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    1 /****************************************************************************
    2 **
    3 ** Copyright (C) 2013 Digia Plc and/or its subsidiary(-ies).
    4 ** Contact: http://www.qt-project.org/legal
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    6 ** This file is part of the Qt Solutions component.
    7 **
    8 ** $QT_BEGIN_LICENSE:BSD$
    9 ** You may use this file under the terms of the BSD license as follows:
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   40 
   41 #include "qtlockedfile.h"
   42 
   43 /*!
   44     \class QtLockedFile
   45 
   46     \brief The QtLockedFile class extends QFile with advisory locking
   47     functions.
   48 
   49     A file may be locked in read or write mode. Multiple instances of
   50     \e QtLockedFile, created in multiple processes running on the same
   51     machine, may have a file locked in read mode. Exactly one instance
   52     may have it locked in write mode. A read and a write lock cannot
   53     exist simultaneously on the same file.
   54 
   55     The file locks are advisory. This means that nothing prevents
   56     another process from manipulating a locked file using QFile or
   57     file system functions offered by the OS. Serialization is only
   58     guaranteed if all processes that access the file use
   59     QLockedFile. Also, while holding a lock on a file, a process
   60     must not open the same file again (through any API), or locks
   61     can be unexpectedly lost.
   62 
   63     The lock provided by an instance of \e QtLockedFile is released
   64     whenever the program terminates. This is true even when the
   65     program crashes and no destructors are called.
   66 */
   67 
   68 /*! \enum QtLockedFile::LockMode
   69 
   70     This enum describes the available lock modes.
   71 
   72     \value ReadLock A read lock.
   73     \value WriteLock A write lock.
   74     \value NoLock Neither a read lock nor a write lock.
   75 */
   76 
   77 /*!
   78     Constructs an unlocked \e QtLockedFile object. This constructor
   79     behaves in the same way as \e QFile::QFile().
   80 
   81     \sa QFile::QFile()
   82 */
   83 QtLockedFile::QtLockedFile()
   84     : QFile()
   85 {
   86 #ifdef Q_OS_WIN
   87     wmutex = 0;
   88     rmutex = 0;
   89 #endif
   90     m_lock_mode = NoLock;
   91 }
   92 
   93 /*!
   94     Constructs an unlocked QtLockedFile object with file \a name. This
   95     constructor behaves in the same way as \e QFile::QFile(const
   96     QString&).
   97 
   98     \sa QFile::QFile()
   99 */
  100 QtLockedFile::QtLockedFile(const QString& name)
  101     : QFile(name)
  102 {
  103 #ifdef Q_OS_WIN
  104     wmutex = 0;
  105     rmutex = 0;
  106 #endif
  107     m_lock_mode = NoLock;
  108 }
  109 
  110 /*!
  111   Opens the file in OpenMode \a mode.
  112 
  113   This is identical to QFile::open(), with the one exception that the
  114   Truncate mode flag is disallowed. Truncation would conflict with the
  115   advisory file locking, since the file would be modified before the
  116   write lock is obtained. If truncation is required, use resize(0)
  117   after obtaining the write lock.
  118 
  119   Returns true if successful; otherwise false.
  120 
  121   \sa QFile::open(), QFile::resize()
  122 */
  123 bool QtLockedFile::open(OpenMode mode)
  124 {
  125     if (mode & QIODevice::Truncate)
  126     {
  127         qWarning("QtLockedFile::open(): Truncate mode not allowed.");
  128         return false;
  129     }
  130     return QFile::open(mode);
  131 }
  132 
  133 /*!
  134     Returns \e true if this object has a in read or write lock;
  135     otherwise returns \e false.
  136 
  137     \sa lockMode()
  138 */
  139 bool QtLockedFile::isLocked() const
  140 {
  141     return m_lock_mode != NoLock;
  142 }
  143 
  144 /*!
  145     Returns the type of lock currently held by this object, or \e
  146     QtLockedFile::NoLock.
  147 
  148     \sa isLocked()
  149 */
  150 QtLockedFile::LockMode QtLockedFile::lockMode() const
  151 {
  152     return m_lock_mode;
  153 }
  154 
  155 /*!
  156     \fn bool QtLockedFile::lock(LockMode mode, bool block = true)
  157 
  158     Obtains a lock of type \a mode. The file must be opened before it
  159     can be locked.
  160 
  161     If \a block is true, this function will block until the lock is
  162     aquired. If \a block is false, this function returns \e false
  163     immediately if the lock cannot be aquired.
  164 
  165     If this object already has a lock of type \a mode, this function
  166     returns \e true immediately. If this object has a lock of a
  167     different type than \a mode, the lock is first released and then a
  168     new lock is obtained.
  169 
  170     This function returns \e true if, after it executes, the file is
  171     locked by this object, and \e false otherwise.
  172 
  173     \sa unlock(), isLocked(), lockMode()
  174 */
  175 
  176 /*!
  177     \fn bool QtLockedFile::unlock()
  178 
  179     Releases a lock.
  180 
  181     If the object has no lock, this function returns immediately.
  182 
  183     This function returns \e true if, after it executes, the file is
  184     not locked by this object, and \e false otherwise.
  185 
  186     \sa lock(), isLocked(), lockMode()
  187 */
  188 
  189 /*!
  190     \fn QtLockedFile::~QtLockedFile()
  191 
  192     Destroys the \e QtLockedFile object. If any locks were held, they
  193     are released.
  194 */