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/* POSIX once-only control.
Copyright (C) 2019-2025 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>, 2019. */
#include <config.h>
/* Specification. */
#include <pthread.h>
#if (defined _WIN32 && ! defined __CYGWIN__) && USE_WINDOWS_THREADS
# include "windows-once.h"
#endif
#if (defined _WIN32 && ! defined __CYGWIN__) && USE_WINDOWS_THREADS
/* Use Windows threads. */
int
pthread_once (pthread_once_t *once_control, void (*initfunction) (void))
{
glwthread_once (once_control, initfunction);
return 0;
}
#elif HAVE_PTHREAD_H
/* Provide workarounds for POSIX threads. */
# if defined __CYGWIN__
# include <stdlib.h>
int
pthread_once (pthread_once_t *once_control, void (*initfunction) (void))
{
/* In this implementation, we reuse the type
typedef struct { pthread_mutex_t mutex; int state; } pthread_once_t;
#define PTHREAD_ONCE_INIT { PTHREAD_MUTEX_INITIALIZER, 0 }
while assigning the following meaning to the state:
state = (<number of waiting threads> << 16) + <1 if done>
In other words:
state = { unsigned int num_threads : 16; unsigned int done : 16; }
*/
struct actual_state
{
_Atomic unsigned short num_threads;
/* done == 0: initial state
done == 1: initfunction executed, lock still active
done == 2: initfunction executed, lock no longer usable */
_Atomic unsigned short done;
};
struct actual_state *state_p = (struct actual_state *) &once_control->state;
/* This test is not necessary. It's only an optimization, to establish
a fast path for the common case that the 'done' word is already > 0. */
if (state_p->done == 0)
{
/* Increment num_threads (atomically), to indicate that this thread will
possibly take the lock. */
state_p->num_threads += 1;
/* Test the 'done' word. */
if (state_p->done == 0)
{
/* The 'done' word is still zero. Now take the lock. */
pthread_mutex_lock (&once_control->mutex);
/* Test the 'done' word again. */
if (state_p->done == 0)
{
/* Execute the initfunction. */
(*initfunction) ();
/* Set the 'done' word to 1 (atomically). */
state_p->done = 1;
}
/* Now the 'done' word is 1. Release the lock. */
pthread_mutex_unlock (&once_control->mutex);
}
/* Here, done is > 0. */
/* Decrement num_threads (atomically). */
if ((state_p->num_threads -= 1) == 0)
{
/* num_threads is now zero, and done is > 0.
No other thread will need to use the lock.
We can therefore destroy the lock, to free resources. */
if (__sync_bool_compare_and_swap (&state_p->done, 1, 2))
pthread_mutex_destroy (&once_control->mutex);
}
}
/* 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 'done' word, once > 0, stays > 0 (since it is never assigned 0).
* The 'done' word is changed from == 0 to > 0 only while the lock
is taken. Therefore, only the first thread that succeeds in taking
the lock executes the initfunction and sets the 'done' word to a
value > 0; the other threads that take the lock do no side effects
between taking and releasing the lock.
* The 'done' word does not change any more once it is 2.
Therefore, it can be changed from 1 to 2 only once.
* pthread_mutex_destroy gets invoked right after 'done' has been changed
from 1 to 2. Therefore, pthread_mutex_destroy gets invoked only once.
* After a moment where num_threads was 0 and done was > 0, no thread can
reach the pthread_mutex_lock invocation. Proof:
- At such a moment, no thread is in the code range between
state_p->num_threads += 1
and
state_p->num_threads -= 1
- After such a moment, some thread can increment num_threads, but from
there they cannot reach the pthread_mutex_lock invocation, because the
if (state_p->done == 0)
test prevents that.
* From this it follows that:
- pthread_mutex_destroy cannot be executed while the lock is taken
(because pthread_mutex_destroy is only executed after a moment where
num_threads was 0 and done was > 0).
- Once pthread_mutex_destroy has been executed, the lock is not used any
more.
*/
return 0;
}
# endif
#else
/* Provide a dummy implementation for single-threaded applications. */
int
pthread_once (pthread_once_t *once_control, void (*initfunction) (void))
{
if (*once_control == 0)
{
*once_control = ~ 0;
initfunction ();
}
return 0;
}
#endif
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