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=head1 NAME |
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|
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Coro::State - first class continuations |
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|
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=head1 SYNOPSIS |
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|
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use Coro::State; |
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|
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$new = new Coro::State sub { |
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print "in coroutine (called with @_), switching back\n"; |
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$new->transfer ($main); |
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print "in coroutine again, switching back\n"; |
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$new->transfer ($main); |
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}, 5; |
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|
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$main = new Coro::State; |
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|
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print "in main, switching to coroutine\n"; |
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$main->transfer ($new); |
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print "back in main, switch to coroutine again\n"; |
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$main->transfer ($new); |
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print "back in main\n"; |
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|
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=head1 DESCRIPTION |
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|
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This module implements coroutines. Coroutines, similar to continuations, |
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allow you to run more than one "thread of execution" in parallel. Unlike |
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threads, there is no parallelism and only voluntary switching is used so |
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locking problems are greatly reduced. |
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|
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This can be used to implement non-local jumps, exception handling, |
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(non-clonable) continuations and more. |
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|
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This module provides only low-level functionality. See L<Coro> and related |
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modules for a higher level process abstraction including scheduling. |
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|
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=head2 MODEL |
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|
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Coro::State implements two different coroutine models: Perl and C. The |
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C coroutines (called cctx's) are basically simplified perl interpreters |
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running/interpreting the Perl coroutines. A single interpreter can run any |
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number of Perl coroutines, so usually there are very few C coroutines. |
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|
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When Perl code calls a C function (e.g. in an extension module) and that |
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C function then calls back into Perl or does a coroutine switch the C |
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coroutine can no longer execute other Perl coroutines, so it stays tied to |
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the specific coroutine until it returns to the original Perl caller, after |
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which it is again available to run other Perl coroutines. |
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|
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The main program always has its own "C coroutine" (which really is |
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*the* Perl interpreter running the whole program), so there will always |
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be at least one additional C coroutine. You can use the debugger (see |
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L<Coro::Debug>) to find out which coroutines are tied to their cctx and |
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which aren't. |
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|
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=head2 MEMORY CONSUMPTION |
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|
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A newly created coroutine that has not been used only allocates a |
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relatively small (a hundred bytes) structure. Only on the first |
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C<transfer> will perl allocate stacks (a few kb, 64 bit architetcures |
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use twice as much, i.e. a few kb :) and optionally a C stack/coroutine |
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(cctx) for coroutines that recurse through C functions. All this is very |
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system-dependent. On my x86-pc-linux-gnu system this amounts to about 2k |
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per (non-trivial but simple) coroutine. |
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|
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You can view the actual memory consumption using Coro::Debug. Keep in mind |
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that a for loop or other block constructs can easily consume 100-200 bytes |
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per nesting level. |
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|
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=cut |
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|
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package Coro::State; |
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|
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use strict; |
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no warnings "uninitialized"; |
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|
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use Carp; |
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|
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use Time::HiRes (); # currently only used for PerlIO::cede |
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|
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our $DIEHOOK; |
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our $WARNHOOK; |
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|
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BEGIN { |
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$DIEHOOK = sub { }; |
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$WARNHOOK = sub { warn $_[0] }; |
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} |
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|
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sub diehook { &$DIEHOOK } |
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sub warnhook { &$WARNHOOK } |
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|
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use XSLoader; |
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|
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BEGIN { |
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our $VERSION = 5.1; |
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|
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# must be done here because the xs part expects it to exist |
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# it might exist already because Coro::Specific created it. |
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$Coro::current ||= { }; |
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|
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{ |
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# save/restore the handlers before/after overwriting %SIG magic |
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local $SIG{__DIE__}; |
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local $SIG{__WARN__}; |
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|
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XSLoader::load __PACKAGE__, $VERSION; |
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} |
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|
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# need to do it after overwriting the %SIG magic |
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$SIG{__DIE__} ||= \&diehook; |
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$SIG{__WARN__} ||= \&warnhook; |
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} |
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|
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use Exporter; |
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use base Exporter::; |
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|
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=head2 GLOBAL VARIABLES |
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|
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=over 4 |
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|
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=item $Coro::State::DIEHOOK |
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|
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This works similarly to C<$SIG{__DIE__}> and is used as the default die |
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hook for newly created coroutines. This is useful if you want some generic |
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logging function that works for all coroutines that don't set their own |
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hook. |
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|
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When Coro::State is first loaded it will install these handlers for the |
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main program, too, unless they have been overwritten already. |
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|
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The default handlers provided will behave like the built-in ones (as if |
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they weren't there). |
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|
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If you don't want to exit your program on uncaught exceptions, you can |
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must not return from your die hook - terminate instead. |
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|
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Note 1: You I<must> store a valid code reference in these variables, |
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C<undef> will I<not> do. |
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|
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Note 2: The value of this variable will be shared among all coroutines, so |
