1 |
=head1 NAME |
2 |
|
3 |
Coro::State - first class continuations |
4 |
|
5 |
=head1 SYNOPSIS |
6 |
|
7 |
use Coro::State; |
8 |
|
9 |
$new = new Coro::State sub { |
10 |
print "in coro (called with @_), switching back\n"; |
11 |
$new->transfer ($main); |
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print "in coro again, switching back\n"; |
13 |
$new->transfer ($main); |
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}, 5; |
15 |
|
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$main = new Coro::State; |
17 |
|
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print "in main, switching to coro\n"; |
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$main->transfer ($new); |
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print "back in main, switch to coro again\n"; |
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$main->transfer ($new); |
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print "back in main\n"; |
23 |
|
24 |
=head1 DESCRIPTION |
25 |
|
26 |
This module implements coro. Coros, similar to threads and continuations, |
27 |
allow you to run more than one "thread of execution" in parallel. Unlike |
28 |
so-called "kernel" threads, there is no parallelism and only voluntary |
29 |
switching is used so locking problems are greatly reduced. The latter is |
30 |
called "cooperative" threading as opposed to "preemptive" threading. |
31 |
|
32 |
This can be used to implement non-local jumps, exception handling, |
33 |
continuation objects and more. |
34 |
|
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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 threads abstraction including a scheduler. |
37 |
|
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=head2 MODEL |
39 |
|
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Coro::State implements two different thread models: Perl and C. The C |
41 |
threads (called cctx's) are basically simplified perl interpreters |
42 |
running/interpreting the Perl threads. A single interpreter can run any |
43 |
number of Perl threads, so usually there are very few C threads. |
44 |
|
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When Perl code calls a C function (e.g. in an extension module) and that C |
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function then calls back into Perl or transfers control to another thread, |
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the C thread can no longer execute other Perl threads, so it stays tied to |
48 |
the specific thread until it returns to the original Perl caller, after |
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which it is again available to run other Perl threads. |
50 |
|
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The main program always has its own "C thread" (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 thread. You can use the debugger (see |
54 |
L<Coro::Debug>) to find out which threads are tied to their cctx and |
55 |
which aren't. |
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|
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=head2 MEMORY CONSUMPTION |
58 |
|
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A newly created Coro::State that has not been used only allocates a |
60 |
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/thread |
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(cctx) for threads 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) Coro::State. |
66 |
|
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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 |
81 |
|
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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 $VERSION = 5.17; |
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|
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# must be done here because the xs part expects it to exist |
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# it might exist already because Coro::Specific created it. |
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$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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|
107 |
XSLoader::load __PACKAGE__, $VERSION; |
108 |
} |
109 |
|
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# need to do it after overwriting the %SIG magic |
111 |
$SIG{__DIE__} ||= \&diehook; |
112 |
$SIG{__WARN__} ||= \&warnhook; |
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} |
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|
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use Exporter; |
116 |
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 $Coro::State::DIEHOOK |
123 |
|
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This works similarly to C<$SIG{__DIE__}> and is used as the default die |
125 |
hook for newly created Coro::States. This is useful if you want some generic |
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logging function that works for all threads that don't set their own |
127 |
hook. |
128 |
|
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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. |
131 |
|
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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 must |
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not return from your die hook - call C<Coro::terminate> instead. |
