QSharedMemory Class Reference

The QSharedMemory class provides access to a shared memory segment.

  1. #include <QSharedMemory>

Inherits: QObject.

This class was introduced in Qt 4.4.

Detailed Description

The QSharedMemory class provides access to a shared memory segment.

QSharedMemory provides access to a shared memory segment by multiple threads and processes. It also provides a way for a single thread or process to lock the memory for exclusive access.

When using this class, be aware of the following platform differences:

  • Windows: QSharedMemory does not "own" the shared memory segment. When all threads or processes that have an instance of QSharedMemory attached to a particular shared memory segment have either destroyed their instance of QSharedMemory or exited, the Windows kernel releases the shared memory segment automatically.
  • Unix: QSharedMemory "owns" the shared memory segment. When the last thread or process that has an instance of QSharedMemory attached to a particular shared memory segment detaches from the segment by destroying its instance of QSharedMemory, the Unix kernel release the shared memory segment. But if that last thread or process crashes without running the QSharedMemory destructor, the shared memory segment survives the crash.
  • HP-UX: Only one attach to a shared memory segment is allowed per process. This means that QSharedMemory should not be used across multiple threads in the same process in HP-UX.
  • Symbian: QSharedMemory does not "own" the shared memory segment. When all threads or processes that have an instance of QSharedMemory attached to a particular shared memory segment have either destroyed their instance of QSharedMemory or exited, the Symbian kernel releases the shared memory segment automatically. Also, access to a shared memory segment cannot be limited to read-only in Symbian.

Remember to lock the shared memory with lock() before reading from or writing to the shared memory, and remember to release the lock with unlock() after you are done.

Unlike QtSharedMemory, QSharedMemory automatically destroys the shared memory segment when the last instance of QSharedMemory is detached from the segment, and no references to the segment remain. Do not mix using QtSharedMemory and QSharedMemory. Port everything to QSharedMemory.

Warning: QSharedMemory changes the key in a Qt-specific way. It is therefore currently not possible to use the shared memory of non-Qt applications with QSharedMemory.

Public Types

Toggle detailsenum QSharedMemory::

AccessModeAccessMode { ReadOnly , ReadWrite 1 ...} { ReadOnly , ReadWrite 1 }

ConstantValueDescription
QSharedMemory::ReadOnly 0 The shared memory segment is read-only. Writing to the shared memory segment is not allowed. An attempt to write to a shared memory segment created with ReadOnly causes the program to abort.
QSharedMemory::ReadWrite 1 Reading and writing the shared memory segment are both allowed.

Look up this member in the source code.

Toggle detailsenum QSharedMemory::

SharedMemoryErrorSharedMemoryError { NoError , PermissionDenied , InvalidSize , KeyError , AlreadyExists , NotFound , LockError , OutOfResources , UnknownError 8 ...} { NoError , PermissionDenied , InvalidSize , KeyError , AlreadyExists , NotFound , LockError , OutOfResources , UnknownError 8 }

ConstantValueDescription
QSharedMemory::NoError 0 No error occurred.
QSharedMemory::PermissionDenied 1 The operation failed because the caller didn't have the required permissions.
QSharedMemory::InvalidSize 2 A create operation failed because the requested size was invalid.
QSharedMemory::KeyError 3 The operation failed because of an invalid key.
QSharedMemory::AlreadyExists 4 A create() operation failed because a shared memory segment with the specified key already existed.
QSharedMemory::NotFound 5 An attach() failed because a shared memory segment with the specified key could not be found.
QSharedMemory::LockError 6 The attempt to lock() the shared memory segment failed because create() or attach() failed and returned false, or because a system error occurred in QSystemSemaphore::acquire().
QSharedMemory::OutOfResources 7 A create() operation failed because there was not enough memory available to fill the request.
QSharedMemory::UnknownError 8 Something else happened and it was bad.

Look up this member in the source code.

    Public Functions

    Toggle details QSharedMemory

    QSharedMemoryQSharedMemory ( const QString &key , QObject *parent=0 ...) ( const QString &key , QObject *parent=0 )

    Constructs a shared memory object with the given parent and with its key set to key. Because its key is set, its create() and attach() functions can be called.

    See also setKey(), create(), and attach().

    Look up this member in the source code.

    Toggle details QSharedMemory

    QSharedMemoryQSharedMemory ( QObject *parent=0 ) ( QObject *parent=0 )

    This function overloads QSharedMemory().

    Constructs a shared memory object with the given parent. The shared memory object's key is not set by the constructor, so the shared memory object does not have an underlying shared memory segment attached. The key must be set with setKey() before create() or attach() can be used.

    See also setKey().

    Look up this member in the source code.

