summaryrefslogtreecommitdiffstats
path: root/src/qt3support/tools/q3semaphore.cpp
blob: 76a6fcd233e8d441f2c0584849b49be48f6bf601 (plain)
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
242
243
244
245
246
247
248
249
250
251
252
253
254
/****************************************************************************
**
** Copyright (C) 2009 Nokia Corporation and/or its subsidiary(-ies).
** Contact: Nokia Corporation (qt-info@nokia.com)
**
** This file is part of the Qt3Support module of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:LGPL$
** No Commercial Usage
** This file contains pre-release code and may not be distributed.
** You may use this file in accordance with the terms and conditions
** contained in the either Technology Preview License Agreement or the
** Beta Release License Agreement.
**
** 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, Nokia gives you certain
** additional rights. These rights are described in the Nokia Qt LGPL
** Exception version 1.0, 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.
**
** If you are unsure which license is appropriate for your use, please
** contact the sales department at http://www.qtsoftware.com/contact.
** $QT_END_LICENSE$
**
****************************************************************************/

#include "q3semaphore.h"

#include "qmutex.h"
#include "qwaitcondition.h"

QT_BEGIN_NAMESPACE

/*!
    \class Q3Semaphore
    \threadsafe
    \brief The Q3Semaphore class provides a robust integer semaphore.

    \compat

    A Q3Semaphore can be used to serialize thread execution, in a
    similar way to a QMutex. A semaphore differs from a mutex, in
    that a semaphore can be accessed by more than one thread at a
    time.

    For example, suppose we have an application that stores data in a
    large tree structure. The application creates 10 threads
    (commonly called a thread pool) to perform searches on the tree.
    When the application searches the tree for some piece of data, it
    uses one thread per base node to do the searching. A semaphore
    could be used to make sure that two threads don't try to search
    the same branch of the tree at the same time.

    A non-computing example of a semaphore would be dining at a
    restaurant. A semaphore is initialized to have a maximum count
    equal to the number of chairs in the restaurant. As people
    arrive, they want a seat. As seats are filled, the semaphore is
    accessed, once per person. As people leave, the access is
    released, 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).

    When a semaphore is created it is given a number which is the
    maximum number of concurrent accesses it will permit. This amount
    may be changed using operator++(), operator--(), operator+=() and
    operator-=(). The number of accesses allowed is retrieved with
    available(), and the total number with total(). Note that the
    incrementing functions will block if there aren't enough available
    accesses. Use tryAccess() if you want to acquire accesses without
    blocking.
*/

#ifdef max
#undef max
#endif

class Q3SemaphorePrivate {
public:
    Q3SemaphorePrivate(int);

    QMutex mutex;
    QWaitCondition cond;

    int value, max;
};


Q3SemaphorePrivate::Q3SemaphorePrivate(int m)
    : value(0), max(m)
{
}


/*!
    Creates a new semaphore. The semaphore can be concurrently
    accessed at most \a maxcount times.
*/
Q3Semaphore::Q3Semaphore(int maxcount)
{
    d = new Q3SemaphorePrivate(maxcount);
}


/*!
    Destroys the semaphore.

    \warning If you destroy a semaphore that has accesses in use the
    resultant behavior is undefined.
*/
Q3Semaphore::~Q3Semaphore()
{
    delete d;
}


/*!
    Postfix ++ operator.

    Try to get access to the semaphore. If \l available() == 0, this
    call will block until it can get access, i.e. until available() \>
    0.
*/
int Q3Semaphore::operator++(int)
{
    QMutexLocker locker(&d->mutex);
    while (d->value >= d->max)
        d->cond.wait(locker.mutex());

    ++d->value;
    if (d->value > d->max)
        d->value = d->max;

    return d->value;
}


/*!
    Postfix -- operator.

    Release access of the semaphore. This wakes all threads waiting
    for access to the semaphore.
*/
int Q3Semaphore::operator--(int)
{
    QMutexLocker locker(&d->mutex);

    --d->value;
    if (d->value < 0)
        d->value = 0;

    d->cond.wakeAll();

    return d->value;
}


/*!
    Try to get access to the semaphore. If \l available() \< \a n, this
    call will block until it can get all the accesses it wants, i.e.
    until available() \>= \a n.
*/
int Q3Semaphore::operator+=(int n)
{
    QMutexLocker locker(&d->mutex);

    if (n < 0 || n > d->max) {
        qWarning("Q3Semaphore::operator+=: parameter %d out of range", n);
        n = n < 0 ? 0 : d->max;
    }

    while (d->value + n > d->max)
        d->cond.wait(locker.mutex());

    d->value += n;

    return d->value;
}


/*!
    Release \a n accesses to the semaphore.
*/
int Q3Semaphore::operator-=(int n)
{
    QMutexLocker locker(&d->mutex);

    if (n < 0 || n > d->value) {
        qWarning("Q3Semaphore::operator-=: parameter %d out of range", n);
        n = n < 0 ? 0 : d->value;
    }

    d->value -= n;
    d->cond.wakeAll();

    return d->value;
}


/*!
    Returns the number of accesses currently available to the
    semaphore.
*/
int Q3Semaphore::available() const
{
    QMutexLocker locker(&d->mutex);
    return d->max - d->value;
}


/*!
    Returns the total number of accesses to the semaphore.
*/
int Q3Semaphore::total() const
{
    QMutexLocker locker(&d->mutex);
    return d->max;
}


/*!
    Try to get access to the semaphore. If \l available() \< \a n, this
    function will return false immediately. If \l available() \>= \a n,
    this function will take \a n accesses and return true. This
    function does \e not block.
*/
bool Q3Semaphore::tryAccess(int n)
{
    QMutexLocker locker(&d->mutex);

    if (d->value + n > d->max)
        return false;

    d->value += n;

    return true;
}

QT_END_NAMESPACE