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|
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Note 1: You I<must> store a valid code reference in these variables, |
139 |
C<undef> will I<not> do. |
140 |
|
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Note 2: The value of this variable will be shared among all threads, so |
142 |
changing its value will change it in all threads that don't have their |
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own die handler. |
144 |
|
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=item $Coro::State::WARNHOOK |
146 |
|
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Similar to above die hook, but augments C<$SIG{__WARN__}>. |
148 |
|
149 |
=back |
150 |
|
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=head2 FUNCTIONS |
152 |
|
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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 Coro::State thread object and return it. The first |
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C<transfer> call to this thread will start execution at the given |
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coderef, with the given arguments. |
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|
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Note that the arguments will not be copied. Instead, as with normal |
162 |
function calls, the thread 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 thread is I<NOT> supported. Neither is calling |
166 |
C<exit> or throwing an uncaught exception. The following paragraphs |
167 |
describe what happens in current versions of Coro. |
168 |
|
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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 thread 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 |
177 |
everywhere. |
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|
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If the coderef is omitted this function will create a new "empty" |
180 |
thread, i.e. a thread that cannot be transfered to but can be used |
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to save the current thread state in (note that this is dangerous, as no |
182 |
reference is taken to ensure that the "current thread state" survives, |
183 |
the caller is responsible to ensure that the cloned state does not go |
184 |
away). |
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|
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The returned object is an empty hash which can be used for any purpose |
187 |
whatsoever, for example when subclassing Coro::State. |
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|
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Certain variables are "localised" to each thread, that is, certain |
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"global" variables are actually per thread. Not everything that would |
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sensibly be localised currently is, and not everything that is localised |
192 |
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 thread, |
195 |
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 |
199 |
$_ undef |
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$@ undef |
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$/ "\n" |
202 |
$SIG{__DIE__} aliased to $Coro::State::DIEHOOK(*) |
203 |
$SIG{__WARN__} aliased to $Coro::State::WARNHOOK(*) |
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(default fh) *STDOUT |
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$^H, %^H zero/empty. |
206 |
$1, $2... all regex results are initially undefined |
207 |
|
208 |
(*) reading the value from %SIG is not supported, but local'ising is. |
209 |
|
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If you feel that something important is missing then tell me. Also |
211 |
remember that every function call that might call C<transfer> (such |
212 |
as C<Coro::Channel::put>) might clobber any global and/or special |
213 |
variables. Yes, this is by design ;) You can always create your own |
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process abstraction model that saves these variables. |
215 |
|
216 |
The easiest way to do this is to create your own scheduling primitive like |
217 |
in the code below, and use it in your threads: |
218 |
|
219 |
sub my_cede { |
220 |
local ($;, ...); |
221 |
Coro::cede; |
222 |
} |
223 |
|
224 |
Another way is to use dynamic winders, see C<Coro::on_enter> and |
225 |
C<Coro::on_leave> for this. |
226 |
|
227 |
=item $prev->transfer ($next) |
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|
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Save the state of the current subroutine in C<$prev> and switch to the |
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thread saved in C<$next>. |
231 |
|
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The "state" of a subroutine includes the scope, i.e. lexical variables and |
233 |
the current execution state (subroutine, stack). |
234 |
|
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=item $state->is_new |
236 |
|
237 |
Returns true iff this Coro::State object is "new", i.e. has never been run |
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yet. Those states basically consist of only the code reference to call and |