    Toggle details QSharedMemory

    ~QSharedMemory~QSharedMemory () ()

    The destructor clears the key, which forces the shared memory object to detach from its underlying shared memory segment. If this shared memory object is the last one connected to the shared memory segment, the detach() operation destroys the shared memory segment.

    See also detach() and isAttached().

    Look up this member in the source code.

    Toggle details bool QSharedMemory

    attachattach ( AccessMode mode=ReadWrite ) ( AccessMode mode=ReadWrite )

    Attempts to attach the process to the shared memory segment identified by the key that was passed to the constructor or to a call to setKey(). The access mode is ReadWrite by default. It can also be ReadOnly. Returns true if the attach operation is successful. If false is returned, call error() to determine which error occurred. After attaching the shared memory segment, a pointer to the shared memory can be obtained by calling data().

    See also isAttached(), detach(), and create().

    Look up this member in the source code.

    Toggle details const void * QSharedMemory

    constDataconstData () ()const

    Returns a const pointer to the contents of the shared memory segment, if one is attached. Otherwise it returns null. Remember to lock the shared memory with lock() before reading from or writing to the shared memory, and remember to release the lock with unlock() after you are done.

    See also attach() and create().

    Look up this member in the source code.

    Toggle details bool QSharedMemory

    createcreate ( int size , AccessMode mode=ReadWrite ( int size , AccessMode mode=ReadWrite )

    Creates a shared memory segment of size bytes with the key passed to the constructor or set with setKey(), attaches to the new shared memory segment with the given access mode, and returns true. If a shared memory segment identified by the key already exists, the attach operation is not performed, and false is returned. When the return value is false, call error() to determine which error occurred.

    See also error().

    Look up this member in the source code.

    Toggle details void * QSharedMemory

    datadata () ()

    Returns a pointer to the contents of the shared memory segment, if one is attached. Otherwise it returns null. Remember to lock the shared memory with lock() before reading from or writing to the shared memory, and remember to release the lock with unlock() after you are done.

    See also attach().

    Look up this member in the source code.

    Toggle details const void * QSharedMemory

    datadata () ()const

    This function overloads data().

    Look up this member in the source code.

    Toggle details bool QSharedMemory

    detachdetach () ()

    Detaches the process from the shared memory segment. If this was the last process attached to the shared memory segment, then the shared memory segment is released by the system, i.e., the contents are destroyed. The function returns true if it detaches the shared memory segment. If it returns false, it usually means the segment either isn't attached, or it is locked by another process.

    See also attach() and isAttached().

    Look up this member in the source code.

    Toggle details SharedMemoryError QSharedMemory

    errorerror () ()const

    Returns a value indicating whether an error occurred, and, if so, which error it was.

    See also errorString().

    Look up this member in the source code.

    Toggle details QString QSharedMemory

    errorStringerrorString () ()const

    Returns a text description of the last error that occurred. If error() returns an error value, call this function to get a text string that describes the error.

    See also error().

    Look up this member in the source code.

    Toggle details bool QSharedMemory

    isAttachedisAttached () ()const

    Returns true if this process is attached to the shared memory segment.

    See also attach() and detach().

    Look up this member in the source code.

    Toggle details QString QSharedMemory

    keykey () ()const

    Returns the key assigned to this shared memory. The key is the identifier used by the operating system to identify the shared memory segment. When QSharedMemory is used for interprocess communication, the key is how each process attaches to the shared memory segment through which the IPC occurs.

    See also setKey().

    Look up this member in the source code.

    Toggle details bool QSharedMemory

    locklock () ()

    This is a semaphore that locks the shared memory segment for access by this process and returns true. If another process has locked the segment, this function blocks until the lock is released. Then it acquires the lock and returns true. If this function returns false, it means either that you have ignored a false return from create() or attach(), or that QSystemSemaphore::acquire() failed due to an unknown system error.

    See also unlock(), data(), and QSystemSemaphore::acquire().

    Look up this member in the source code.

    Toggle details void QSharedMemory

    setKeysetKey ( const QString &key ) ( const QString &key )

    Sets a new key for this shared memory object. If key and the current key are the same, the function returns without doing anything. If the shared memory object is attached to an underlying shared memory segment, it will detach from it before setting the new key. This function does not do an attach().

    See also key() and isAttached().

    Look up this member in the source code.

    Toggle details int QSharedMemory

    sizesize () ()const

    Returns the size of the attached shared memory segment. If no shared memory segment is attached, 0 is returned.

    See also create() and attach().

    Look up this member in the source code.

    Toggle details bool QSharedMemory

    unlockunlock () ()

    Releases the lock on the shared memory segment and returns true, if the lock is currently held by this process. If the segment is not locked, or if the lock is held by another process, nothing happens and false is returned.

    See also lock().

    Look up this member in the source code.

      Notes provided by the Qt Community

      No notes