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Branched from the monolithic repo, Qt master branch, at commit
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1 files changed, 363 insertions, 0 deletions

diff --git a/src/corelib/kernel/qsystemsemaphore.cpp b/src/corelib/kernel/qsystemsemaphore.cpp
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+/****************************************************************************
+**
+** Copyright (C) 2011 Nokia Corporation and/or its subsidiary(-ies).
+** All rights reserved.
+** Contact: Nokia Corporation (qt-info@nokia.com)
+**
+** This file is part of the QtCore 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 Technology Preview License Agreement accompanying
+** this package.
+**
+** 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 1.1, included in the file LGPL_EXCEPTION.txt in this package.
+**
+** If you have questions regarding the use of this file, please contact
+** Nokia at qt-info@nokia.com.
+**
+**
+**
+**
+**
+**
+**
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+#include "qsystemsemaphore.h"
+#include "qsystemsemaphore_p.h"
+#include <qglobal.h>
+
+QT_BEGIN_NAMESPACE
+
+#ifndef QT_NO_SYSTEMSEMAPHORE
+
+/*!
+  \class QSystemSemaphore
+  \since 4.4
+
+  \brief The QSystemSemaphore class provides a general counting system semaphore.
+
+  A semaphore is a generalization of a mutex. While a mutex can be
+  locked only once, a semaphore can be acquired multiple times.
+  Typically, a semaphore is used to protect a certain number of
+  identical resources.
+
+  Like its lighter counterpart QSemaphore, a QSystemSemaphore can be
+  accessed from multiple \l {QThread} {threads}. Unlike QSemaphore, a
+  QSystemSemaphore can also be accessed from multiple \l {QProcess}
+  {processes}. This means QSystemSemaphore is a much heavier class, so
+  if your application doesn't need to access your semaphores across
+  multiple processes, you will probably want to use QSemaphore.
+
+  Semaphores support two fundamental operations, acquire() and release():
+
+  acquire() tries to acquire one resource. If there isn't a resource
+  available, the call blocks until a resource becomes available. Then
+  the resource is acquired and the call returns.
+
+  release() releases one resource so it can be acquired by another
+  process. The function can also be called with a parameter n > 1,
+  which releases n resources.
+
+  A system semaphore is created with a string key that other processes
+  can use to use the same semaphore.
+
+  Example: Create a system semaphore
+  \snippet doc/src/snippets/code/src_corelib_kernel_qsystemsemaphore.cpp 0
+
+  A typical application of system semaphores is for controlling access
+  to a circular buffer shared by a producer process and a consumer
+  processes.
+
+  \section1 Platform-Specific Behavior
+
+  When using this class, be aware of the following platform
+  differences:
+
+  \bold{Windows:} QSystemSemaphore does not own its underlying system
+  semaphore. Windows owns it. This means that when all instances of
+  QSystemSemaphore for a particular key have been destroyed, either by
+  having their destructors called, or because one or more processes
+  crash, Windows removes the underlying system semaphore.
+
+  \bold{Unix:}
+
+  \list
+  \o QSystemSemaphore owns the underlying system semaphore
+  in Unix systems. This means that the last process having an instance of
+  QSystemSemaphore for a particular key must remove the underlying
+  system semaphore in its destructor. If the last process crashes
+  without running the QSystemSemaphore destructor, Unix does not
+  automatically remove the underlying system semaphore, and the
+  semaphore survives the crash. A subsequent process that constructs a
+  QSystemSemaphore with the same key will then be given the existing
+  system semaphore. In that case, if the QSystemSemaphore constructor
+  has specified its \l {QSystemSemaphore::AccessMode} {access mode} as
+  \l {QSystemSemaphore::} {Open}, its initial resource count will not
+  be reset to the one provided but remain set to the value it received
+  in the crashed process. To protect against this, the first process
+  to create a semaphore for a particular key (usually a server), must
+  pass its \l {QSystemSemaphore::AccessMode} {access mode} as \l
+  {QSystemSemaphore::} {Create}, which will force Unix to reset the
+  resource count in the underlying system semaphore.
+
+  \o When a process using QSystemSemaphore terminates for
+  any reason, Unix automatically reverses the effect of all acquire
+  operations that were not released. Thus if the process acquires a
+  resource and then exits without releasing it, Unix will release that
+  resource.