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the arguments, but consumes very little other resources. New states will |
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automatically get assigned a perl interpreter when they are transfered to. |
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|
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=item $state->is_destroyed |
243 |
|
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Returns true iff the Coro::State object has been destroyed (by |
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C<cancel>), i.e. it's resources freed because they were C<cancel>'d (or |
246 |
C<terminate>'d). |
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|
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=item $state->cancel |
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|
250 |
Forcefully destructs the given Coro::State. While you can keep the |
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reference, and some memory is still allocated, the Coro::State object is |
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effecticely dead, destructors have been freed, it cannot be transfered to |
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anymore. |
254 |
|
255 |
=item $state->throw ([$scalar]) |
256 |
|
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See L<< Coro->throw >>. |
258 |
|
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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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|
267 |
=item $state->eval ($string) |
268 |
|
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Like C<call>, but eval's the string. Dangerous. |
270 |
|
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=item $state->swap_defsv |
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|
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=item $state->swap_defav |
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|
275 |
Swap the current C<$_> (swap_defsv) or C<@_> (swap_defav) with the |
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equivalent in the saved state of C<$state>. This can be used to give the |
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coro a defined content for C<@_> and C<$_> before transfer'ing to it. |
278 |
|
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=item $state->trace ($flags) |
280 |
|
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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. |
283 |
|
284 |
=item $bytes = $state->rss |
285 |
|
286 |
Returns the memory allocated by the coro (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 |
289 |
use. |
290 |
|
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=item $state->has_cctx |
292 |
|
293 |
Returns whether the state currently uses a cctx/C context. An active |
294 |
state always has a cctx, as well as the main program. Other states only |
295 |
use a cctxts when needed. |
296 |
|
297 |
=item Coro::State::force_cctx |
298 |
|
299 |
Forces the allocation of a C context for the currently running coro |
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(if not already done). Apart from benchmarking there is little point |
301 |
in doing so, however. |
302 |
|
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=item $ncctx = Coro::State::cctx_count |
304 |
|
305 |
Returns the number of C-level coro allocated. If this number is |
306 |
very high (more than a dozen) it might help to identify points of C-level |
307 |
recursion in your code and moving this into a separate coro. |
308 |
|
309 |
=item $nidle = Coro::State::cctx_idle |
310 |
|
311 |
Returns the number of allocated but idle (free for reuse) C level |
312 |
coro. Currently, Coro will limit the number of idle/unused cctxs to |
313 |
8. |
314 |
|
315 |
=item $old = Coro::State::cctx_stacksize [$new_stacksize] |
316 |
|
317 |
Returns the current C stack size and optionally sets the new I<minimum> |
318 |
stack size to C<$new_stacksize> I<long>s. Existing stacks will not |
319 |
be changed, but Coro will try to replace smaller stacks as soon as |
320 |
possible. Any Coro::State that starts to use a stack after this call is |
321 |
guaranteed this minimum stack size. |
322 |
|
323 |
Please note that coros will only need to use a C-level stack if the |
324 |
interpreter recurses or calls a function in a module that calls back into |
325 |
the interpreter, so use of this feature is usually never needed. |
326 |
|
327 |
=item $old = Coro::State::cctx_max_idle [$new_count] |
328 |
|
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Coro caches C contexts that are not in use currently, as creating them |
330 |
from scratch has some overhead. |
331 |
|
332 |
This function returns the current maximum number of idle C contexts and |
333 |
optionally sets the new amount. The count must be at least C<1>, with the |
334 |
default being C<4>. |
335 |
|
336 |
=item @states = Coro::State::list |
337 |
|
338 |
Returns a list of all states currently allocated. |
339 |
|
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=item $was_enabled = Coro::State::enable_times [$enable] |
341 |
|
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Enables/disables/queries the current state of per-thread real and |
343 |
cpu-time gathering. |
344 |
