The QSharedMemory class provides access to a shared memory segment. More...

#include <qsharedmemory.h>

Inheritance diagram for QSharedMemory:

Collaboration diagram for QSharedMemory:

Public Types
enum	AccessMode { ReadOnly , ReadWrite }

enum	SharedMemoryError { NoError , PermissionDenied , InvalidSize , KeyError , AlreadyExists , NotFound , LockError , OutOfResources , UnknownError }

Public Member Functions
	QSharedMemory (QObject *parent=nullptr)

	QSharedMemory (const QString &key, QObject *parent=nullptr)

	~QSharedMemory ()

void	setKey (const QString &key)

QString	key () const

void	setNativeKey (const QString &key)

QString	nativeKey () const

bool	create (qsizetype size, AccessMode mode=ReadWrite)

qsizetype	size () const

bool	attach (AccessMode mode=ReadWrite)

bool	isAttached () const

bool	detach ()

void *	data ()

const void *	constData () const

const void *	data () const

bool	lock ()

bool	unlock ()

SharedMemoryError	error () const

QString	errorString () const

Public Member Functions inherited from QObject
Q_INVOKABLE	QObject (QObject *parent=nullptr)

virtual	~QObject ()

virtual bool	event (QEvent *event)

virtual bool	eventFilter (QObject watched, QEvent event)

QString	objectName () const

void	setObjectName (const QString &name)

QBindable< QString >	bindableObjectName ()

bool	isWidgetType () const

bool	isWindowType () const

bool	signalsBlocked () const noexcept

bool	blockSignals (bool b) noexcept

QThread *	thread () const

void	moveToThread (QThread *thread)

int	startTimer (int interval, Qt::TimerType timerType=Qt::CoarseTimer)

void	killTimer (int id)

template<typename T >
T	findChild (const QString &aName=QString(), Qt::FindChildOptions options=Qt::FindChildrenRecursively) const

template<typename T >
QList< T >	findChildren (const QString &aName, Qt::FindChildOptions options=Qt::FindChildrenRecursively) const

template<typename T >
QList< T >	findChildren (Qt::FindChildOptions options=Qt::FindChildrenRecursively) const

const QObjectList &	children () const

void	setParent (QObject *parent)

void	installEventFilter (QObject *filterObj)

void	removeEventFilter (QObject *obj)

QMetaObject::Connection	connect (const QObject sender, const char signal, const char *member, Qt::ConnectionType type=Qt::AutoConnection) const

bool	disconnect (const char signal=nullptr, const QObject receiver=nullptr, const char *member=nullptr) const

bool	disconnect (const QObject receiver, const char member=nullptr) const

void	dumpObjectTree () const

void	dumpObjectInfo () const

bool	setProperty (const char *name, const QVariant &value)

QVariant	property (const char *name) const

QList< QByteArray >	dynamicPropertyNames () const

QBindingStorage *	bindingStorage ()

const QBindingStorage *	bindingStorage () const

QObject *	parent () const

bool	inherits (const char *classname) const

Related Functions
(Note that these are not member functions.)
	QSharedMemory

Related Functions inherited from QObject
template< class T > T	qobject_cast (const QObject *object)

template< typename T > T	qFindChildqFindChildren (const QObject *obj, const QString &name)()

template< typename T > QList< T >	qFindChildrenqFindChildren (const QObject *obj, const QString &name)()

	QObjectList

Additional Inherited Members
Public Slots inherited from QObject
void	deleteLater ()

Signals inherited from QObject
void	destroyed (QObject *=nullptr)

void	objectNameChanged (const QString &objectName, QPrivateSignal)

