QSharedMemory Class

Member Function Documentation

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

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

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

QSharedMemory::QSharedMemory(QObject *parent = nullptr)

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

See also setNativeKey().

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

This is an overloaded function.

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 setNativeKey(), create(), and attach().

[virtual noexcept] 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().

bool QSharedMemory::attach(QSharedMemory::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 setNativeKey(). 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().

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. The value returned by this function will not change until a detach happens, so it is safe to store this pointer.

If the memory operations are not atomic, you may lock the shared memory with lock() before reading from or writing, but remember to release the lock with unlock() after you are done.

See also attach() and create().

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

Creates a shared memory segment of size bytes with the key passed to the constructor or set with setNativeKey(), then 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().

void *QSharedMemory::data()

Returns a pointer to the contents of the shared memory segment, if one is attached. Otherwise it returns null. The value returned by this function will not change until a detach happens, so it is safe to store this pointer.

If the memory operations are not atomic, you may lock the shared memory with lock() before reading from or writing, but remember to release the lock with unlock() after you are done.

See also attach().

const void *QSharedMemory::data() const

This function overloads data().

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() and isAttached().

QSharedMemory::SharedMemoryError QSharedMemory::error() const

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

See also errorString().

QString QSharedMemory::errorString() 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().

bool QSharedMemory::isAttached() const

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

See also attach() and detach().

QString QSharedMemory::key() const

Returns the legacy 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() and setNativeKey().

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(), and QSystemSemaphore::acquire().

[since 6.6] QNativeIpcKey QSharedMemory::nativeIpcKey() const

Returns the key type for this shared memory object. The key type complements the nativeKey() as 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 Native IPC Keys for more information.

This function was introduced in Qt 6.6.

See also nativeKey() and setNativeKey().

QString QSharedMemory::nativeKey() const

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 Native IPC Keys for more information.

See also setNativeKey() and nativeIpcKey().

void QSharedMemory::setKey(const QString &key)

This is an overloaded function.

Sets the legacy key for this shared memory object. If key is the same as the current key, the function returns without doing anything. Otherwise, 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().

You can call key() to retrieve the legacy key. This function is mostly the same as:

 shm.setNativeKey(QSharedMemory::legacyNativeKey(key));

except that it enables obtaining the legacy key using key().

See also key(), nativeKey(), and isAttached().

[since 6.6] void QSharedMemory::setNativeKey(const QNativeIpcKey &key)

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. Otherwise, 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().

This function is useful if the native key was shared from another process. See Native IPC Keys for more information.

Portable native keys can be obtained using platformSafeKey().

You can call nativeKey() to retrieve the native key.

This function was introduced in Qt 6.6.

See also nativeKey(), nativeIpcKey(), and isAttached().

void QSharedMemory::setNativeKey(const QString &key, QNativeIpcKey::Type type = QNativeIpcKey::legacyDefaultTypeForOs())

Sets the native, platform specific, key for this shared memory object of type type (the type parameter has been available since Qt 6.6). If key is the same as the current native key, the function returns without doing anything. Otherwise, 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().

This function is useful if the native key was shared from another process, though the application must take care to ensure the key type matches what the other process expects. See Native IPC Keys for more information.

Portable native keys can be obtained using platformSafeKey().

You can call nativeKey() to retrieve the native key.

See also nativeKey(), nativeIpcKey(), and isAttached().

qsizetype QSharedMemory::size() 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().

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().