path: root/README.threads

NOTE: This documentation describes the style of threading that was
available in Perl 5.005.  Perl 5.6.0 introduced the early beginnings of
interpreter-based threads support, also known as ithreads, and in Perl
5.8.0 the interpeter threads became available from perl level through
the threads and threads::shared modules (in Perl 5.6 ithreads are
available only internally and to XS extension builders, and used
by the Win32 port for emulating fork()).

If you really want the older support for threads described below,
it is enabled with:

    sh Configure -Dusethreads -Duse5005threads

Be warned that the old 5.005 implementation of threads is known
to be quite buggy, and unmaintained, which means that the bugs
are there to stay.  (We are not mean by not fixing the bugs:
the bugs are just really, really, really hard to fix.  Honest.)

The rest of this document only applies to the use5005threads style of
threads, and the comments on what works on which platform are highly
obsolete and preserved here for archaeology buffs only.  The
architecture specific hints files do all the necessary option
tweaking automatically during Configure, both for the 5.005 threads
and for the new interpreter threads.

---------------------------------------------------------------------------

Support for threading is still in the highly experimental stages.  There
are known race conditions that show up under high contention on SMP
machines.  Internal implementation is still subject to changes.
It is not recommended for production use at this time.

---------------------------------------------------------------------------

Building

If your system is in the following list you should be able to just:

    ./Configure -Dusethreads -Duse5005threads -des
    make

and ignore the rest of this "Building" section.  If not, continue
from the "Problems" section.

	* Linux 2.* (with the LinuxThreads library installed:
	  that's the linuxthreads and linuxthreads-devel RPMs
	  for RedHat)

	* Tru64 UNIX (formerly Digital UNIX formerly DEC OSF/1)
	  (see additional note below)

	* Solaris 2.* for recentish x (2.5 is OK)

	* IRIX 6.2 or newer. 6.2 will require a few OS patches.
	  IMPORTANT: Without patch 2401 (or its replacement),
	  a kernel bug in IRIX 6.2 will cause your machine to
	  panic and crash when running threaded perl.
	  IRIX 6.3 and up should be OK. See lower down for patch details.

	* AIX 4.1.5 or newer.

	* FreeBSD 2.2.8 or newer.

	* OpenBSD

	* NeXTstep, OpenStep

	* OS/2

	* DOS DJGPP

	* VM/ESA

---------------------------------------------------------------------------

Problems

If the simple way doesn't work or you are using another platform which
you believe supports POSIX.1c threads then read on.  Additional
information may be in a platform-specific "hints" file in the hints/
subdirectory.

On platforms that use Configure to build perl, omit the -d from your
./Configure arguments. For example, use:

    ./Configure -Dusethreads -Duse5005threads

When Configure prompts you for ccflags, insert any other arguments in
there that your compiler needs to use POSIX threads (-D_REENTRANT,
-pthreads, -threads, -pthread, -thread, are good guesses). When
Configure prompts you for linking flags, include any flags required
for threading (usually nothing special is required here).  Finally,
when Configure prompts you for libraries, include any necessary
libraries (e.g. -lpthread).  Pay attention to the order of libraries.
It is probably necessary to specify your threading library *before*
your standard C library, e.g.  it might be necessary to have -lpthread
-lc, instead of -lc -lpthread.  You may also need to use -lc_r instead
of -lc.

Once you have specified all your compiler flags, you can have Configure
accept all the defaults for the remainder of the session by typing  &-d
at any Configure prompt.

Some additional notes (some of these may be obsolete now, other items
may be handled automatically):

For Digital Unix 4.x:
    Add -pthread to ccflags
    Add -pthread to ldflags
    Add -lpthread -lc_r to lddlflags

    For some reason, the extra includes for pthreads make Digital UNIX
    complain fatally about the sbrk() declaration in perl's malloc.c
    so use the native malloc, e.g.  sh Configure -Uusemymalloc, or
    manually edit your config.sh as follows:
	Change usemymalloc to n
	Zap mallocobj and mallocsrc (foo='')
	Change d_mymalloc to undef

For Digital Unix 3.x (Formerly DEC OSF/1):
    Add -DOLD_PTHREADS_API to ccflags
    If compiling with the GNU cc compiler, remove -threads from ccflags

    (The following should be done automatically if you call Configure
      with the -Dusethreads option).
    Add -lpthread -lmach -lc_r to libs (in the order specified).