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changing its value will change it in all coroutines that don't have their |
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own die handler. |
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|
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=item $Coro::State::WARNHOOK |
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|
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Similar to above die hook, but augments C<$SIG{__WARN__}>. |
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|
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=back |
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|
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=head2 FUNCTIONS |
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|
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=over 4 |
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|
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=item $coro = new Coro::State [$coderef[, @args...]] |
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|
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Create a new coroutine and return it. The first C<transfer> call to this |
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coroutine will start execution at the given coderef, with the given |
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arguments. |
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|
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Note that the arguments will not be copied. Instead, as with normal |
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function calls, the coroutine receives passed arguments by reference, so |
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make sure you don't change them in unexpected ways. |
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|
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Returning from such a coroutine is I<NOT> supported. Neither is calling |
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C<exit> or throwing an uncaught exception. The following paragraphs |
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describe what happens in current versions of Coro. |
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|
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If the subroutine returns the program will be terminated as if execution |
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of the main program ended. |
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|
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If it throws an exception the program will terminate unless the exception |
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is caught, exactly like in the main program. |
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|
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Calling C<exit> in a coroutine does the same as calling it in the main |
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program, but due to libc bugs on many BSDs, this doesn't work reliable |
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everywhere. |
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|
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If the coderef is omitted this function will create a new "empty" |
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coroutine, i.e. a coroutine that cannot be transfered to but can be used |
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to save the current coroutine state in (note that this is dangerous, as no |
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reference is taken to ensure that the "current coroutine state" survives, |
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the caller is responsible to ensure that the cloned state does not go |
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away). |
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|
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The returned object is an empty hash which can be used for any purpose |
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whatsoever, for example when subclassing Coro::State. |
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|
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Certain variables are "localised" to each coroutine, that is, certain |
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"global" variables are actually per coroutine. Not everything that would |
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sensibly be localised currently is, and not everything that is localised |
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makes sense for every application, and the future might bring changes. |
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|
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The following global variables can have different values per coroutine, |
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and have the stated initial values: |
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|
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Variable Initial Value |
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@_ whatever arguments were passed to the Coro |
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$_ undef |
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$@ undef |
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$/ "\n" |
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$SIG{__DIE__} aliased to $Coro::State::DIEHOOK(*) |
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$SIG{__WARN__} aliased to $Coro::State::WARNHOOK(*) |
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(default fh) *STDOUT |
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$^H, %^H zero/empty. |
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$1, $2... all regex results are initially undefined |
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|
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(*) reading the value from %SIG is not supported, but local'ising is. |
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|
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If you feel that something important is missing then tell me. Also |
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remember that every function call that might call C<transfer> (such |
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as C<Coro::Channel::put>) might clobber any global and/or special |
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variables. Yes, this is by design ;) You can always create your own |
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process abstraction model that saves these variables. |
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|
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The easiest way to do this is to create your own scheduling primitive like |
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in the code below, and use it in your coroutines: |
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|
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sub my_cede { |
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local ($;, ...); |
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Coro::cede; |
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} |
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|
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=cut |
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|
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=item $state->throw ([$scalar]) |
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|
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See L<< Coro->throw >>. |
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|
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=item $state->call ($coderef) |
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|
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Try to call the given C<$coderef> in the context of the given state. This |
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works even when the state is currently within an XS function, and can |
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be very dangerous. You can use it to acquire stack traces etc. (see the |
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Coro::Debug module for more details). The coderef MUST NOT EVER transfer |
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to another state. |
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|
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=item $state->eval ($string) |
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|
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Like C<call>, but eval's the string. Dangerous. |
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|
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=item $state->swap_defsv |
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|
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=item $state->swap_defav |
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|
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Swap the current C<$_> (swap_defsv) or C<@_> (swap_defav) with the |
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equivalent in the saved state of C<$state>. This can be used to give the |
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coroutine a defined content for C<@_> and C<$_> before transfer'ing to it. |
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|
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=item $state->trace ($flags) |
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|
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Internal function to control tracing. I just mention this so you can stay |
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away from abusing it. |
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|
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=item $prev->transfer ($next) |
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|
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Save the state of the current subroutine in C<$prev> and switch to the |
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coroutine saved in C<$next>. |
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|
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The "state" of a subroutine includes the scope, i.e. lexical variables and |
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the current execution state (subroutine, stack). |
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|
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=item $bytes = $state->rss |
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|
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Returns the memory allocated by the coroutine (which includes static |
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structures, various perl stacks but NOT local variables, arguments or any |
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C context data). This is a rough indication of how much memory it might |
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use. |
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|
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=item $state->has_cctx |
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|
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Returns whether the state currently uses a cctx/C context. An active |
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state always has a cctx, as well as the main program. Other states only |
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use a cctxts when needed. |
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|
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=item Coro::State::force_cctx |
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|