|
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When enabled, the real time and the cpu time (user + system time) |
346 |
spent in each thread is accumulated. If disabled, then the accumulated |
347 |
times will stay as they are (they start at 0). |
348 |
|
349 |
Currently, cpu time is only measured on GNU/Linux systems, all other |
350 |
systems only gather real time. |
351 |
|
352 |
Enabling time profiling slows down thread switching by a factor of 2 to |
353 |
10, depending on platform on hardware. |
354 |
|
355 |
The times will be displayed when running C<Coro::Debug::command "ps">, and |
356 |
cna be queried by calling C<< $state->times >>. |
357 |
|
358 |
=item ($real, $cpu) = $state->times |
359 |
|
360 |
Returns the real time and cpu times spent in the given C<$state>. See |
361 |
C<Coro::State::enable_times> for more info. |
362 |
|
363 |
=item $clone = $state->clone |
364 |
|
365 |
This exciting method takes a Coro::State object and clones it, i.e., it |
366 |
creates a copy. This makes it possible to restore a state more than once, |
367 |
and even return to states that have returned or have been terminated. |
368 |
|
369 |
Since its only known purpose is for intellectual self-gratification, and |
370 |
because it is a difficult piece of code, it is not enabled by default, and |
371 |
not supported. |
372 |
|
373 |
Here are a few little-known facts: First, coros *are* full/true/real |
374 |
continuations. Secondly Coro::State objects (without clone) *are* first |
375 |
class continuations. Thirdly, nobody has ever found a use for the full |
376 |
power of call/cc that isn't better (faster, easier, more efficiently) |
377 |
implemented differently, and nobody has yet found a useful control |
378 |
construct that can't be implemented without it already, just much faster |
379 |
and with fewer resources. And lastly, Scheme's call/cc doesn't support |
380 |
using call/cc to implement threads. |
381 |
|
382 |
Among the games you can play with this is implementing a scheme-like |
383 |
call-with-current-continuation, as the following code does (well, with |
384 |
small differences). |
385 |
|
386 |
# perl disassociates from local lexicals on frame exit, |
387 |
# so use a global variable for return values. |
388 |
my @ret; |
389 |
|
390 |
sub callcc($@) { |
391 |
my ($func, @arg) = @_; |
392 |
|
393 |
my $continuation = new Coro::State; |
394 |
$continuation->transfer (new Coro::State sub { |
395 |
my $escape = sub { |
396 |
@ret = @_; |
397 |
Coro::State->new->transfer ($continuation->clone); |
398 |
}; |
399 |
$escape->($func->($escape, @arg)); |
400 |
}); |
401 |
|
402 |
my @ret_ = @ret; @ret = (); |
403 |
wantarray ? @ret_ : pop @ret_ |
404 |
} |
405 |
|
406 |
Which could be used to implement a loop like this: |
407 |
|
408 |
async { |
409 |
my $n; |
410 |
my $l = callcc sub { $_[0] }; |
411 |
|
412 |
$n++; |
413 |
print "iteration $n\n"; |
414 |
|
415 |
$l->($l) unless $n == 10; |
416 |
}; |
417 |
|
418 |
If you find this confusing, then you already understand the coolness of |
419 |
call/cc: It can turn anything into spaghetti code real fast. |
420 |
|
421 |
Besides, call/cc is much less useful in a Perl-like dynamic language (with |
422 |
references, and its scoping rules) then in, say, scheme. |
423 |
|
424 |
Now, the known limitations of C<clone>: |
425 |
|
426 |
It probably only works on perl 5.10; it cannot clone a coro inside |
427 |
the substition operator (but windows perl can't fork from there either) |
428 |
and some other contexts, and C<abort ()> is the preferred mechanism to |
429 |
signal errors. It cannot clone a state that has a c context attached |
430 |
(implementing clone on the C level is too hard for me to even try), |
431 |
which rules out calling call/cc from the main coro. It cannot |
432 |
clone a context that hasn't even been started yet. It doesn't work with |
433 |
C<-DDEBUGGING> (but what does). It probably also leaks, and sometimes |
434 |
triggers a few assertions inside Coro. Most of these limitations *are* |
435 |
fixable with some effort, but that's pointless just to make a point that |
436 |
it could be done. |
437 |
|
438 |
The current implementation could without doubt be optimised to be a |
439 |
constant-time operation by doing lazy stack copying, if somebody were |
440 |
insane enough to invest the time. |
441 |
|
442 |
=cut |
443 |
|
444 |
# used by Coro::Debug only atm. |
445 |
sub debug_desc { |
446 |
$_[0]{desc} |
447 |
} |
448 |
|
449 |
# for very deep reasons, we must initialise $Coro::main here. |
450 |
|
451 |
{ |
452 |
package Coro; |
453 |
|
454 |
our $main; # main coro |
455 |
our $current; # current coro |
456 |
|
457 |
$main = Coro::new Coro::; |
458 |
|
459 |
$main->{desc} = "[main::]"; |
460 |
|
461 |
# maybe some other module used Coro::Specific before... |
462 |
$main->{_specific} = $current->{_specific} |
463 |
if $current; |
464 |
|
465 |
_set_current $main; |
466 |
} |
467 |
|
468 |
1; |
469 |
|
470 |
=back |
471 |
|
472 |
=head1 BUGS |
473 |
|
474 |
This module is not thread-safe. You must only ever use this module from |
475 |
the same thread (this requirement might be removed in the future). |
476 |
|
477 |
=head1 SEE ALSO |
478 |
|
479 |
L<Coro>. |
480 |
|
481 |
=head1 AUTHOR |
482 |
|
483 |
Marc Lehmann <schmorp@schmorp.de> |
484 |
http://home.schmorp.de/ |
485 |
|
486 |
=cut |
487 |
|