1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
|
/****************************************************************************
**
** Copyright (C) 2013 Digia Plc and/or its subsidiary(-ies).
** Contact: http://www.qt-project.org/legal
**
** This file is part of the QtCore module of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:LGPL$
** Commercial License Usage
** Licensees holding valid commercial Qt licenses may use this file in
** accordance with the commercial license agreement provided with the
** Software or, alternatively, in accordance with the terms contained in
** a written agreement between you and Digia. For licensing terms and
** conditions see http://qt.digia.com/licensing. For further information
** use the contact form at http://qt.digia.com/contact-us.
**
** GNU Lesser General Public License Usage
** Alternatively, this file may be used under the terms of the GNU Lesser
** General Public License version 2.1 as published by the Free Software
** Foundation and appearing in the file LICENSE.LGPL included in the
** packaging of this file. Please review the following information to
** ensure the GNU Lesser General Public License version 2.1 requirements
** will be met: http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html.
**
** In addition, as a special exception, Digia gives you certain additional
** rights. These rights are described in the Digia Qt LGPL Exception
** version 1.1, included in the file LGPL_EXCEPTION.txt in this package.
**
** GNU General Public License Usage
** Alternatively, this file may be used under the terms of the GNU
** General Public License version 3.0 as published by the Free Software
** Foundation and appearing in the file LICENSE.GPL included in the
** packaging of this file. Please review the following information to
** ensure the GNU General Public License version 3.0 requirements will be
** met: http://www.gnu.org/copyleft/gpl.html.
**
**
** $QT_END_LICENSE$
**
****************************************************************************/
#include "qsemaphore.h"
#ifndef QT_NO_THREAD
#include "qmutex.h"
#include "qwaitcondition.h"
#include "qelapsedtimer.h"
#include "qdatetime.h"
QT_BEGIN_NAMESPACE
/*!
\class QSemaphore
\inmodule QtCore
\brief The QSemaphore class provides a general counting semaphore.
\threadsafe
\ingroup thread
A semaphore is a generalization of a mutex. While a mutex can
only be locked once, it's possible to acquire a semaphore
multiple times. Semaphores are typically used to protect a
certain number of identical resources.
Semaphores support two fundamental operations, acquire() and
release():
\list
\li acquire(\e{n}) tries to acquire \e n resources. If there aren't
that many resources available, the call will block until this
is the case.
\li release(\e{n}) releases \e n resources.
\endlist
There's also a tryAcquire() function that returns immediately if
it cannot acquire the resources, and an available() function that
returns the number of available resources at any time.
Example:
\snippet code/src_corelib_thread_qsemaphore.cpp 0
A typical application of semaphores is for controlling access to
a circular buffer shared by a producer thread and a consumer
thread. The \l{threads/semaphores}{Semaphores} example shows how
to use QSemaphore to solve that problem.
A non-computing example of a semaphore would be dining at a
restaurant. A semaphore is initialized with the number of chairs
in the restaurant. As people arrive, they want a seat. As seats
are filled, available() is decremented. As people leave, the
available() is incremented, allowing more people to enter. If a
party of 10 people want to be seated, but there are only 9 seats,
those 10 people will wait, but a party of 4 people would be
seated (taking the available seats to 5, making the party of 10
people wait longer).
\sa QMutex, QWaitCondition, QThread, {Semaphores Example}
*/
class QSemaphorePrivate {
public:
inline QSemaphorePrivate(int n) : avail(n) { }
QMutex mutex;
QWaitCondition cond;
int avail;
};
/*!
Creates a new semaphore and initializes the number of resources
it guards to \a n (by default, 0).
\sa release(), available()
*/
QSemaphore::QSemaphore(int n)
{
Q_ASSERT_X(n >= 0, "QSemaphore", "parameter 'n' must be non-negative");
d = new QSemaphorePrivate(n);
}
/*!
Destroys the semaphore.
\warning Destroying a semaphore that is in use may result in
undefined behavior.
*/
QSemaphore::~QSemaphore()
{ delete d; }
/*!
Tries to acquire \c n resources guarded by the semaphore. If \a n
> available(), this call will block until enough resources are
available.
\sa release(), available(), tryAcquire()
*/
void QSemaphore::acquire(int n)
{
Q_ASSERT_X(n >= 0, "QSemaphore::acquire", "parameter 'n' must be non-negative");
QMutexLocker locker(&d->mutex);
while (n > d->avail)
d->cond.wait(locker.mutex());
d->avail -= n;
}
/*!
Releases \a n resources guarded by the semaphore.
This function can be used to "create" resources as well. For
example:
\snippet code/src_corelib_thread_qsemaphore.cpp 1
\sa acquire(), available()
*/
void QSemaphore::release(int n)
{
Q_ASSERT_X(n >= 0, "QSemaphore::release", "parameter 'n' must be non-negative");
QMutexLocker locker(&d->mutex);
d->avail += n;
d->cond.wakeAll();
}
/*!
Returns the number of resources currently available to the
semaphore. This number can never be negative.
\sa acquire(), release()
*/
int QSemaphore::available() const
{
QMutexLocker locker(&d->mutex);
return d->avail;
}
/*!
Tries to acquire \c n resources guarded by the semaphore and
returns true on success. If available() < \a n, this call
immediately returns false without acquiring any resources.
Example:
\snippet code/src_corelib_thread_qsemaphore.cpp 2
\sa acquire()
*/
bool QSemaphore::tryAcquire(int n)
{
Q_ASSERT_X(n >= 0, "QSemaphore::tryAcquire", "parameter 'n' must be non-negative");
QMutexLocker locker(&d->mutex);
if (n > d->avail)
return false;
d->avail -= n;
return true;
}
/*!
Tries to acquire \c n resources guarded by the semaphore and
returns true on success. If available() < \a n, this call will
wait for at most \a timeout milliseconds for resources to become
available.
Note: Passing a negative number as the \a timeout is equivalent to
calling acquire(), i.e. this function will wait forever for
resources to become available if \a timeout is negative.
Example:
\snippet code/src_corelib_thread_qsemaphore.cpp 3
\sa acquire()
*/
bool QSemaphore::tryAcquire(int n, int timeout)
{
Q_ASSERT_X(n >= 0, "QSemaphore::tryAcquire", "parameter 'n' must be non-negative");
QMutexLocker locker(&d->mutex);
if (timeout < 0) {
while (n > d->avail)
d->cond.wait(locker.mutex());
} else {
QElapsedTimer timer;
timer.start();
while (n > d->avail) {
const qint64 elapsed = timer.elapsed();
if (timeout - elapsed <= 0
|| !d->cond.wait(locker.mutex(), timeout - elapsed))
return false;
}
}
d->avail -= n;
return true;
}
QT_END_NAMESPACE
#endif // QT_NO_THREAD
|
