diff options
Diffstat (limited to 'gnu')
| -rw-r--r-- | gnu/usr.bin/perl/ext/DB_File/DB_File.pm | 2 | ||||
| -rw-r--r-- | gnu/usr.bin/perl/ext/Encode/lib/Encode/Supported.pod | 2 | ||||
| -rw-r--r-- | gnu/usr.bin/perl/ext/IO/lib/IO/Socket.pm | 2 | ||||
| -rw-r--r-- | gnu/usr.bin/perl/pod/perldelta.pod | 6 | ||||
| -rw-r--r-- | gnu/usr.bin/perl/pod/perlintern.pod | 276 | ||||
| -rw-r--r-- | gnu/usr.bin/perl/pod/perlthrtut.pod | 1094 |
6 files changed, 847 insertions, 535 deletions
diff --git a/gnu/usr.bin/perl/ext/DB_File/DB_File.pm b/gnu/usr.bin/perl/ext/DB_File/DB_File.pm index df189eb1cda..d072da2dad8 100644 --- a/gnu/usr.bin/perl/ext/DB_File/DB_File.pm +++ b/gnu/usr.bin/perl/ext/DB_File/DB_File.pm @@ -1436,7 +1436,7 @@ Returns the number of elements in the array. =item B<$X-E<gt>splice(offset, length, elements);> -Returns a splice of the the array. +Returns a splice of the array. =back diff --git a/gnu/usr.bin/perl/ext/Encode/lib/Encode/Supported.pod b/gnu/usr.bin/perl/ext/Encode/lib/Encode/Supported.pod index ec5da49332c..e76f7fb6f78 100644 --- a/gnu/usr.bin/perl/ext/Encode/lib/Encode/Supported.pod +++ b/gnu/usr.bin/perl/ext/Encode/lib/Encode/Supported.pod @@ -182,7 +182,7 @@ This is also covered in Encode::Byte even though it is not an Note that Vietnamese is listed above. Also read "Encoding vs Charset" below. Also note that these are implemented in distinct modules by -countries, due the the size concerns (simplified Chinese is mapped +countries, due the size concerns (simplified Chinese is mapped to 'CN', continental China, while traditional Chinese is mapped to 'TW', Taiwan). Please refer to their respective documentataion pages. diff --git a/gnu/usr.bin/perl/ext/IO/lib/IO/Socket.pm b/gnu/usr.bin/perl/ext/IO/lib/IO/Socket.pm index f047eb152a4..7f434d94877 100644 --- a/gnu/usr.bin/perl/ext/IO/lib/IO/Socket.pm +++ b/gnu/usr.bin/perl/ext/IO/lib/IO/Socket.pm @@ -421,7 +421,7 @@ C<use> declaration will fail at compile time. =item connected -If the socket is in a connected state the the peer address is returned. +If the socket is in a connected state the peer address is returned. If the socket is not in a connected state then undef will be returned. =item protocol diff --git a/gnu/usr.bin/perl/pod/perldelta.pod b/gnu/usr.bin/perl/pod/perldelta.pod index f66c126528f..e3e220228e7 100644 --- a/gnu/usr.bin/perl/pod/perldelta.pod +++ b/gnu/usr.bin/perl/pod/perldelta.pod @@ -416,7 +416,7 @@ for more information about UTF-8. =item * If your environment variables (LC_ALL, LC_CTYPE, LANG, LANGUAGE) look -like you want to use UTF-8 (any of the the variables match C</utf-?8/i>), +like you want to use UTF-8 (any of the variables match C</utf-?8/i>), your STDIN, STDOUT, STDERR handles and the default open layer (see L<open>) are marked as UTF-8. (This feature, like other new features that combine Unicode and I/O, work only if you are using @@ -3361,7 +3361,7 @@ use the bundled C compiler.) Perl 5.8.0 doesn't build in AmigaOS. It broke at some point during the ithreads work and we could not find Amiga experts to unbreak the -problems. Perl 5.6.1 still works for AmigaOS (as does the the 5.7.2 +problems. Perl 5.6.1 still works for AmigaOS (as does the 5.7.2 development release). =head2 BeOS @@ -3690,7 +3690,7 @@ from the CPAN. Perl 5.8 unfortunately does not build anymore on AmigaOS; this broke accidentally at some point. Since there are not that many Amiga developers available, we could not get this fixed and tested in time -for 5.8.0. Perl 5.6.1 still works for AmigaOS (as does the the 5.7.2 +for 5.8.0. Perl 5.6.1 still works for AmigaOS (as does the 5.7.2 development release). The C<PerlIO::Scalar> and C<PerlIO::Via> (capitalised) were renamed as diff --git a/gnu/usr.bin/perl/pod/perlintern.pod b/gnu/usr.bin/perl/pod/perlintern.pod index 58eeac6e954..b5ef36d5a22 100644 --- a/gnu/usr.bin/perl/pod/perlintern.pod +++ b/gnu/usr.bin/perl/pod/perlintern.pod @@ -1,23 +1,287 @@ =head1 NAME -perlintern - autogenerated documentation of purely B<internal> +perlintern - autogenerated documentation of purely B<internal> Perl functions =head1 DESCRIPTION -This file is the autogenerated documentation of functions in the -Perl intrepreter that are documented using Perl's internal documentation -format but are not marked as part of the Perl API. In other words, +This file is the autogenerated documentation of functions in the +Perl interpreter that are documented using Perl's internal documentation +format but are not marked as part of the Perl API. In other words, B<they are not for use in extensions>! + +=head1 Global Variables + =over 8 +=item PL_DBsingle + +When Perl is run in debugging mode, with the B<-d> switch, this SV is a +boolean which indicates whether subs are being single-stepped. +Single-stepping is automatically turned on after every step. This is the C +variable which corresponds to Perl's $DB::single variable. See +C<PL_DBsub>. + + SV * PL_DBsingle + +=for hackers +Found in file intrpvar.h + +=item PL_DBsub + +When Perl is run in debugging mode, with the B<-d> switch, this GV contains +the SV which holds the name of the sub being debugged. This is the C +variable which corresponds to Perl's $DB::sub variable. See +C<PL_DBsingle>. + + GV * PL_DBsub + +=for hackers +Found in file intrpvar.h + +=item PL_DBtrace + +Trace variable used when Perl is run in debugging mode, with the B<-d> +switch. This is the C variable which corresponds to Perl's $DB::trace +variable. See C<PL_DBsingle>. + + SV * PL_DBtrace + +=for hackers +Found in file intrpvar.h + +=item PL_dowarn + +The C variable which corresponds to Perl's $^W warning variable. + + bool PL_dowarn + +=for hackers +Found in file intrpvar.h + +=item PL_last_in_gv + +The GV which was last used for a filehandle input operation. (C<< <FH> >>) + + GV* PL_last_in_gv + +=for hackers +Found in file thrdvar.h + +=item PL_ofs_sv + +The output field separator - C<$,> in Perl space. + + SV* PL_ofs_sv + +=for hackers +Found in file thrdvar.h + +=item PL_rs + +The input record separator - C<$/> in Perl space. + + SV* PL_rs + +=for hackers +Found in file thrdvar.h + + +=back + +=head1 GV Functions + +=over 8 + +=item is_gv_magical + +Returns C<TRUE> if given the name of a magical GV. + +Currently only useful internally when determining if a GV should be +created even in rvalue contexts. + +C<flags> is not used at present but available for future extension to +allow selecting particular classes of magical variable. + + bool is_gv_magical(char *name, STRLEN len, U32 flags) + +=for hackers +Found in file gv.c + + +=back + +=head1 IO Functions + +=over 8 + +=item start_glob + +Function called by C<do_readline> to spawn a glob (or do the glob inside +perl on VMS). This code used to be inline, but now perl uses C<File::Glob> +this glob starter is only used by miniperl during the build process. +Moving it away shrinks pp_hot.c; shrinking pp_hot.c helps speed perl up. + + PerlIO* start_glob(SV* pattern, IO *io) + +=for hackers +Found in file doio.c + + +=back + +=head1 Pad Data Structures + +=over 8 + +=item CvPADLIST + +CV's can have CvPADLIST(cv) set to point to an AV. + +For these purposes "forms" are a kind-of CV, eval""s are too (except they're +not callable at will and are always thrown away after the eval"" is done +executing). + +XSUBs don't have CvPADLIST set - dXSTARG fetches values from PL_curpad, +but that is really the callers pad (a slot of which is allocated by +every entersub). + +The CvPADLIST AV has does not have AvREAL set, so REFCNT of component items +is managed "manual" (mostly in op.c) rather than normal av.c rules. +The items in the AV are not SVs as for a normal AV, but other AVs: + +0'th Entry of the CvPADLIST is an AV which represents the "names" or rather +the "static type information" for lexicals. + +The CvDEPTH'th entry of CvPADLIST AV is an AV which is the stack frame at that +depth of recursion into the CV. +The 0'th slot of a frame AV is an AV which is @_. +other entries are storage for variables and op targets. + +During compilation: +C<PL_comppad_name> is set to the names AV. +C<PL_comppad> is set to the frame AV for the frame CvDEPTH == 1. +C<PL_curpad> is set to body of the frame AV (i.e. AvARRAY(PL_comppad)). + +Itterating over the names AV itterates over all possible pad +items. Pad slots that are SVs_PADTMP (targets/GVs/constants) end up having +&PL_sv_undef "names" (see pad_alloc()). + +Only my/our variable (SVs_PADMY/SVs_PADOUR) slots get valid names. +The rest are op targets/GVs/constants which are statically allocated +or resolved at compile time. These don't have names by which they +can be looked up from Perl code at run time through eval"" like +my/our variables can be. Since they can't be looked up by "name" +but only by their index allocated at compile time (which is usually +in PL_op->op_targ), wasting a name SV for them doesn't make sense. + +The SVs in the names AV have their PV being the name of the variable. +NV+1..IV inclusive is a range of cop_seq numbers for which the name is valid. +For typed lexicals name SV is SVt_PVMG and SvSTASH points at the type. + +If SvFAKE is set on the name SV then slot in the frame AVs are +a REFCNT'ed references to a lexical from "outside". + +If the 'name' is '&' the corresponding entry in frame AV +is a CV representing a possible closure. +(SvFAKE and name of '&' is not a meaningful combination currently but could +become so if C<my sub foo {}> is implemented.) + + AV * CvPADLIST(CV *cv) + +=for hackers +Found in file