QSharedMemory

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

Inheritance diagram of PySide6.QtCore.QSharedMemory

Synopsis

Functions

Detailed Description

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:

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.

class PySide6.QtCore.QSharedMemory([parent=None])

PySide6.QtCore.QSharedMemory(key[, parent=None])

Parameters

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

See also

setKey()

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.

PySide6.QtCore.QSharedMemory.AccessMode

Constant

Description

QSharedMemory.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 causes the program to abort.

QSharedMemory.ReadWrite

Reading and writing the shared memory segment are both allowed.

PySide6.QtCore.QSharedMemory.SharedMemoryError

Constant

Description

QSharedMemory.NoError

No error occurred.

QSharedMemory.PermissionDenied

The operation failed because the caller didn’t have the required permissions.

QSharedMemory.InvalidSize

A create operation failed because the requested size was invalid.

QSharedMemory.KeyError

The operation failed because of an invalid key.

QSharedMemory.AlreadyExists

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

QSharedMemory.NotFound

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

QSharedMemory.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 acquire() .

QSharedMemory.OutOfResources

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

QSharedMemory.UnknownError

Something else happened and it was bad.

PySide6.QtCore.QSharedMemory.attach([mode=ReadWrite])
Parameters

modeAccessMode

Return type

bool

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 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() .

PySide6.QtCore.QSharedMemory.constData()
Return type

void

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()

PySide6.QtCore.QSharedMemory.create(size[, mode=ReadWrite])
Parameters
Return type

bool

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 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()

PySide6.QtCore.QSharedMemory.detach()
Return type

bool

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.

PySide6.QtCore.QSharedMemory.error()
Return type

SharedMemoryError

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

See also

errorString()

PySide6.QtCore.QSharedMemory.errorString()
Return type

str

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()

PySide6.QtCore.QSharedMemory.isAttached()
Return type

bool

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

See also

attach() detach()

PySide6.QtCore.QSharedMemory.key()
Return type

str

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() .

PySide6.QtCore.QSharedMemory.lock()
Return type

bool

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 acquire() failed due to an unknown system error.

See also

unlock() data() acquire()

PySide6.QtCore.QSharedMemory.nativeKey()
Return type

str

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.

PySide6.QtCore.QSharedMemory.setKey(key)
Parameters

key – str

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 detach from it before setting the new key. This function does not do an attach() .

PySide6.QtCore.QSharedMemory.setNativeKey(key)
Parameters

key – str

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 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.

PySide6.QtCore.QSharedMemory.size()
Return type

qsizetype

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()

PySide6.QtCore.QSharedMemory.unlock()
Return type

bool

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()