Static Public Member Functions inherited from QObject
static QMetaObject::Connection	connect (const QObject sender, const char signal, const QObject receiver, const char member, Qt::ConnectionType=Qt::AutoConnection)

static QMetaObject::Connection	connect (const QObject sender, const QMetaMethod &signal, const QObject receiver, const QMetaMethod &method, Qt::ConnectionType type=Qt::AutoConnection)

template<typename Func1 , typename Func2 >
static QMetaObject::Connection	connect (const typename QtPrivate::FunctionPointer< Func1 >::Object sender, Func1 signal, const typename QtPrivate::FunctionPointer< Func2 >::Object receiver, Func2 slot, Qt::ConnectionType type=Qt::AutoConnection)

static QMetaObject::Connection ::type	connect (const typename QtPrivate::FunctionPointer< Func1 >::Object *sender, Func1 signal, Func2 slot)

static QMetaObject::Connection ::type	connect (const typename QtPrivate::FunctionPointer< Func1 >::Object sender, Func1 signal, const QObject context, Func2 slot, Qt::ConnectionType type=Qt::AutoConnection)

template<typename Func1 , typename Func2 >
static std::enable_if< QtPrivate::FunctionPointer< Func2 >::ArgumentCount==-1 &&!std::is_convertible_v< Func2, const char * >, QMetaObject::Connection >::type	connect (const typename QtPrivate::FunctionPointer< Func1 >::Object *sender, Func1 signal, Func2 slot)

template<typename Func1 , typename Func2 >
static std::enable_if< QtPrivate::FunctionPointer< Func2 >::ArgumentCount==-1 &&!std::is_convertible_v< Func2, const char * >, QMetaObject::Connection >::type	connect (const typename QtPrivate::FunctionPointer< Func1 >::Object sender, Func1 signal, const QObject context, Func2 slot, Qt::ConnectionType type=Qt::AutoConnection)

static bool	disconnect (const QObject sender, const char signal, const QObject receiver, const char member)

static bool	disconnect (const QObject sender, const QMetaMethod &signal, const QObject receiver, const QMetaMethod &member)

static bool	disconnect (const QMetaObject::Connection &)

template<typename Func1 , typename Func2 >
static bool	disconnect (const typename QtPrivate::FunctionPointer< Func1 >::Object sender, Func1 signal, const typename QtPrivate::FunctionPointer< Func2 >::Object receiver, Func2 slot)

template<typename Func1 >
static bool	disconnect (const typename QtPrivate::FunctionPointer< Func1 >::Object sender, Func1 signal, const QObject receiver, void **zero)

Protected Member Functions inherited from QObject
QObject *	sender () const

int	senderSignalIndex () const

int	receivers (const char *signal) const

bool	isSignalConnected (const QMetaMethod &signal) const

virtual void	timerEvent (QTimerEvent *event)

virtual void	childEvent (QChildEvent *event)

virtual void	customEvent (QEvent *event)

virtual void	connectNotify (const QMetaMethod &signal)

virtual void	disconnectNotify (const QMetaMethod &signal)

	QObject (QObjectPrivate &dd, QObject *parent=nullptr)

Protected Attributes inherited from QObject
QScopedPointer< QObjectData >	d_ptr

Properties inherited from QObject
QString	objectName
	the name of this object More...

Detailed Description

The QSharedMemory class provides access to a shared memory segment.

\inmodule QtCore

Since: 4.4

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:

\list

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.

Apple platforms: Sandboxed applications (including apps shipped through the Apple App Store) require the use of POSIX shared memory (instead of System V shared memory), which adds a number of limitations, including:

\list

The key must be in the form { <application group identifier>/<custom identifier>}, as documented \l {https://developer.apple.com/library/archive/documentation/Security/Conceptual/AppSandboxDesignGuide/AppSandboxInDepth/AppSandboxInDepth.html#//apple_ref/doc/uid/TP40011183-CH3-SW24} {here} and \l {https://developer.apple.com/documentation/bundleresources/entitlements/com_apple_security_application-groups} {here}.

The key length is limited to 30 characters.