cv.h + + +=back + +=head1 Stack Manipulation Macros + +=over 8 + +=item djSP + +Declare Just C<SP>. This is actually identical to C<dSP>, and declares +a local copy of perl's stack pointer, available via the C<SP> macro. +See C<SP>. (Available for backward source code compatibility with the +old (Perl 5.005) thread model.) + + djSP; + +=for hackers +Found in file pp.h + +=item LVRET + +True if this op will be the return value of an lvalue subroutine + +=for hackers +Found in file pp.h + + +=back + +=head1 SV Manipulation Functions + +=over 8 + +=item report_uninit + +Print appropriate "Use of uninitialized variable" warning + + void report_uninit() + +=for hackers +Found in file sv.c + +=item sv_add_arena + +Given a chunk of memory, link it to the head of the list of arenas, +and split it into a list of free SVs. + + void sv_add_arena(char* ptr, U32 size, U32 flags) + +=for hackers +Found in file sv.c + +=item sv_clean_all + +Decrement the refcnt of each remaining SV, possibly triggering a +cleanup. This function may have to be called multiple times to free +SVs which are in complex self-referential hierarchies. + + I32 sv_clean_all() + +=for hackers +Found in file sv.c + +=item sv_clean_objs + +Attempt to destroy all objects not yet freed + + void sv_clean_objs() + +=for hackers +Found in file sv.c + +=item sv_free_arenas + +Deallocate the memory used by all arenas. Note that all the individual SV +heads and bodies within the arenas must already have been freed. + + void sv_free_arenas() + +=for hackers +Found in file sv.c + + =back =head1 AUTHORS -The autodocumentation system was orignally added to the Perl core by -Benjamin Stuhl. Documentation is by whoever was kind enough to +The autodocumentation system was originally added to the Perl core by +Benjamin Stuhl. Documentation is by whoever was kind enough to document their functions. =head1 SEE ALSO diff --git a/gnu/usr.bin/perl/pod/perlthrtut.pod b/gnu/usr.bin/perl/pod/perlthrtut.pod index f2ca3bda644..c4af43a6fa7 100644 --- a/gnu/usr.bin/perl/pod/perlthrtut.pod +++ b/gnu/usr.bin/perl/pod/perlthrtut.pod @@ -4,9 +4,33 @@ perlthrtut - tutorial on threads in Perl =head1 DESCRIPTION -One of the most prominent new features of Perl 5.005 is the inclusion -of threads. Threads make a number of things a lot easier, and are a -very useful addition to your bag of programming tricks. +B<NOTE>: this tutorial describes the new Perl threading flavour +introduced in Perl 5.6.0 called interpreter threads, or B<ithreads> +for short. In this model each thread runs in its own Perl interpreter, +and any data sharing between threads must be explicit. + +There is another older Perl threading flavour called the 5.005 model, +unsurprisingly for 5.005 versions of Perl. The old model is known to +have problems, deprecated, and will probably be removed around release +5.10. You are strongly encouraged to migrate any existing 5.005 +threads code to the new model as soon as possible. + +You can see which (or neither) threading flavour you have by +running C<perl -V> and looking at the C<Platform> section. +If you have C<useithreads=define> you have ithreads, if you +have C<use5005threads=define> you have 5.005 threads. +If you have neither, you don't have any thread support built in. +If you have both, you are in trouble. + +The user-level interface to the 5.005 threads was via the L<Threads> +class, while ithreads uses the L<threads> class. Note the change in case. + +=head1 Status + +The ithreads code has been available since Perl 5.6.0, and is considered +stable. The user-level interface to ithreads (the L<threads> classes) +appeared in the 5.8.0 release, and as of this time is considered stable +although it should be treated with caution as with all new features. =head1 What Is A Thread Anyway? @@ -14,44 +38,44 @@ A thread is a flow of control through a program with a single execution point. Sounds an awful lot like a process, doesn't it? Well, it should. -Threads are one of the pieces of a process. Every process has at least +Threads are one of the pieces of a process. Every process has at least one thread and, up until now, every process running Perl had only one -thread. With 5.005, though, you can create extra threads. We're going +thread. With 5.8, though, you can create extra threads. We're going to show you how, when, and why. =head1 Threaded Program Models There are three basic ways that you can structure a threaded -program. Which model you choose depends on what you need your program -to do. For many non-trivial threaded programs you'll need to choose +program. Which model you choose depends on what you need your program +to do. For many non-trivial threaded programs you'll need to choose different models for different pieces of your program. =head2 Boss/Worker The boss/worker model usually has one `boss' thread and one or more -`worker' threads. The boss thread gathers or generates tasks that need +`worker' threads. The boss thread gathers or generates tasks that need to be done, then parcels those tasks out to the appropriate worker thread. This model is common in GUI and server programs, where a main thread waits for some event and then passes that event to the appropriate -worker threads for processing. Once the event has been passed on, the +worker threads for processing. Once the event has been passed on, the boss thread goes back to waiting for another event. -The boss thread does relatively little work. While tasks aren't +The boss thread does relatively little work. While tasks aren't necessarily performed faster than with any other method, it tends to have the best user-response times. =head2 Work Crew In the work crew model, several threads are created that do -essentially the same thing to different pieces of data. It closely +essentially the same thing to different pieces of data. It closely mirrors classical parallel processing and vector processors, where a large array of processors do the exact same thing to many pieces of data. This model is particularly useful if the system running the program -will distribute multiple threads across different processors. It can +will distribute multiple threads across different processors. It can also be useful in ray tracing or rendering engines, where the individual threads can pass on interim results to give the user visual feedback. @@ -60,75 +84,75 @@ feedback. The pipeline model divides up a task into a series of steps, and passes the results of one step on to the thread processing the -next. Each thread does one thing to each piece of data and passes the +next. Each thread does one thing to each piece of data and passes the results to the next thread in line. This model makes the most sense if you have multiple processors so two or more threads will be executing in parallel, though it can often -make sense in other contexts as well. It tends to keep the individual +make sense in other contexts as well. It tends to keep the individual tasks small and simple, as well as allowing some parts of the pipeline to block (on I/O or system calls, for example) while other parts keep -going. If you're running different parts of the pipeline on different +going. If you're running different parts of the pipeline on different processors you may also take advantage of the caches on each processor. This model is also handy for a form of recursive programming where, rather than having a subroutine call itself, it instead creates -another thread. Prime and Fibonacci generators both map well to this +another thread. Prime and Fibonacci generators both map well to this form of the pipeline model. (A version of a prime number generator is presented later on.) =head1 Native threads -There are several different ways to implement threads on a system. How +There are several different ways to implement threads on a system. How threads are implemented depends both on the vendor and, in some cases, -the version of the operating system. Often the first implementation +the version of the operating system. Often the first implementation will be relatively simple, but later versions of the OS will be more sophisticated. While the information in this section is useful, it's not necessary, so you can skip it if you don't feel up to it. -There are three basic categories of threads-user-mode threads, kernel +There are three basic categories of threads: user-mode threads, kernel threads, and multiprocessor kernel threads. User-mode threads are threads that live entirely within a program and -its libraries. In this model, the OS knows nothing about threads. As +its libraries. In this model, the OS knows nothing about threads. As far as it's concerned, your process is just a process. This is the easiest way to implement threads, and the way most OSes -start. The big disadvantage is that, since the OS knows nothing about -threads, if one thread blocks they all do. Typical blocking activities +start. The big disadvantage is that, since the OS knows nothing about +threads, if one thread blocks they all do. Typical blocking activities include most system calls, most I/O, and things like sleep(). -Kernel threads are the next step in thread evolution. The OS knows +Kernel threads are the next step in thread evolution. The OS knows about kernel threads, and makes allowances for them. The main difference between a kernel thread and a user-mode thread is -blocking. With kernel threads, things that block a single thread don't -block other threads. This is not the case with