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