QSharedMemory¶
- PyQt5.QtCore.QSharedMemory
Inherits from QObject.
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.
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.
Enums¶
- AccessMode
Member
Value
Description
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 causes the program to abort.
ReadWrite 1
Reading and writing the shared memory segment are both allowed.
- SharedMemoryError
Member
Value
Description
AlreadyExists 4
A create() operation failed because a shared memory segment with the specified key already existed.
InvalidSize 2
A create operation failed because the requested size was invalid.
KeyError 3
The operation failed because of an invalid key.
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 acquire().
NoError 0
No error occurred.
NotFound 5
An attach() failed because a shared memory segment with the specified key could not be found.
OutOfResources 7
A create() operation failed because there was not enough memory available to fill the request.
PermissionDenied 1
The operation failed because the caller didn’t have the required permissions.
UnknownError 8
Something else happened and it was bad.
Methods¶
- __init__(parent: QObject = None)
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
- __init__(str, parent: QObject = None)
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.
- attach(mode: AccessMode = ReadWrite) → 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().See also
- constData() → sip.voidptr
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.
- create(int, mode: AccessMode = ReadWrite) → 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 andfalse
is returned. When the return value isfalse
, call error() to determine which error occurred.See also
- data() → sip.voidptr
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
- detach() → 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 returnsfalse
, it usually means the segment either isn’t attached, or it is locked by another process.See also
- error() → SharedMemoryError
Returns a value indicating whether an error occurred, and, if so, which error it was.
See also
- errorString() → str
Returns a text description of the last error that occurred. If error() returns an SharedMemoryError, call this function to get a text string that describes the error.
See also
- isAttached() → bool
Returns
true
if this process is attached to the shared memory segment.
- key() → 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().
See also
- lock() → 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 returnstrue
. If this function returnsfalse
, 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.
- nativeKey() → 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.
See also
- setKey(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().
See also
- setNativeKey(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.
See also
- size() → int
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().
- unlock() → 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