105 lines
4.7 KiB
C
105 lines
4.7 KiB
C
/* Once-only control (native Windows implementation).
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Copyright (C) 2005-2024 Free Software Foundation, Inc.
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This file is free software: you can redistribute it and/or modify
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it under the terms of the GNU Lesser General Public License as
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published by the Free Software Foundation; either version 2.1 of the
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License, or (at your option) any later version.
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This file is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public License
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along with this program. If not, see <https://www.gnu.org/licenses/>. */
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/* Written by Bruno Haible <bruno@clisp.org>, 2005.
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Based on GCC's gthr-win32.h. */
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#include <config.h>
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/* Specification. */
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#include "windows-once.h"
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#include <stdlib.h>
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void
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glwthread_once (glwthread_once_t *once_control, void (*initfunction) (void))
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{
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if (once_control->inited <= 0)
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{
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InterlockedIncrement (&once_control->num_threads);
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/* If once_control->started is == -1, set it to 0. */
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if (InterlockedCompareExchange (&once_control->started, 0, -1) < 0)
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{
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/* This thread is the first one to come to this once_control. */
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InitializeCriticalSection (&once_control->lock);
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EnterCriticalSection (&once_control->lock);
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once_control->inited = 0;
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initfunction ();
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once_control->inited = 1;
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LeaveCriticalSection (&once_control->lock);
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}
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else
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{
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/* Some other thread has already started the initialization.
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Yield the CPU while waiting for the other thread to finish
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initializing and taking the lock. */
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while (once_control->inited < 0)
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Sleep (0);
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if (once_control->inited <= 0)
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{
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/* Take the lock. This blocks until the other thread has
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finished calling the initfunction. */
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EnterCriticalSection (&once_control->lock);
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LeaveCriticalSection (&once_control->lock);
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if (!(once_control->inited > 0))
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abort ();
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}
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}
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/* Here once_control->started == 0 and once_control->inited > 0. */
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if (InterlockedDecrement (&once_control->num_threads) == 0)
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/* once_control->num_threads is now zero, and
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once_control->started == 0 and once_control->inited > 0.
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No other thread will need to use the lock.
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We can therefore destroy the lock, to free resources. */
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/* If once_control->inited is == 1, set it to 2. */
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if (InterlockedCompareExchange (&once_control->inited, 2, 1) == 1)
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DeleteCriticalSection (&once_control->lock);
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}
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/* Proof of correctness:
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* num_threads is incremented and then decremented by some threads.
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Therefore, num_threads always stays >= 0, and is == 0 at the end.
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* The first thread to go through the once_control->started fence
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initializes the lock and moves inited from <= 0 to > 0. The other
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threads don't move inited from <= 0 to > 0.
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* started, once == 0, stays == 0.
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* inited, once > 0, stays > 0 (since at the place where it is assigned 0,
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it cannot be > 0).
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* inited does not change any more once it is 2.
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Therefore, it can be changed from 1 to 2 only once.
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* DeleteCriticalSection gets invoked right after inited has been changed
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from 1 to 2. Therefore, DeleteCriticalSection gets invoked only once.
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* After a moment where num_threads was 0 and started was 0 and
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inited was > 0, no thread can reach an InitializeCriticalSection or
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EnterCriticalSection invocation. Proof:
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- At such a moment, no thread is in the code range between
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InterlockedIncrement (&once_control->num_threads)
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and
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InterlockedDecrement (&once_control->num_threads)
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- After such a moment, some thread can increment num_threads, but from
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there they cannot reach the InitializeCriticalSection invocation,
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because the once_control->started test prevents that, and they cannot
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reach the EnterCriticalSection invocation in the other branch because
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the
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if (once_control->inited <= 0)
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test prevents that.
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* From this it follows that:
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- DeleteCriticalSection cannot be executed while the lock is taken
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(because DeleteCriticalSection is only executed after a moment where
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num_threads was 0 and started was 0 and inited was > 0).
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- Once DeleteCriticalSection has been executed, the lock is not used any
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more.
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*/
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}
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