user-mode threads, +blocking. With kernel threads, things that block a single thread don't +block other threads. This is not the case with user-mode threads, where the kernel blocks at the process level and not the thread level. This is a big step forward, and can give a threaded program quite a performance boost over non-threaded programs. Threads that block performing I/O, for example, won't block threads that are doing other -things. Each process still has only one thread running at once, +things. Each process still has only one thread running at once, though, regardless of how many CPUs a system might have. Since kernel threading can interrupt a thread at any time, they will uncover some of the implicit locking assumptions you may make in your -program. For example, something as simple as C<$a = $a + 2> can behave -unpredictably with kernel threads if C<$a> is visible to other -threads, as another thread may have changed C<$a> between the time it +program. For example, something as simple as C<$a = $a + 2> can behave +unpredictably with kernel threads if $a is visible to other +threads, as another thread may have changed $a between the time it was fetched on the right hand side and the time the new value is stored. -Multiprocessor Kernel Threads are the final step in thread -support. With multiprocessor kernel threads on a machine with multiple +Multiprocessor kernel threads are the final step in thread +support. With multiprocessor kernel threads on a machine with multiple CPUs, the OS may schedule two or more threads to run simultaneously on different CPUs. This can give a serious performance boost to your threaded program, -since more than one thread will be executing at the same time. As a +since more than one thread will be executing at the same time. As a tradeoff, though, any of those nagging synchronization issues that might not have shown with basic kernel threads will appear with a vengeance. @@ -138,14 +162,14 @@ different OSes (and different thread implementations for a particular OS) allocate CPU cycles to threads in different ways. Cooperative multitasking systems have running threads give up control -if one of two things happen. If a thread calls a yield function, it -gives up control. It also gives up control if the thread does -something that would cause it to block, such as perform I/O. In a +if one of two things happen. If a thread calls a yield function, it +gives up control. It also gives up control if the thread does +something that would cause it to block, such as perform I/O. In a cooperative multitasking implementation, one thread can starve all the others for CPU time if it so chooses. Preemptive multitasking systems interrupt threads at regular intervals -while the system decides which thread should run next. In a preemptive +while the system decides which thread should run next. In a preemptive multitasking system, one thread usually won't monopolize the CPU. On some systems, there can be cooperative and preemptive threads @@ -153,123 +177,145 @@ running simultaneously. (Threads running with realtime priorities often behave cooperatively, for example, while threads running at normal priorities behave preemptively.) -=head1 What kind of threads are perl threads? +=head1 What kind of threads are Perl threads? If you have experience with other thread implementations, you might -find that things aren't quite what you expect. It's very important to +find that things aren't quite what you expect. It's very important to remember when dealing with Perl threads that Perl Threads Are Not X Threads, for all values of X. They aren't POSIX threads, or -DecThreads, or Java's Green threads, or Win32 threads. There are +DecThreads, or Java's Green threads, or Win32 threads. There are similarities, and the broad concepts are the same, but if you start looking for implementation details you're going to be either -disappointed or confused. Possibly both. +disappointed or confused. Possibly both. This is not to say that Perl threads are completely different from -everything that's ever come before--they're not. Perl's threading -model owes a lot to other thread models, especially POSIX. Just as -Perl is not C, though, Perl threads are not POSIX threads. So if you +everything that's ever come before--they're not. Perl's threading +model owes a lot to other thread models, especially POSIX. Just as +Perl is not C, though, Perl threads are not POSIX threads. So if you find yourself looking for mutexes, or thread priorities, it's time to step back a bit and think about what you want to do and how Perl can do it. -=head1 Threadsafe Modules +However it is important to remember that Perl threads cannot magically +do things unless your operating systems threads allows it. So if your +system blocks the entire process on sleep(), Perl usually will as well. + +Perl Threads Are Different. + +=head1 Thread-Safe Modules The addition of threads has changed Perl's internals substantially. There are implications for people who write -modules--especially modules with XS code or external libraries. While -most modules won't encounter any problems, modules that aren't -explicitly tagged as thread-safe should be tested before being used in -production code. +modules with XS code or external libraries. However, since perl data is +not shared among threads by default, Perl modules stand a high chance of +being thread-safe or can be made thread-safe easily. Modules that are not +tagged as thread-safe should be tested or code reviewed before being used +in production code. Not all modules that you might use are thread-safe, and you should always assume a module is unsafe unless the documentation says -otherwise. This includes modules that are distributed as part of the -core. Threads are a beta feature, and even some of the standard +otherwise. This includes modules that are distributed as part of the +core. Threads are a new feature, and even some of the standard modules aren't thread-safe. +Even if a module is thread-safe, it doesn't mean that the module is optimized +to work well with threads. A module could possibly be rewritten to utilize +the new features in threaded Perl to increase performance in a threaded +environment. + If you're using a module that's not thread-safe for some reason, you -can protect yourself by using semaphores and lots of programming -discipline to control access to the module. Semaphores are covered -later in the article. Perl Threads Are Different +can protect yourself by using it from one, and only one thread at all. +If you need multiple threads to access such a module, you can use semaphores and +lots of programming discipline to control access to it. Semaphores +are covered in L</"Basic semaphores">. + +See also L</"Thread-Safety of System Libraries">. =head1 Thread Basics -The core Thread module provides the basic functions you need to write -threaded programs. In the following sections we'll cover the basics, -showing you what you need to do to create a threaded program. After -that, we'll go over some of the features of the Thread module that +The core L<threads> module provides the basic functions you need to write +threaded programs. In the following sections we'll cover the basics, +showing you what you need to do to create a threaded program. After +that, we'll go over some of the features of the L<threads> module that make threaded programming easier. =head2 Basic Thread Support -Thread support is a Perl compile-time option-it's something that's +Thread support is a Perl compile-time option - it's something that's turned on or off when Perl is built at your site, rather than when your programs are compiled. If your Perl wasn't compiled with thread support enabled, then any attempt to use threads will fail. -Remember that the threading support in 5.005 is in beta release, and -should be treated as such. You should expect that it may not function -entirely properly, and the thread interface may well change some -before it is a fully supported, production release. The beta version -shouldn't be used for mission-critical projects. Having said that, -threaded Perl is pretty nifty, and worth a look. - Your programs can use the Config module to check whether threads are enabled. If your program can't run without them, you can say something like: - $Config{usethreads} or die "Recompile Perl with threads to run this program."; + $Config{useithreads} or die "Recompile Perl with threads to run this program."; A possibly-threaded program using a possibly-threaded module might have code like this: - use Config; - use MyMod; - - if ($Config{usethreads}) { - # We have threads - require MyMod_threaded; - import MyMod_threaded; - } else { - require MyMod_unthreaded; - import MyMod_unthreaded; - } + use Config; + use MyMod; + + BEGIN { + if ($Config{useithreads}) { + # We have threads + require MyMod_threaded; + import MyMod_threaded; + } else { + require MyMod_unthreaded; + import MyMod_unthreaded; + } + } Since code that runs both with and without threads is usually pretty messy, it's best to isolate the thread-specific code in its own -module. In our example above, that's what MyMod_threaded is, and it's +module. In our example above, that's what MyMod_threaded is, and it's only imported if we're running on a threaded Perl. +=head2 A Note about the Examples + +Although thread support is considered to be stable, there are still a number +of quirks that may startle you when you try out any of the examples