+
+  \o Symbian: QSystemSemaphore behaves the same as Windows semaphores.
+  In other words, the operating system owns the semaphore and ignores
+  QSystemSemaphore::AccessMode.
+
+  \endlist
+
+  \sa QSharedMemory, QSemaphore
+ */
+
+/*!
+  Requests a system semaphore for the specified \a key. The parameters
+  \a initialValue and \a mode are used according to the following
+  rules, which are system dependent.
+
+  In Unix, if the \a mode is \l {QSystemSemaphore::} {Open} and the
+  system already has a semaphore identified by \a key, that semaphore
+  is used, and the semaphore's resource count is not changed, i.e., \a
+  initialValue is ignored. But if the system does not already have a
+  semaphore identified by \a key, it creates a new semaphore for that
+  key and sets its resource count to \a initialValue.
+
+  In Unix, if the \a mode is \l {QSystemSemaphore::} {Create} and the
+  system already has a semaphore identified by \a key, that semaphore
+  is used, and its resource count is set to \a initialValue. If the
+  system does not already have a semaphore identified by \a key, it
+  creates a new semaphore for that key and sets its resource count to
+  \a initialValue.
+
+  In Windows and in Symbian, \a mode is ignored, and the system always tries to
+  create a semaphore for the specified \a key. If the system does not
+  already have a semaphore identified as \a key, it creates the
+  semaphore and sets its resource count to \a initialValue. But if the
+  system already has a semaphore identified as \a key it uses that
+  semaphore and ignores \a initialValue.
+
+  The \l {QSystemSemaphore::AccessMode} {mode} parameter is only used
+  in Unix systems to handle the case where a semaphore survives a
+  process crash. In that case, the next process to allocate a
+  semaphore with the same \a key will get the semaphore that survived
+  the crash, and unless \a mode is \l {QSystemSemaphore::} {Create},
+  the resource count will not be reset to \a initialValue but will
+  retain the initial value it had been given by the crashed process.
+
+  \sa acquire(), key()
+ */
+QSystemSemaphore::QSystemSemaphore(const QString &key, int initialValue, AccessMode mode)
+    : d(new QSystemSemaphorePrivate)
+{
+    setKey(key, initialValue, mode);
+}
+
+/*!
+  The destructor destroys the QSystemSemaphore object, but the
+  underlying system semaphore is not removed from the system unless
+  this instance of QSystemSemaphore is the last one existing for that
+  system semaphore.
+
+  Two important side effects of the destructor depend on the system.
+  In Windows, if acquire() has been called for this semaphore but not
+  release(), release() will not be called by the destructor, nor will
+  the resource be released when the process exits normally. This would
+  be a program bug which could be the cause of a deadlock in another
+  process trying to acquire the same resource. In Unix, acquired
+  resources that are not released before the destructor is called are
+  automatically released when the process exits.
+*/
+QSystemSemaphore::~QSystemSemaphore()
+{
+    d->cleanHandle();
+}
+
+/*!
+  \enum QSystemSemaphore::AccessMode
+
+  This enum is used by the constructor and setKey(). Its purpose is to
+  enable handling the problem in Unix implementations of semaphores
+  that survive a crash. In Unix, when a semaphore survives a crash, we
+  need a way to force it to reset its resource count, when the system
+  reuses the semaphore. In Windows and in Symbian, where semaphores can't survive a
+  crash, this enum has no effect.
+
+  \value Open If the semaphore already exists, its initial resource
+  count is not reset. If the semaphore does not already exist, it is
+  created and its initial resource count set.
+
+  \value Create QSystemSemaphore takes ownership of the semaphore and
+  sets its resource count to the requested value, regardless of
+  whether the semaphore already exists by having survived a crash.
+  This value should be passed to the constructor, when the first
+  semaphore for a particular key is constructed and you know that if
+  the semaphore already exists it could only be because of a crash. In
+  Windows and in Symbian, where a semaphore can't survive a crash, Create and Open
+  have the same behavior.
+*/
+
+/*!