On process exit, the named shared memory entries are not cleaned up, so restarting the application and re-creating the shared memory under the same name will fail. To work around this, fall back to attaching to the existing shared memory entry:

QSharedMemory shm("DEVTEAMID.app-group/shared");
if (!shm.create(42) && shm.error() == QSharedMemory::AlreadyExists)
    shm.attach();

\endlist

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.

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.

Warning: QSharedMemory changes the key in a Qt-specific way, unless otherwise specified. Interoperation with non-Qt applications is achieved by first creating a default shared memory with QSharedMemory() and then setting a native key with setNativeKey(). When using native keys, shared memory is not protected against multiple accesses on it (for example, unable to lock()) and a user-defined mechanism should be used to achieve such protection.

Definition at line 58 of file qsharedmemory.h.

Member Enumeration Documentation

◆ AccessMode

enum QSharedMemory::AccessMode

\value ReadOnly 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.

\value ReadWrite Reading and writing the shared memory segment are both allowed.

Enumerator
ReadOnly
ReadWrite

Definition at line 69 of file qsharedmemory.h.

◆ SharedMemoryError

enum QSharedMemory::SharedMemoryError

\value NoError No error occurred.

\value PermissionDenied The operation failed because the caller didn't have the required permissions.

\value InvalidSize A create operation failed because the requested size was invalid.

\value KeyError The operation failed because of an invalid key.

\value AlreadyExists A create() operation failed because a shared memory segment with the specified key already existed.

\value NotFound An attach() failed because a shared memory segment with the specified key could not be found.

\value LockError 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().

\value OutOfResources A create() operation failed because there was not enough memory available to fill the request.

\value UnknownError Something else happened and it was bad.

Enumerator
NoError
PermissionDenied
InvalidSize
KeyError
AlreadyExists
NotFound
LockError
OutOfResources
UnknownError

Definition at line 75 of file qsharedmemory.h.

Constructor & Destructor Documentation

◆ QSharedMemory() [1/2]

QSharedMemory::QSharedMemory ( QObject * parent = nullptr )

Definition at line 212 of file qsharedmemory.cpp.

◆ QSharedMemory() [2/2]

QSharedMemory::QSharedMemory	(	const QString &	key,
		QObject *	parent = `nullptr`
	)

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(), attach()

Definition at line 230 of file qsharedmemory.cpp.

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◆ ~QSharedMemory()

QSharedMemory::~QSharedMemory ( )

The destructor clears the key, which forces the shared memory object to \l {detach()} {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(), isAttached()

Definition at line 252 of file qsharedmemory.cpp.

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Member Function Documentation

◆ attach()

bool QSharedMemory::attach ( 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() or setNativeKey(). The access mode is \l {QSharedMemory::} {ReadWrite} by default. It can also be \l {QSharedMemory::} {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(), create()

Definition at line 474 of file qsharedmemory.cpp.

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◆ constData()

const void * QSharedMemory::constData ( ) 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(), create()

Definition at line 553 of file qsharedmemory.cpp.

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◆ create()

bool QSharedMemory::create	(	qsizetype	size,
		AccessMode	mode = `ReadWrite`
	)

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

See also: error()

Definition at line 400 of file qsharedmemory.cpp.

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◆ data() [1/2]

void * QSharedMemory::data ( )

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()

Definition at line 538 of file qsharedmemory.cpp.

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◆ data() [2/2]

const void * QSharedMemory::data ( ) const

Definition at line 562 of file qsharedmemory.cpp.

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◆ detach()

bool QSharedMemory::detach ( )

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(), isAttached()

Definition at line 514 of file qsharedmemory.cpp.

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◆ error()

QSharedMemory::SharedMemoryError QSharedMemory::error ( ) const

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

See also: errorString()

Definition at line 655 of file qsharedmemory.cpp.

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◆ errorString()

QString QSharedMemory::errorString ( ) const

Returns a text description of the last error that occurred. If error() returns an \l {QSharedMemory::SharedMemoryError} {error value}, call this function to get a text string that describes the error.

See also: error()

Definition at line 669 of file qsharedmemory.cpp.