below. +In a real situation, care should be taken that all threads are finished +executing before the program exits. That care has B<not> been taken in these +examples in the interest of simplicity. Running these examples "as is" will +produce error messages, usually caused by the fact that there are still +threads running when the program exits. You should not be alarmed by this. +Future versions of Perl may fix this problem. + =head2 Creating Threads -The Thread package provides the tools you need to create new -threads. Like any other module, you need to tell Perl you want to use -it; use Thread imports all the pieces you need to create basic +The L<threads> package provides the tools you need to create new +threads. Like any other module, you need to tell Perl that you want to use +it; C<use threads> imports all the pieces you need to create basic threads. -The simplest, straightforward way to create a thread is with new(): +The simplest, most straightforward way to create a thread is with new(): - use Thread; + use threads; - $thr = new Thread \&sub1; + $thr = threads->new(\&sub1); sub sub1 { print "In the thread\n"; } The new() method takes a reference to a subroutine and creates a new -thread, which starts executing in the referenced subroutine. Control +thread, which starts executing in the referenced subroutine. Control then passes both to the subroutine and the caller. If you need to, your program can pass parameters to the subroutine as -part of the thread startup. Just include the list of parameters as -part of the C<Thread::new> call, like this: +part of the thread startup. Just include the list of parameters as +part of the C<threads::new> call, like this: + + use threads; - use Thread; $Param3 = "foo"; - $thr = new Thread \&sub1, "Param 1", "Param 2", $Param3; - $thr = new Thread \&sub1, @ParamList; - $thr = new Thread \&sub1, qw(Param1 Param2 $Param3); + $thr = threads->new(\&sub1, "Param 1", "Param 2", $Param3); + $thr = threads->new(\&sub1, @ParamList); + $thr = threads->new(\&sub1, qw(Param1 Param2 Param3)); sub sub1 { my @InboundParameters = @_; @@ -278,78 +324,55 @@ part of the C<Thread::new> call, like this: } -The subroutine runs like a normal Perl subroutine, and the call to new -Thread returns whatever the subroutine returns. - -The last example illustrates another feature of threads. You can spawn -off several threads using the same subroutine. Each thread executes +The last example illustrates another feature of threads. You can spawn +off several threads using the same subroutine. Each thread executes the same subroutine, but in a separate thread with a separate environment and potentially separate arguments. -The other way to spawn a new thread is with async(), which is a way to -spin off a chunk of code like eval(), but into its own thread: - - use Thread qw(async); - - $LineCount = 0; - - $thr = async { - while(<>) {$LineCount++} - print "Got $LineCount lines\n"; - }; - - print "Waiting for the linecount to end\n"; - $thr->join; - print "All done\n"; - -You'll notice we did a use Thread qw(async) in that example. async is -not exported by default, so if you want it, you'll either need to -import it before you use it or fully qualify it as -Thread::async. You'll also note that there's a semicolon after the -closing brace. That's because async() treats the following block as an -anonymous subroutine, so the semicolon is necessary. - -Like eval(), the code executes in the same context as it would if it -weren't spun off. Since both the code inside and after the async start -executing, you need to be careful with any shared resources. Locking -and other synchronization techniques are covered later. +C<create()> is a synonym for C<new()>. =head2 Giving up control There are times when you may find it useful to have a thread -explicitly give up the CPU to another thread. Your threading package +explicitly give up the CPU to another thread. Your threading package might not support preemptive multitasking for threads, for example, or -you may be doing something compute-intensive and want to make sure -that the user-interface thread gets called frequently. Regardless, +you may be doing something processor-intensive and want to make sure +that the user-interface thread gets called frequently. Regardless, there are times that you might want a thread to give up the processor. Perl's threading package provides the yield() function that does this. yield() is pretty straightforward, and works like this: - use Thread qw(yield async); - async { - my $foo = 50; - while ($foo--) { print "first async\n" } - yield; - $foo = 50; - while ($foo--) { print "first async\n" } - }; - async { - my $foo = 50; - while ($foo--) { print "second async\n" } - yield; - $foo = 50; - while ($foo--) { print "second async\n" } - }; + use threads; + + sub loop { + my $thread = shift; + my $foo = 50; + while($foo--) { print "in thread $thread\n" } + threads->yield; + $foo = 50; + while($foo--) { print "in thread $thread\n" } + } + + my $thread1 = threads->new(\&loop, 'first'); + my $thread2 = threads->new(\&loop, 'second'); + my $thread3 = threads->new(\&loop, 'third'); + +It is important to remember that yield() is only a hint to give up the CPU, +it depends on your hardware, OS and threading libraries what actually happens. +Therefore it is important to note that one should not build the scheduling of +the threads around yield() calls. It might work on your platform but it won't +work on another platform. =head2 Waiting For A Thread To Exit -Since threads are also subroutines, they can return values. To wait -for a thread to exit and extract any scalars it might return, you can -use the join() method. +Since threads are also subroutines, they can return values. To wait +for a thread to exit and extract any values it might return, you can +use the join() method: - use Thread; - $thr = new Thread \&sub1; + use threads; + + $thr = threads->new(\&sub1); @ReturnData = $thr->join; print "Thread returned @ReturnData"; @@ -357,54 +380,33 @@ use the join() method. sub sub1 { return "Fifty-six", "foo", 2; } In the example above, the join() method returns as soon as the thread -ends. In addition to waiting for a thread to finish and gathering up +ends. In addition to waiting for a thread to finish and gathering up any values that the thread might have returned, join() also performs any OS cleanup necessary for the thread. That cleanup might be important, especially for long-running programs that spawn lots of -threads. If you don't want the return values and don't want to wait +threads. If you don't want the return values and don't want to wait for the thread to finish, you should call the detach() method -instead. detach() is covered later in the article. - -=head2 Errors In Threads - -So what happens when an error occurs in a thread? Any errors that -could be caught with eval() are postponed until the thread is -joined. If your program never joins, the errors appear when your -program exits. - -Errors deferred until a join() can be caught with eval(): - - use Thread qw(async); - $thr = async {$b = 3/0}; # Divide by zero error - $foo = eval {$thr->join}; - if ($@) { - print "died with error $@\n"; - } else { - print "Hey, why aren't you dead?\n"; - } - -eval() passes any results from the joined thread back unmodified, so -if you want the return value of the thread, this is your only chance -to get them. +instead, as described next. =head2 Ignoring A Thread -join() does three things:it waits for a thread to exit, cleans up -after it, and returns any data the thread may have produced. But what +join() does three things: it waits for a thread to exit, cleans up +after it, and returns any data the thread may have produced. But what if you're not interested in the thread's return values, and you don't really care when the thread finishes? All you want is for the thread to get cleaned up after when it's done. -In this case, you use the detach() method. Once a thread is detached, +In this case, you use the detach() method. Once a thread is detached, it'll run until it's finished, then Perl will clean up after it automatically. - use Thread; - $thr = new Thread \&sub1; # Spawn the thread + use threads; + + $thr = threads->new(\&sub1); # Spawn the thread $thr->detach; # Now we officially don't care any more - sub sub1 { + sub sub1 { $a = 0; while (1) { $a++; @@ -413,234 +415,324 @@ automatically. } } - -Once a thread is detached, it may not be joined, and any output that -it might have produced (if it was done and waiting for a join) is +Once a thread is detached, it may not be joined, and any return data +that it might have produced (if it was done and waiting for a join) is lost. =head1 Threads And Data Now that we've covered the basics of threads, it's time for our next -topic: data. Threading introduces a couple of complications to data +topic: data. Threading introduces a couple of complications to data access that non-threaded programs never need to worry about. =head2 Shared And Unshared Data -The single most important thing to remember when using threads is that -all threads potentially have access to all the data anywhere in your -program. While this is true with a nonthreaded Perl program as well, -it's especially important to remember with a threaded program, since -more than one thread can be accessing this data at once. - -Perl's scoping rules don't change because you're using threads. If a -subroutine (or block, in the case of async()) could