD_ROOT environment variable to point to the perl build directory.
+
+In other words, it is generally not a good idea to try to build a perl
+with a shared library if $archlib/CORE/$libperl already exists from a
+previous build.
+
+A good workaround is to specify a different directory for the
+architecture-dependent library for your -DDEBUGGING version of perl.
+You can do this by changing all the *archlib* variables in config.sh to
+point to your new architecture-dependent library.
+
+=head3 Environment access
+
+Perl often needs to write to the program's environment, such as when C<%ENV>
+is assigned to. Many implementations of the C library function C<putenv()>
+leak memory, so where possible perl will manipulate the environment directly
+to avoid these leaks. The default is now to perform direct manipulation
+whenever perl is running as a stand alone interpreter, and to call the safe
+but potentially leaky C<putenv()> function when the perl interpreter is
+embedded in another application. You can force perl to always use C<putenv()>
+by compiling with -DPERL_USE_SAVE_PUTENV. You can force an embedded perl to
+use direct manipulation by setting C<PL_use_safe_putenv = 0;> after the
+C<perl_construct()> call.