+  This function works the same as the constructor. It reconstructs
+  this QSystemSemaphore object. If the new \a key is different from
+  the old key, calling this function is like calling the destructor of
+  the semaphore with the old key, then calling the constructor to
+  create a new semaphore with the new \a key. The \a initialValue and
+  \a mode parameters are as defined for the constructor.
+
+  \sa QSystemSemaphore(), key()
+ */
+void QSystemSemaphore::setKey(const QString &key, int initialValue, AccessMode mode)
+{
+    if (key == d->key && mode == Open)
+        return;
+    d->error = NoError;
+    d->errorString = QString();
+#if !defined(Q_OS_WIN) && !defined(Q_OS_SYMBIAN)
+    // optimization to not destroy/create the file & semaphore
+    if (key == d->key && mode == Create && d->createdSemaphore && d->createdFile) {
+        d->initialValue = initialValue;
+        d->unix_key = -1;
+        d->handle(mode);
+        return;
+    }
+#endif
+    d->cleanHandle();
+    d->key = key;
+    d->initialValue = initialValue;
+    // cache the file name so it doesn't have to be generated all the time.
+    d->fileName = d->makeKeyFileName();
+    d->handle(mode);
+}
+
+/*!
+  Returns the key assigned to this system semaphore. The key is the
+  name by which the semaphore can be accessed from other processes.
+
+  \sa setKey()
+ */
+QString QSystemSemaphore::key() const
+{
+    return d->key;
+}
+
+/*!
+  Acquires one of the resources guarded by this semaphore, if there is
+  one available, and returns true. If all the resources guarded by this
+  semaphore have already been acquired, the call blocks until one of
+  them is released by another process or thread having a semaphore
+  with the same key.
+
+  If false is returned, a system error has occurred. Call error()
+  to get a value of QSystemSemaphore::SystemSemaphoreError that
+  indicates which error occurred.
+
+  \sa release()
+ */
+bool QSystemSemaphore::acquire()
+{
+    return d->modifySemaphore(-1);
+}
+
+/*!
+  Releases \a n resources guarded by the semaphore. Returns true
+  unless there is a system error.
+
+  Example: Create a system semaphore having five resources; acquire
+  them all and then release them all.
+
+  \snippet doc/src/snippets/code/src_corelib_kernel_qsystemsemaphore.cpp 1
+
+  This function can also "create" resources. For example, immediately
+  following the sequence of statements above, suppose we add the
+  statement:
+
+  \snippet doc/src/snippets/code/src_corelib_kernel_qsystemsemaphore.cpp 2
+
+  Ten new resources are now guarded by the semaphore, in addition to
+  the five that already existed. You would not normally use this
+  function to create more resources.
+
+  \sa acquire()
+ */
+bool QSystemSemaphore::release(int n)
+{
+    if (n == 0)
+        return true;
+    if (n < 0) {
+        qWarning("QSystemSemaphore::release: n is negative.");
+        return false;
+    }
+    return d->modifySemaphore(n);
+}
+
+/*!
+  Returns a value indicating whether an error occurred, and, if so,
+  which error it was.
+
+  \sa errorString()
+ */
+QSystemSemaphore::SystemSemaphoreError QSystemSemaphore::error() const
+{
+    return d->error;
+}
+
+/*!
+  \enum QSystemSemaphore::SystemSemaphoreError
+
+  \value NoError No error occurred.
+
+  \value PermissionDenied The operation failed because the caller
+  didn't have the required permissions.
+
+  \value KeyError The operation failed because of an invalid key.
+
+  \value AlreadyExists The operation failed because a system
+  semaphore with the specified key already existed.
+
+  \value NotFound The operation failed because a system semaphore
+  with the specified key could not be found.
+
+  \value OutOfResources The operation failed because there was
+  not enough memory available to fill the request.
+
+  \value UnknownError Something else happened and it was bad.
+*/
+
+/*!
+  Returns a text description of the last error that occurred. If
+  error() returns an \l {QSystemSemaphore::SystemSemaphoreError} {error
+  value}, call this function to get a text string that describes the
+  error.
+
+  \sa error()
+ */
+QString QSystemSemaphore::errorString() const
+{
+    return d->errorString;
+}
+
+#endif // QT_NO_SYSTEMSEMAPHORE
+
+QT_END_NAMESPACE