see a variable if -you weren't running with threads, it can see it if you are. This is -especially important for the subroutines that create, and makes my -variables even more important. Remember--if your variables aren't -lexically scoped (declared with C<my>) you're probably sharing it between -threads. +The biggest difference between Perl ithreads and the old 5.005 style +threading, or for that matter, to most other threading systems out there, +is that by default, no data is shared. When a new perl thread is created, +all the data associated with the current thread is copied to the new +thread, and is subsequently private to that new thread! +This is similar in feel to what happens when a UNIX process forks, +except that in this case, the data is just copied to a different part of +memory within the same process rather than a real fork taking place. + +To make use of threading however, one usually wants the threads to share +at least some data between themselves. This is done with the +L<threads::shared> module and the C< : shared> attribute: -=head2 Thread Pitfall: Races + use threads; + use threads::shared; + + my $foo : shared = 1; + my $bar = 1; + threads->new(sub { $foo++; $bar++ })->join; + + print "$foo\n"; #prints 2 since $foo is shared + print "$bar\n"; #prints 1 since $bar is not shared + +In the case of a shared array, all the array's elements are shared, and for +a shared hash, all the keys and values are shared. This places +restrictions on what may be assigned to shared array and hash elements: only +simple values or references to shared variables are allowed - this is +so that a private variable can't accidentally become shared. A bad +assignment will cause the thread to die. For example: + + use threads; + use threads::shared; + + my $var = 1; + my $svar : shared = 2; + my %hash : shared; + + ... create some threads ... + + $hash{a} = 1; # all threads see exists($hash{a}) and $hash{a} == 1 + $hash{a} = $var # okay - copy-by-value: same effect as previous + $hash{a} = $svar # okay - copy-by-value: same effect as previous + $hash{a} = \$svar # okay - a reference to a shared variable + $hash{a} = \$var # This will die + delete $hash{a} # okay - all threads will see !exists($hash{a}) + +Note that a shared variable guarantees that if two or more threads try to +modify it at the same time, the internal state of the variable will not +become corrupted. However, there are no guarantees beyond this, as +explained in the next section. + +=head2 Thread Pitfalls: Races While threads bring a new set of useful tools, they also bring a -number of pitfalls. One pitfall is the race condition: +number of pitfalls. One pitfall is the race condition: - use Thread; - $a = 1; - $thr1 = Thread->new(\&sub1); - $thr2 = Thread->new(\&sub2); + use threads; + use threads::shared; - sleep 10; + my $a : shared = 1; + $thr1 = threads->new(\&sub1); + $thr2 = threads->new(\&sub2); + + $thr1->join; + $thr2->join; print "$a\n"; - sub sub1 { $foo = $a; $a = $foo + 1; } - sub sub2 { $bar = $a; $a = $bar + 1; } + sub sub1 { my $foo = $a; $a = $foo + 1; } + sub sub2 { my $bar = $a; $a = $bar + 1; } What do you think $a will be? The answer, unfortunately, is "it depends." Both sub1() and sub2() access the global variable $a, once -to read and once to write. Depending on factors ranging from your +to read and once to write. Depending on factors ranging from your thread implementation's scheduling algorithm to the phase of the moon, $a can be 2 or 3. Race conditions are caused by unsynchronized access to shared -data. Without explicit synchronization, there's no way to be sure that +data. Without explicit synchronization, there's no way to be sure that nothing has happened to the shared data between the time you access it -and the time you update it. Even this simple code fragment has the +and the time you update it. Even this simple code fragment has the possibility of error: - use Thread qw(async); - $a = 2; - async{ $b = $a; $a = $b + 1; }; - async{ $c = $a; $a = $c + 1; }; - -Two threads both access $a. Each thread can potentially be interrupted -at any point, or be executed in any order. At the end, $a could be 3 + use threads; + my $a : shared = 2; + my $b : shared; + my $c : shared; + my $thr1 = threads->create(sub { $b = $a; $a = $b + 1; }); + my $thr2 = threads->create(sub { $c = $a; $a = $c + 1; }); + $thr1->join; + $thr2->join; + +Two threads both access $a. Each thread can potentially be interrupted +at any point, or be executed in any order. At the end, $a could be 3 or 4, and both $b and $c could be 2 or 3. +Even C<$a += 5> or C<$a++> are not guaranteed to be atomic. + Whenever your program accesses data or resources that can be accessed by other threads, you must take steps to coordinate access or risk -data corruption and race conditions. +data inconsistency and race conditions. Note that Perl will protect its +internals from your race conditions, but it won't protect you from you. + +=head1 Synchronization and control + +Perl provides a number of mechanisms to coordinate the interactions +between themselves and their data, to avoid race conditions and the like. +Some of these are designed to resemble the common techniques used in thread +libraries such as C<pthreads>; others are Perl-specific. Often, the +standard techniques are clumsy and difficult to get right (such as +condition waits). Where possible, it is usually easier to use Perlish +techniques such as queues, which remove some of the hard work involved. =head2 Controlling access: lock() -The lock() function takes a variable (or subroutine, but we'll get to -that later) and puts a lock on it. No other thread may lock the -variable until the locking thread exits the innermost block containing -the lock. Using lock() is straightforward: - - use Thread qw(async); - $a = 4; - $thr1 = async { - $foo = 12; - { - lock ($a); # Block until we get access to $a - $b = $a; - $a = $b * $foo; - } - print "\$foo was $foo\n"; - }; - $thr2 = async { - $bar = 7; - { - lock ($a); # Block until we can get access to $a - $c = $a; - $a = $c * $bar; - } - print "\$bar was $bar\n"; - }; - $thr1->join; - $thr2->join; - print "\$a is $a\n"; +The lock() function takes a shared variable and puts a lock on it. +No other thread may lock the variable until the variable is unlocked +by the thread holding the lock. Unlocking happens automatically +when the locking thread exits the outermost block that contains +C<lock()> function. Using lock() is straightforward: this example has +several threads doing some calculations in parallel, and occasionally +updating a running total: + + use threads; + use threads::shared; + + my $total : shared = 0; + + sub calc { + for (;;) { + my $result; + # (... do some calculations and set $result ...) + { + lock($total); # block until we obtain the lock + $total += $result; + } # lock implicitly released at end of scope + last if $result == 0; + } + } + + my $thr1 = threads->new(\&calc); + my $thr2 = threads->new(\&calc); + my $thr3 = threads->new(\&calc); + $thr1->join; + $thr2->join; + $thr3->join; + print "total=$total\n"; + lock() blocks the thread until the variable being locked is -available. When lock() returns, your thread can be sure that no other -thread can lock that variable until the innermost block containing the +available. When lock() returns, your thread can be sure that no other +thread can lock that variable until the outermost block containing the lock exits. It's important to note that locks don't prevent access to the variable -in question, only lock attempts. This is in keeping with Perl's +in question, only lock attempts. This is in keeping with Perl's longstanding tradition of courteous programming, and the advisory file -locking that flock() gives you. Locked subroutines behave differently, -however. We'll cover that later in the article. +locking that flock() gives you. -You may lock arrays and hashes as well as scalars. Locking an array, +You may lock arrays and hashes as well as scalars. Locking an array, though, will not block subsequent locks on array elements, just lock attempts on the array itself. -Finally, locks are recursive, which means it's okay for a thread to -lock a variable more than once. The lock will last until the outermost -lock() on the variable goes out of scope. +Locks are recursive, which means it's okay for a thread to +lock a variable more than once. The lock will last until the outermost +lock() on the variable goes out of scope. For example: + + my $x : shared; + doit(); + + sub doit { + { + { + lock($x); # wait for lock + lock($x); # NOOP - we already have the lock + { + lock($x); # NOOP + { + lock($x); # NOOP + lockit_some_more(); + } + } + } # *** implicit unlock here *** + } + } + + sub lockit_some_more { + lock($x); # NOOP + } # nothing happens here + +Note that there is no unlock() function - the only way to unlock a +variable is to allow it to go out of scope. + +A lock can either be used to guard the data contained within the variable +being locked, or it can be used to guard something else, like a section +of code. In this latter case, the variable in question does not hold any +useful data, and exists only for the purpose of being locked. In this +respect, the variable behaves like the mutexes and basic semaphores of +traditional thread libraries. -=head2 Thread Pitfall: Deadlocks +=head2 A Thread Pitfall: Deadlocks -Locks are a handy tool to synchronize access to data. Using them -properly is the key to