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Forces the allocation of a C context for the currently running coroutine |
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(if not already done). Apart from benchmarking there is little point |
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in doing so, however. |
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|
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=item $ncctx = Coro::State::cctx_count |
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|
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Returns the number of C-level coroutines allocated. If this number is |
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very high (more than a dozen) it might help to identify points of C-level |
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recursion in your code and moving this into a separate coroutine. |
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|
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=item $nidle = Coro::State::cctx_idle |
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|
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Returns the number of allocated but idle (free for reuse) C level |
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coroutines. Currently, Coro will limit the number of idle/unused cctxs to |
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8. |
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|
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=item $old = Coro::State::cctx_stacksize [$new_stacksize] |
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|
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Returns the current C stack size and optionally sets the new I<minimum> |
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stack size to C<$new_stacksize> I<long>s. Existing stacks will not |
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be changed, but Coro will try to replace smaller stacks as soon as |
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possible. Any Coro::State that starts to use a stack after this call is |
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guaranteed this minimum stack size. |
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|
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Please note that Coroutines will only need to use a C-level stack if the |
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interpreter recurses or calls a function in a module that calls back into |
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the interpreter, so use of this feature is usually never needed. |
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|
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=item $old = Coro::State::cctx_max_idle [$new_count] |
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|
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Coro caches C contexts that are not in use currently, as creating them |
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from scratch has some overhead. |
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|
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This function returns the current maximum number of idle C contexts and |
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optionally sets the new amount. The count must be at least C<1>, with the |
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default being C<4>. |
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|
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=item @states = Coro::State::list |
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|
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Returns a list of all states currently allocated. |
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|
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=item $clone = $state->clone |
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|
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This exciting method takes a Coro::State object and clones, i.e., creates |
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a copy. This makes it possible to restore a state more than once. |
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|
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Since its only known purpose is intellectual self-gratification, and |
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because it is a difficult piece of code, it is not enabled by default, and |
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not supported. |
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|
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Among the games you can play with this is implement a scheme-like call/cc, |
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as the following code does (well, with small differences). |
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|
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sub callcc(&@) { |
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my ($func, @arg) = @_; |
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|
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my $state = new Coro::State; |
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my $wrapper; $wrapper = new Coro::State sub { |
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my $escape = sub { |
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@arg = @_; |
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$wrapper->transfer ($state->clone); |
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}; |
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$escape->($func->($escape, @arg)); |
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}; |
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|
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$state->transfer ($wrapper); |
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@arg |
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} |
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|
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Here are a few little-known facts: First, coroutines *are* full/true/real |
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continuations. Secondly Coro::State objects (without clone) *are* first |
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class continuations. Thirdly, nobody has ever found a use for the full |
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power of call/cc that isn't better (faster, easier, more efficient) |
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implemented differently, and nobody has yet found a useful control |
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construct that can't be implemented without it already, just much faster |
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and with fewer resources. |
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|
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Besides, call/cc is much less useful in a Perl-like dynamic language (with |
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references, and its scoping rules) then in, say, scheme. |
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|
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Now, the known limitations of C<clone>: |
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|
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It probably only works on perl 5.10; it cannot clone a coroutine inside |
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the substition operator (but windows perl can't fork from there either) |
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and some other contexts, and C<abort ()> is the preferred mechanism to |
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signal errors. It cannot clone a state that has a c context attached |
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(implementing clone on the C level is too hard for me to even try), |
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which rules out calling call/cc from the main coroutine. It cannot |
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clone a context that hasn't even been started yet. It doesn't work with |
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C<-DDEBUGGING> (but what does). It probably also leaks, and sometimes |
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triggers a few assertions inside Coro. Most of these limitations *are* |
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fixable with some effort, but that's pointless just to make a point that |
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it could be done. |
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|
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=cut |
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|
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# used by Coro::Debug only atm. |
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sub debug_desc { |
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$_[0]{desc} |
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} |
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|
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# for very deep reasons, we must initialise $Coro::main here. |
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|
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{ |
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package Coro; |
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|
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our $main; # main coroutine |
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our $current; # current coroutine |
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|
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$main = Coro::State::new Coro::; |
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|
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$main->{desc} = "[main::]"; |
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|
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# maybe some other module used Coro::Specific before... |
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$main->{_specific} = $current->{_specific} |
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if $current; |
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|
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_set_current $main; |
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} |
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|
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1; |
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|
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=back |
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|
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=head1 BUGS |
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|
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This module is not thread-safe. You must only ever use this module from |
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the same thread (this requirement might be removed in the future). |
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|
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=head1 SEE ALSO |
| 407 |
|
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L<Coro>. |
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|
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=head1 AUTHOR |
| 411 |
|
| 412 |
Marc Lehmann <schmorp@schmorp.de> |
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http://home.schmorp.de/ |
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|
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=cut |
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|