For IRIX:
    (This should all be done automatically by the hint file).
    Add -lpthread to libs
    For IRIX 6.2, you have to have the following patches installed:
	1404 Irix 6.2 Posix 1003.1b man pages
	1645 IRIX 6.2 & 6.3 POSIX header file updates
	2000 Irix 6.2 Posix 1003.1b support modules
	2254 Pthread library fixes
	2401 6.2 all platform kernel rollup
    IMPORTANT: Without patch 2401, a kernel bug in IRIX 6.2 will
    cause your machine to panic and crash when running threaded perl.
    IRIX 6.3 and up should be OK.

    For IRIX 6.3 and 6.4 the pthreads should work out of the box.
    Thanks to Hannu Napari <Hannu.Napari@hut.fi> for the IRIX
    pthreads patches information.

For AIX:
    (This should all be done automatically by the hint file).
    Change cc to xlc_r or cc_r.
    Add -DNEED_PTHREAD_INIT to ccflags and cppflags
    Add -lc_r to libswanted
    Change -lc in lddflags to be -lpthread -lc_r -lc

For Win32:
    See README.win32, and the notes at the beginning of win32/Makefile
    or win32/makefile.mk.

Now you can do a
    make

When you succeed in compiling and testing ("make test" after your
build) a threaded Perl in a platform previously unknown to support
threaded perl, please let perlbug@perl.com know about your victory.
Explain what you did in painful detail.

---------------------------------------------------------------------------

O/S specific bugs

Irix 6.2:  See the Irix warning above.

LinuxThreads 0.5 has a bug which can cause file descriptor 0 to be
closed after a fork() leading to many strange symptoms. Version 0.6
has this fixed but the following patch can be applied to 0.5 for now:

----------------------------- cut here -----------------------------
--- linuxthreads-0.5/pthread.c.ORI	Mon Oct  6 13:55:50 1997
+++ linuxthreads-0.5/pthread.c	Mon Oct  6 13:57:24 1997
@@ -312,8 +312,10 @@
   free(pthread_manager_thread_bos);
   pthread_manager_thread_bos = pthread_manager_thread_tos = NULL;
   /* Close the two ends of the pipe */
-  close(pthread_manager_request);
-  close(pthread_manager_reader);
+  if (pthread_manager_request >= 0) {
+    close(pthread_manager_request);
+    close(pthread_manager_reader);
+  }
   pthread_manager_request = pthread_manager_reader = -1;
   /* Update the pid of the main thread */
   self->p_pid = getpid();
----------------------------- cut here -----------------------------


Building the Thread extension

The Thread extension is now part of the main perl distribution tree.
If you did Configure -Dusethreads -Duse5005threads then it will have been
added to the list of extensions automatically.

You can try some of the tests with
    cd ext/Thread
    perl create.t
    perl join.t
    perl lock.t
    perl io.t
etc.
The io one leaves a thread reading from the keyboard on stdin so
as the ping messages appear you can type lines and see them echoed.

Try running the main perl test suite too. There are known
failures for some of the DBM/DB extensions (if their underlying
libraries were not compiled to be thread-aware).

---------------------------------------------------------------------------

Bugs

* FAKE_THREADS should produce a working perl but the Thread
extension won't build with it yet.  (FAKE_THREADS has not been
tested at all in recent times.)

* There may still be races where bugs show up under contention.