safe shared data. Unfortunately, locks aren't -without their dangers. Consider the following code: +Locks are a handy tool to synchronize access to data, and using them +properly is the key to safe shared data. Unfortunately, locks aren't +without their dangers, especially when multiple locks are involved. +Consider the following code: - use Thread qw(async yield); - $a = 4; - $b = "foo"; - async { + use threads; + + my $a : shared = 4; + my $b : shared = "foo"; + my $thr1 = threads->new(sub { lock($a); - yield; + threads->yield; sleep 20; - lock ($b); - }; - async { lock($b); - yield; + }); + my $thr2 = threads->new(sub { + lock($b); + threads->yield; sleep 20; - lock ($a); - }; + lock($a); + }); -This program will probably hang until you kill it. The only way it -won't hang is if one of the two async() routines acquires both locks -first. A guaranteed-to-hang version is more complicated, but the +This program will probably hang until you kill it. The only way it +won't hang is if one of the two threads acquires both locks +first. A guaranteed-to-hang version is more complicated, but the principle is the same. -The first thread spawned by async() will grab a lock on $a then, a -second or two later, try to grab a lock on $b. Meanwhile, the second -thread grabs a lock on $b, then later tries to grab a lock on $a. The -second lock attempt for both threads will block, each waiting for the -other to release its lock. +The first thread will grab a lock on $a, then, after a pause during which +the second thread has probably had time to do some work, try to grab a +lock on $b. Meanwhile, the second thread grabs a lock on $b, then later +tries to grab a lock on $a. The second lock attempt for both threads will +block, each waiting for the other to release its lock. This condition is called a deadlock, and it occurs whenever two or more threads are trying to get locks on resources that the others -own. Each thread will block, waiting for the other to release a lock -on a resource. That never happens, though, since the thread with the +own. Each thread will block, waiting for the other to release a lock +on a resource. That never happens, though, since the thread with the resource is itself waiting for a lock to be released. -There are a number of ways to handle this sort of problem. The best +There are a number of ways to handle this sort of problem. The best way is to always have all threads acquire locks in the exact same -order. If, for example, you lock variables $a, $b, and $c, always lock -$a before $b, and $b before $c. It's also best to hold on to locks for +order. If, for example, you lock variables $a, $b, and $c, always lock +$a before $b, and $b before $c. It's also best to hold on to locks for as short a period of time to minimize the risks of deadlock. +The other synchronization primitives described below can suffer from +similar problems. + =head2 Queues: Passing Data Around A queue is a special thread-safe object that lets you put data in one end and take it out the other without having to worry about -synchronization issues. They're pretty straightforward, and look like +synchronization issues. They're pretty straightforward, and look like this: - use Thread qw(async); + use threads; use Thread::Queue; - my $DataQueue = new Thread::Queue; - $thr = async { + my $DataQueue = Thread::Queue->new; + $thr = threads->new(sub { while ($DataElement = $DataQueue->dequeue) { print "Popped $DataElement off the queue\n"; } - }; + }); $DataQueue->enqueue(12); $DataQueue->enqueue("A", "B", "C"); $DataQueue->enqueue(\$thr); sleep 10; $DataQueue->enqueue(undef); + $thr->join; -You create the queue with new Thread::Queue. Then you can add lists of -scalars onto the end with enqueue(), and pop scalars off the front of -it with dequeue(). A queue has no fixed size, and can grow as needed -to hold everything pushed on to it. +You create the queue with C<new Thread::Queue>. Then you can +add lists of scalars onto the end with enqueue(), and pop scalars off +the front of it with dequeue(). A queue has no fixed size, and can grow +as needed to hold everything pushed on to it. If a queue is empty, dequeue() blocks until another thread enqueues -something. This makes queues ideal for event loops and other +something. This makes queues ideal for event loops and other communications between threads. -=head1 Threads And Code - -In addition to providing thread-safe access to data via locks and -queues, threaded Perl also provides general-purpose semaphores for -coarser synchronization than locks provide and thread-safe access to -entire subroutines. - =head2 Semaphores: Synchronizing Data Access -Semaphores are a kind of generic locking mechanism. Unlike lock, which -gets a lock on a particular scalar, Perl doesn't associate any -particular thing with a semaphore so you can use them to control -access to anything you like. In addition, semaphores can allow more -than one thread to access a resource at once, though by default -semaphores only allow one thread access at a time. +Semaphores are a kind of generic locking mechanism. In their most basic +form, they behave very much like lockable scalars, except that thay +can't hold data, and that they must be explicitly unlocked. In their +advanced form, they act like a kind of counter, and can allow multiple +threads to have the 'lock' at any one time. -=over 4 +=head2 Basic semaphores -=item Basic semaphores - -Semaphores have two methods, down and up. down decrements the resource -count, while up increments it. down calls will block if the -semaphore's current count would decrement below zero. This program +Semaphores have two methods, down() and up(): down() decrements the resource +count, while up increments it. Calls to down() will block if the +semaphore's current count would decrement below zero. This program gives a quick demonstration: - use Thread qw(yield); + use threads qw(yield); use Thread::Semaphore; + my $semaphore = new Thread::Semaphore; - $GlobalVariable = 0; + my $GlobalVariable : shared = 0; - $thr1 = new Thread \&sample_sub, 1; - $thr2 = new Thread \&sample_sub, 2; - $thr3 = new Thread \&sample_sub, 3; + $thr1 = new threads \&sample_sub, 1; + $thr2 = new threads \&sample_sub, 2; + $thr3 = new threads \&sample_sub, 3; sub sample_sub { my $SubNumber = shift @_; @@ -659,42 +751,65 @@ gives a quick demonstration: } } -The three invocations of the subroutine all operate in sync. The + $thr1->join; + $thr2->join; + $thr3->join; + +The three invocations of the subroutine all operate in sync. The semaphore, though, makes sure that only one thread is accessing the global variable at once. -=item Advanced Semaphores +=head2 Advanced Semaphores By default, semaphores behave like locks, letting only one thread -down() them at a time. However, there are other uses for semaphores. +down() them at a time. However, there are other uses for semaphores. + +Each semaphore has a counter attached to it. By default, semaphores are +created with the counter set to one, down() decrements the counter by +one, and up() increments by one. However, we can override any or all +of these defaults simply by passing in different values: + + use threads; + use Thread::Semaphore; + my $semaphore = Thread::Semaphore->new(5); + # Creates a semaphore with the counter set to five + + $thr1 = threads->new(\&sub1); + $thr2 = threads->new(\&sub1); -Each semaphore has a counter attached to it. down() decrements the -counter and up() increments the counter. By default, semaphores are -created with the counter set to one, down() decrements by one, and -up() increments by one. If down() attempts to decrement the counter -below zero, it blocks until the counter is large enough. Note that -while a semaphore can be created with a starting count of zero, any -up() or down() always changes the counter by at least -one. $semaphore->down(0) is the same as $semaphore->down(1). + sub sub1 { + $semaphore->down(5); # Decrements the counter by five + # Do stuff here + $semaphore->up(5); # Increment the counter by five + } + + $thr1->detach; + $thr2->detach; + +If down() attempts to decrement the counter below zero, it blocks until +the counter is large enough. Note that while a semaphore can be created +with a starting count of zero, any up() or down() always changes the +counter by at least one, and so $semaphore->down(0) is the same as +$semaphore->down(1). The question, of course, is why would you do something like this? Why create a semaphore with a starting count that's not one, or why decrement/increment it by more than one? The answer is resource -availability. Many resources that you want to manage access for can be +availability. Many resources that you want to manage access for can be safely used by more than one thread at once. -For example, let's take a GUI driven program. It has a semaphore that +For example, let's take a GUI driven program. It has a semaphore that it uses to synchronize access to the display, so only one thread is -ever drawing at once. Handy, but of course you don't want any thread -to start drawing until things are properly set up. In this case, you +ever drawing at once. Handy, but of course you don't want any thread +to start drawing until things are properly set up. In this case, you can create a semaphore with a counter set to zero, and up it when things are ready for drawing. Semaphores with counters greater than one are also useful for -establishing quotas. Say, for example, that you have a number