◆ isAttached()

bool QSharedMemory::isAttached ( ) const

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

See also: attach(), detach()

Definition at line 498 of file qsharedmemory.cpp.

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◆ key()

QString QSharedMemory::key ( ) const

Returns the key assigned with setKey() to this shared memory, or a null key if no key has been assigned, or if the segment is using a nativeKey(). The key is the identifier used by Qt applications to identify the shared memory segment.

You can find the native, platform specific, key used by the operating system by calling nativeKey().

See also: setKey(), setNativeKey()

Definition at line 366 of file qsharedmemory.cpp.

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◆ lock()

bool QSharedMemory::lock ( )

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 that you have ignored a false return from create() or attach(), that you have set the key with setNativeKey() or that QSystemSemaphore::acquire() failed due to an unknown system error.

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

Definition at line 580 of file qsharedmemory.cpp.

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◆ nativeKey()

QString QSharedMemory::nativeKey ( ) const

Since: 4.8

Returns the native, platform specific, key for this shared memory object. The native key is the identifier used by the operating system to identify the shared memory segment.

You can use the native key to access shared memory segments that have not been created by Qt, or to grant shared memory access to non-Qt applications.

See also: setKey(), setNativeKey()

Definition at line 384 of file qsharedmemory.cpp.

◆ setKey()

void QSharedMemory::setKey ( const QString & key )

Sets the platform independent key for this shared memory object. If key is the same as the current key, the function returns without doing anything.

You can call key() to retrieve the platform independent key. Internally, QSharedMemory converts this key into a platform specific key. If you instead call nativeKey(), you will get the platform specific, converted key.

If the shared memory object is attached to an underlying shared memory segment, it will \l {detach()} {detach} from it before setting the new key. This function does not do an attach().

See also: key(), nativeKey(), isAttached()

Definition at line 271 of file qsharedmemory.cpp.

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◆ setNativeKey()

void QSharedMemory::setNativeKey ( const QString & key )

Since: 4.8

Sets the native, platform specific, key for this shared memory object. If key is the same as the current native key, the function returns without doing anything. If all you want is to assign a key to a segment, you should call setKey() instead.

You can call nativeKey() to retrieve the native key. If a native key has been assigned, calling key() will return a null string.

If the shared memory object is attached to an underlying shared memory segment, it will \l {detach()} {detach} from it before setting the new key. This function does not do an attach().

The application will not be portable if you set a native key.

See also: nativeKey(), key(), isAttached()

Definition at line 303 of file qsharedmemory.cpp.

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◆ size()

qsizetype QSharedMemory::size ( ) const

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

Note: The size of the segment may be larger than the requested size that was passed to create().

See also: create(), attach()

Definition at line 444 of file qsharedmemory.cpp.

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◆ unlock()

bool QSharedMemory::unlock ( )

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()

Definition at line 605 of file qsharedmemory.cpp.

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Friends And Related Function Documentation

◆ QSharedMemory()

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() or setNativeKey() before create() or attach() can be used.

See also: setKey()

The documentation for this class was generated from the following files:

src/corelib/kernel/qsharedmemory.h
src/corelib/kernel/qsharedmemory.cpp

Public Types

Public Member Functions

Related Functions

Additional Inherited Members

Detailed Description

Member Enumeration Documentation

◆ AccessMode

◆ SharedMemoryError

Constructor & Destructor Documentation

◆ QSharedMemory() [1/2]

◆ QSharedMemory() [2/2]

◆ ~QSharedMemory()

Member Function Documentation

◆ attach()

◆ constData()

◆ create()

◆ data() [1/2]

◆ data() [2/2]

◆ detach()

◆ error()

◆ errorString()

◆ isAttached()

◆ key()

◆ lock()

◆ nativeKey()

◆ setKey()

◆ setNativeKey()

◆ size()

◆ unlock()

Friends And Related Function Documentation

◆ QSharedMemory()