---------------------------------------------------------------------------

Debugging

Use the -DS command-line option to turn on debugging of the
multi-threading code. Under Linux, that also turns on a quick
hack I did to grab a bit of extra information from segfaults.
If you have a fancier gdb/threads setup than I do then you'll
have to delete the lines in perl.c which say
    #if defined(DEBUGGING) && defined(USE_5005THREADS) && defined(__linux__)
        DEBUG_S(signal(SIGSEGV, (void(*)(int))catch_sigsegv););
    #endif

---------------------------------------------------------------------------

Background

Some old globals (e.g. stack_sp, op) and some old per-interpreter
variables (e.g. tmps_stack, cxstack) move into struct thread.
All fields of struct thread which derived from original perl
variables have names of the form Tfoo. For example, stack_sp becomes
the field Tstack_sp of struct thread. For those fields which moved
from original perl, thread.h does
    #define foo (thr->Tfoo)
This means that all functions in perl which need to use one of these
fields need an (automatic) variable thr which points at the current
thread's struct thread. For pp_foo functions, it is passed around as
an argument, for other functions they do
    dTHR;
which declares and initialises thr from thread-specific data
via pthread_getspecific. If a function fails to compile with an
error about "no such variable thr", it probably just needs a dTHR
at the top.


Fake threads

For FAKE_THREADS, thr is a global variable and perl schedules threads
by altering thr in between appropriate ops. The next and prev fields
of struct thread keep all fake threads on a doubly linked list and
the next_run and prev_run fields keep all runnable threads on a
doubly linked list. Mutexes are stubs for FAKE_THREADS. Condition
variables are implemented as a list of waiting threads.


Mutexes and condition variables

The API is via macros MUTEX_{INIT,LOCK,UNLOCK,DESTROY} and
COND_{INIT,WAIT,SIGNAL,BROADCAST,DESTROY}.

A mutex is only required to be a simple, fast mutex (e.g. it does not
have to be recursive). It is only ever held across very short pieces
of code. Condition variables are only ever signalled/broadcast while
their associated mutex is held. (This constraint simplifies the
implementation of condition variables in certain porting situations.)
For POSIX threads, perl mutexes and condition variables correspond to
POSIX ones.  For FAKE_THREADS, mutexes are stubs and condition variables
are implemented as lists of waiting threads. For FAKE_THREADS, a thread
waits on a condition variable by removing itself from the runnable
list, calling SCHEDULE to change thr to the next appropriate
runnable thread and returning op (i.e. the new threads next op).
This means that fake threads can only block while in PP code.
A PP function which contains a COND_WAIT must be prepared to
handle such restarts and can use the field "private" of struct
thread to record its state. For fake threads, COND_SIGNAL and
COND_BROADCAST work by putting back all the threads on the
condition variables list into the run queue. Note that a mutex
must *not* be held while returning from a PP function.

Perl locks and condition variables are both implemented as a
condpair_t structure, containing a mutex, an "owner" condition
variable, an owner thread field and another condition variable).
The structure is attached by 'm' magic to any SV. pp_lock locks
such an object by waiting on the ownercond condition variable until
the owner field is zero and then setting the owner field to its own
thread pointer. The lock is semantically recursive so if the owner
field already matches the current thread then pp_lock returns
straight away. If the owner field has to be filled in then
unlock_condpair is queued as an end-of-block destructor and
that function zeroes out the owner field and signals the ownercond
condition variable, thus waking up any other thread that wants to
lock it. When used as a condition variable, the condpair is locked
(involving the above wait-for-ownership and setting the owner field)
and the spare condition variable field is used for waiting on.


Thread states


              $t->join
R_JOINABLE ---------------------> R_JOINED >----\
    |      \  pthread_join(t)         |  ^      |
    |       \                         |  | join | pthread_join
    |        \                        |  |      |
    |         \                       |  \------/
    |          \                      |
    |           \                     |
    |  $t->detach\ pthread_detach     |
    |            _\|                  |
ends|             R_DETACHED     ends | unlink
    |                       \         |
    |                   ends \ unlink |
    |                         \       |
    |                          \      |
    |                           \     |
    |                            \    |
    |                             \   |
    V    join          detach     _\| V
ZOMBIE ----------------------------> DEAD
       pthread_join   pthread_detach
       and unlink     and unlink



Malcolm Beattie
mbeattie@sable.ox.ac.uk
Last updated: 27 November 1997

Configure-related info updated 16 July 1998 by
Andy Dougherty <doughera@lafayette.edu>

Other minor updates 10 Feb 1999 by
Gurusamy Sarathy

More platforms added 26 Jul 1999 by
Jarkko Hietaniemi