of -threads that can do I/O at once. You don't want all the threads +establishing quotas. Say, for example, that you have a number of +threads that can do I/O at once. You don't want all the threads reading or writing at once though, since that can potentially swamp -your I/O channels, or deplete your process' quota of filehandles. You +your I/O channels, or deplete your process' quota of filehandles. You can use a semaphore initialized to the number of concurrent I/O requests (or open files) that you want at any one time, and have your threads quietly block and unblock themselves. @@ -702,173 +817,32 @@ threads quietly block and unblock themselves. Larger increments or decrements are handy in those cases where a thread needs to check out or return a number of resources at once. -=back - -=head2 Attributes: Restricting Access To Subroutines - -In addition to synchronizing access to data or resources, you might -find it useful to synchronize access to subroutines. You may be -accessing a singular machine resource (perhaps a vector processor), or -find it easier to serialize calls to a particular subroutine than to -have a set of locks and sempahores. - -One of the additions to Perl 5.005 is subroutine attributes. The -Thread package uses these to provide several flavors of -serialization. It's important to remember that these attributes are -used in the compilation phase of your program so you can't change a -subroutine's behavior while your program is actually running. - -=head2 Subroutine Locks - -The basic subroutine lock looks like this: - - sub test_sub { - use attrs qw(locked); - } - -This ensures that only one thread will be executing this subroutine at -any one time. Once a thread calls this subroutine, any other thread -that calls it will block until the thread in the subroutine exits -it. A more elaborate example looks like this: - - use Thread qw(yield); - - new Thread \&thread_sub, 1; - new Thread \&thread_sub, 2; - new Thread \&thread_sub, 3; - new Thread \&thread_sub, 4; - - sub sync_sub { - use attrs qw(locked); - my $CallingThread = shift @_; - print "In sync_sub for thread $CallingThread\n"; - yield; - sleep 3; - print "Leaving sync_sub for thread $CallingThread\n"; - } - - sub thread_sub { - my $ThreadID = shift @_; - print "Thread $ThreadID calling sync_sub\n"; - sync_sub($ThreadID); - print "$ThreadID is done with sync_sub\n"; - } - -The use attrs qw(locked) locks sync_sub(), and if you run this, you -can see that only one thread is in it at any one time. - -=head2 Methods - -Locking an entire subroutine can sometimes be overkill, especially -when dealing with Perl objects. When calling a method for an object, -for example, you want to serialize calls to a method, so that only one -thread will be in the subroutine for a particular object, but threads -calling that subroutine for a different object aren't blocked. The -method attribute indicates whether the subroutine is really a method. - - use Thread; - - sub tester { - my $thrnum = shift @_; - my $bar = new Foo; - foreach (1..10) { - print "$thrnum calling per_object\n"; - $bar->per_object($thrnum); - print "$thrnum out of per_object\n"; - yield; - print "$thrnum calling one_at_a_time\n"; - $bar->one_at_a_time($thrnum); - print "$thrnum out of one_at_a_time\n"; - yield; - } - } - - foreach my $thrnum (1..10) { - new Thread \&tester, $thrnum; - } - - package Foo; - sub new { - my $class = shift @_; - return bless [@_], $class; - } - - sub per_object { - use attrs qw(locked method); - my ($class, $thrnum) = @_; - print "In per_object for thread $thrnum\n"; - yield; - sleep 2; - print "Exiting per_object for thread $thrnum\n"; - } - - sub one_at_a_time { - use attrs qw(locked); - my ($class, $thrnum) = @_; - print "In one_at_a_time for thread $thrnum\n"; - yield; - sleep 2; - print "Exiting one_at_a_time for thread $thrnum\n"; - } - -As you can see from the output (omitted for brevity; it's 800 lines) -all the threads can be in per_object() simultaneously, but only one -thread is ever in one_at_a_time() at once. - -=head2 Locking A Subroutine - -You can lock a subroutine as you would lock a variable. Subroutine -locks work the same as a C<use attrs qw(locked)> in the subroutine, -and block all access to the subroutine for other threads until the -lock goes out of scope. When the subroutine isn't locked, any number -of threads can be in it at once, and getting a lock on a subroutine -doesn't affect threads already in the subroutine. Getting a lock on a -subroutine looks like this: - - lock(\&sub_to_lock); - -Simple enough. Unlike use attrs, which is a compile time option, -locking and unlocking a subroutine can be done at runtime at your -discretion. There is some runtime penalty to using lock(\&sub) instead -of use attrs qw(locked), so make sure you're choosing the proper -method to do the locking. +=head2 cond_wait() and cond_signal() -You'd choose lock(\&sub) when writing modules and code to run on both -threaded and unthreaded Perl, especially for code that will run on -5.004 or earlier Perls. In that case, it's useful to have subroutines -that should be serialized lock themselves if they're running threaded, -like so: - - package Foo; - use Config; - $Running_Threaded = 0; - - BEGIN { $Running_Threaded = $Config{'usethreads'} } - - sub sub1 { lock(\&sub1) if $Running_Threaded } - - -This way you can ensure single-threadedness regardless of which -version of Perl you're running. +These two functions can be used in conjunction with locks to notify +co-operating threads that a resource has become available. They are +very similar in use to the functions found in C<pthreads>. However +for most purposes, queues are simpler to use and more intuitive. See +L<threads::shared> for more details. =head1 General Thread Utility Routines We've covered the workhorse parts of Perl's threading package, and with these tools you should be well on your way to writing threaded -code and packages. There are a few useful little pieces that didn't +code and packages. There are a few useful little pieces that didn't really fit in anyplace else. =head2 What Thread Am I In? -The Thread->self method provides your program with a way to get an -object representing the thread it's currently in. You can use this -object in the same way as the ones returned from the thread creation. +The C<< threads->self >> class method provides your program with a way to +get an object representing the thread it's currently in. You can use this +object in the same way as the ones returned from thread creation. =head2 Thread IDs tid() is a thread object method that returns the thread ID of the -thread the object represents. Thread IDs are integers, with the main -thread in a program being 0. Currently Perl assigns a unique tid to +thread the object represents. Thread IDs are integers, with the main +thread in a program being 0. Currently Perl assigns a unique tid to every thread ever created in your program, assigning the first thread to be created a tid of 1, and increasing the tid by 1 for each new thread that's created. @@ -878,46 +852,49 @@ thread that's created. The equal() method takes two thread objects and returns true if the objects represent the same thread, and false if they don't. +Thread objects also have an overloaded == comparison so that you can do +comparison on them as you would with normal objects. + =head2 What Threads Are Running? -Thread->list returns a list of thread objects, one for each thread -that's currently running. Handy for a number of things, including -cleaning up at the end of your program: +C<< threads->list >> returns a list of thread objects, one for each thread +that's currently running and not detached. Handy for a number of things, +including cleaning up at the end of your program: # Loop through all the threads - foreach $thr (Thread->list) { + foreach $thr (threads->list) { # Don't join the main thread or ourselves - if ($thr->tid && !Thread::equal($thr, Thread->self)) { + if ($thr->tid && !threads::equal($thr, threads->self)) { $thr->join; } } -The example above is just for illustration. It isn't strictly -necessary to join all the threads you create, since Perl detaches all -the threads before it exits. +If some threads have not finished running when the main Perl thread +ends, Perl will warn you about it and die, since it is impossible for Perl +to clean up itself while other threads are running =head1 A Complete Example Confused yet? It's time for an example program to show some of the -things we've covered. This program finds prime numbers using threads. +things we've covered. This program finds prime numbers using threads. 1 #!/usr/bin/perl -w 2 # prime-pthread, courtesy of Tom Christiansen 3 4 use strict; 5 - 6 use Thread; + 6 use threads; 7 use Thread::Queue; 8 9 my $stream = new Thread::Queue; - 10 my $kid = new Thread(\&check_num, $stream, 2); + 10 my $kid = new threads(\&check_num, $stream, 2); 11 12 for my $i ( 3 .. 1000 ) { 13 $stream->enqueue($i); 14 } 15 16 $stream->enqueue(undef); - 17 $kid->join(); + 17 $kid->join; 18 19 sub check_num { 20 my ($upstream, $cur_prime) = @_; @@ -929,67 +906,126 @@ things we've covered. This program finds prime numbers using threads. 