- Qt 6.9
- Qt Core
- C++ Classes
- QSharedMemory
- Qt 6.9.3 Reference Documentation

# QSharedMemory Class

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

Header: | `#include <QSharedMemory>`
CMake: | `find_package(Qt6 REQUIRED COMPONENTS Core)` `target_link_libraries(mytarget PRIVATE Qt6::Core)`
qmake: | `QT += core`
Inherits: | QObject

- List of all members, including inherited members

## Public Types

enum | AccessMode { ReadOnly, ReadWrite }
enum | SharedMemoryError { NoError, PermissionDenied, InvalidSize, KeyError, AlreadyExists, …, UnknownError }

## Public Functions

| QSharedMemory(const QString &key, QObject *parent = nullptr)
| QSharedMemory(QObject *parent = nullptr)
| QSharedMemory(const QNativeIpcKey &key, QObject *parent = nullptr)
virtual | ~QSharedMemory()
bool | attach(QSharedMemory::AccessMode mode = ReadWrite)
const void * | constData() const
bool | create(qsizetype size, QSharedMemory::AccessMode mode = ReadWrite)
void * | data()
const void * | data() const
bool | detach()
QSharedMemory::SharedMemoryError | error() const
QString | errorString() const
bool | isAttached() const
QString | key() const
bool | lock()
`(since 6.6)` QNativeIpcKey | nativeIpcKey() const
QString | nativeKey() const
void | setKey(const QString &key)
`(since 6.6)` void | setNativeKey(const QNativeIpcKey &key)
void | setNativeKey(const QString &key, QNativeIpcKey::Type type = QNativeIpcKey::legacyDefaultTypeForOs())
qsizetype | size() const
bool | unlock()

## Detailed Description

QSharedMemory provides access to a shared memory segment by multiple threads and processes. Shared memory segments are identified by a key, represented by QNativeIpcKey. A key can be created in a cross-platform manner by using platformSafeKey().

One QSharedMemory object must create() the segment and this call specifies the size of the segment. All other processes simply attach() to the segment that must already exist. After either operation is successful, the application may call data() to obtain a pointer to the data.

To support non-atomic operations, QSharedMemory provides API to gain exclusive access: you may lock the shared memory with lock() before reading from or writing to the shared memory, but remember to release the lock with unlock() after you are done.

By default, 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.

For details on the key types, platform-specific limitations, and interoperability with older or non-Qt applications, see the Native IPC Keys documentation. That includes important information for sandboxed applications on Apple platforms, including all apps obtained via the Apple App Store.

See also Inter-Process Communication and QSystemSemaphore.

## Member Type Documentation

### enum QSharedMemory::AccessMode

Constant | Value | Description
--- | --- | ---
`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.

### enum QSharedMemory::SharedMemoryError

Constant | Value | Description
--- | --- | ---
`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.

## 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)

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.

This function overloads QSharedMemory::QSharedMemory().

See also setNativeKey().

### QSharedMemory::QSharedMemory(const QNativeIpcKey &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.

This is an overloaded function.

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 QSharedMemory::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)

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.

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

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().