26 $downstream->enqueue($num); 27 } else { 28 print "Found prime $num\n"; - 29 $kid = new Thread(\&check_num, $downstream, $num); + 29 $kid = new threads(\&check_num, $downstream, $num); 30 } 31 } 32 $downstream->enqueue(undef) if $kid; - 33 $kid->join() if $kid; + 33 $kid->join if $kid; 34 } -This program uses the pipeline model to generate prime numbers. Each +This program uses the pipeline model to generate prime numbers. Each thread in the pipeline has an input queue that feeds numbers to be checked, a prime number that it's responsible for, and an output queue -that it funnels numbers that have failed the check into. If the thread +into which it funnels numbers that have failed the check. If the thread has a number that's failed its check and there's no child thread, then -the thread must have found a new prime number. In that case, a new +the thread must have found a new prime number. In that case, a new child thread is created for that prime and stuck on the end of the pipeline. -This probably sounds a bit more confusing than it really is, so lets +This probably sounds a bit more confusing than it really is, so let's go through this program piece by piece and see what it does. (For those of you who might be trying to remember exactly what a prime number is, it's a number that's only evenly divisible by itself and 1) The bulk of the work is done by the check_num() subroutine, which takes a reference to its input queue and a prime number that it's -responsible for. After pulling in the input queue and the prime that +responsible for. After pulling in the input queue and the prime that the subroutine's checking (line 20), we create a new queue (line 22) and reserve a scalar for the thread that we're likely to create later (line 21). The while loop from lines 23 to line 31 grabs a scalar off the input queue and checks against the prime this thread is responsible -for. Line 24 checks to see if there's a remainder when we modulo the -number to be checked against our prime. If there is one, the number +for. Line 24 checks to see if there's a remainder when we modulo the +number to be checked against our prime. If there is one, the number must not be evenly divisible by our prime, so we need to either pass it on to the next thread if we've created one (line 26) or create a new thread if we haven't. -The new thread creation is line 29. We pass on to it a reference to +The new thread creation is line 29. We pass on to it a reference to the queue we've created, and the prime number we've found. Finally, once the loop terminates (because we got a 0 or undef in the queue, which serves as a note to die), we pass on the notice to our -child and wait for it to exit if we've created a child (Lines 32 and +child and wait for it to exit if we've created a child (lines 32 and 37). Meanwhile, back in the main thread, we create a queue (line 9) and the initial child thread (line 10), and pre-seed it with the first prime: -2. Then we queue all the numbers from 3 to 1000 for checking (lines +2. Then we queue all the numbers from 3 to 1000 for checking (lines 12-14), then queue a die notice (line 16) and wait for the first child -thread to terminate (line 17). Because a child won't die until its +thread to terminate (line 17). Because a child won't die until its child has died, we know that we're done once we return from the join. -That's how it works. It's pretty simple; as with many Perl programs, +That's how it works. It's pretty simple; as with many Perl programs, the explanation is much longer than the program. +=head1 Performance considerations + +The main thing to bear in mind when comparing ithreads to other threading +models is the fact that for each new thread created, a complete copy of +all the variables and data of the parent thread has to be taken. Thus +thread creation can be quite expensive, both in terms of memory usage and +time spent in creation. The ideal way to reduce these costs is to have a +relatively short number of long-lived threads, all created fairly early +on - before the base thread has accumulated too much data. Of course, this +may not always be possible, so compromises have to be made. However, after +a thread has been created, its performance and extra memory usage should +be little different than ordinary code. + +Also note that under the current implementation, shared variables +use a little more memory and are a little slower than ordinary variables. + +=head1 Process-scope Changes + +Note that while threads themselves are separate execution threads and +Perl data is thread-private unless explicitly shared, the threads can +affect process-scope state, affecting all the threads. + +The most common example of this is changing the current working +directory using chdir(). One thread calls chdir(), and the working +directory of all the threads changes. + +Even more drastic example of a process-scope change is chroot(): +the root directory of all the threads changes, and no thread can +undo it (as opposed to chdir()). + +Further examples of process-scope changes include umask() and +changing uids/gids. + +Thinking of mixing fork() and threads? Please lie down and wait +until the feeling passes-- but in case you really want to know, +the semantics is that fork() duplicates all the threads. +(In UNIX, at least, other platforms will do something different.) + +Similarly, mixing signals and threads should not be attempted. +Implementations are platform-dependent, and even the POSIX +semantics may not be what you expect (and Perl doesn't even +give you the full POSIX API). + +=head1 Thread-Safety of System Libraries + +Whether various library calls are thread-safe is outside the control +of Perl. Calls often suffering from not being thread-safe include: +localtime(), gmtime(), get{gr,host,net,proto,serv,pw}*(), readdir(), +rand(), and srand() -- in general, calls that depend on some global +external state. + +If the system Perl is compiled in has thread-safe variants of such +calls, they will be used. Beyond that, Perl is at the mercy of +the thread-safety or -unsafety of the calls. Please consult your +C library call documentation. + +In some platforms the thread-safe interfaces may fail if the result +buffer is too small (for example getgrent() may return quite large +group member lists). Perl will retry growing the result buffer +a few times, but only up to 64k (for safety reasons). + =head1 Conclusion A complete thread tutorial could fill a book (and has, many times), -but this should get you well on your way. The final authority on how -Perl's threads behave is the documention bundled with the Perl -distribution, but with what we've covered in this article, you should -be well on your way to becoming a threaded Perl expert. +but with what we've covered in this introduction, you should be well +on your way to becoming a threaded Perl expert. =head1 Bibliography @@ -1000,8 +1036,8 @@ Here's a short bibliography courtesy of Jürgen Christoffel: Birrell, Andrew D. An Introduction to Programming with Threads. Digital Equipment Corporation, 1989, DEC-SRC Research Report #35 online as -http://www.research.digital.com/SRC/staff/birrell/bib.html (highly -recommended) +http://gatekeeper.dec.com/pub/DEC/SRC/research-reports/abstracts/src-rr-035.html +(highly recommended) Robbins, Kay. A., and Steven Robbins. Practical Unix Programming: A Guide to Concurrency, Communication, and @@ -1025,7 +1061,7 @@ LoVerso. Programming under Mach. Addison-Wesley, 1994, ISBN 0-201-52739-1. Tanenbaum, Andrew S. Distributed Operating Systems. Prentice Hall, -1995, ISBN 0-13-143934-0 (great textbook). +1995, ISBN 0-13-219908-4 (great textbook). Silberschatz, Abraham, and Peter B. Galvin. Operating System Concepts, 4th ed. Addison-Wesley, 1995, ISBN 0-201-59292-4 @@ -1035,29 +1071,41 @@ Silberschatz, Abraham, and Peter B. Galvin. Operating System Concepts, Arnold, Ken and James Gosling. The Java Programming Language, 2nd ed. Addison-Wesley, 1998, ISBN 0-201-31006-6. +comp.programming.threads FAQ, +L<http://www.serpentine.com/~bos/threads-faq/> + Le Sergent, T. and B. Berthomieu. "Incremental MultiThreaded Garbage Collection on Virtually Shared Memory Architectures" in Memory Management: Proc. of the International Workshop IWMM 92, St. Malo, France, September 1992, Yves Bekkers and Jacques Cohen, eds. Springer, 1992, ISBN 3540-55940-X (real-life thread applications). +Artur Bergman, "Where Wizards Fear To Tread", June 11, 2002, +L<http://www.perl.com/pub/a/2002/06/11/threads.html> + =head1 Acknowledgements Thanks (in no particular order) to Chaim Frenkel, Steve Fink, Gurusamy Sarathy, Ilya Zakharevich, Benjamin Sugars, Jürgen Christoffel, Joshua Pritikin, and Alan Burlison, for their help in reality-checking and -polishing this article. Big thanks to Tom Christiansen for his rewrite +polishing this article. Big thanks to Tom Christiansen for his rewrite of the prime number generator. =head1 AUTHOR -Dan Sugalski E<lt>sugalskd@ous.eduE<gt> +Dan Sugalski E<lt>dan@sidhe.org<gt> + +Slightly modified by Arthur Bergman to fit the new thread model/module. + +Reworked slightly by Jörg Walter E<lt>jwalt@cpan.org<gt> to be more concise +about thread-safety of perl code. =head1 Copyrights -This article originally appeared in The Perl Journal #10, and is -copyright 1998 The Perl Journal. It appears courtesy of Jon Orwant and -The Perl Journal. This document may be distributed under the same terms -as Perl itself. +The original version of this article originally appeared in The Perl +Journal #10, and is copyright 1998 The Perl Journal. It appears courtesy +of Jon Orwant and The Perl Journal. This document may be distributed +under the same terms as Perl itself. +For more information please see L<threads> and L<threads::shared>. |