Added in API level 27

Summary: Inherited Constants | Fields | Methods | Inherited Methods

SharedMemory

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Kotlin |Java

public final class SharedMemory
extends Object implements Closeable, Parcelable

java.lang.Object
↳	android.os.SharedMemory

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SharedMemory enables the creation, mapping, and protection control over anonymous shared memory.

Summary

Inherited constants

From interface android.os.Parcelable int CONTENTS_FILE_DESCRIPTOR Descriptor bit used with describeContents(): indicates that the Parcelable object's flattened representation includes a file descriptor. int PARCELABLE_WRITE_RETURN_VALUE Flag for use with writeToParcel(Parcel, int): the object being written is a return value, that is the result of a function such as "Parcelable someFunction()", "void someFunction(out Parcelable)", or "void someFunction(inout Parcelable)".

Fields
public static final Creator<SharedMemory>	CREATOR

Public methods
void	close() Close the backing FileDescriptor of this SharedMemory instance.
static SharedMemory	create(String name, int size) Creates an anonymous SharedMemory instance with the provided debug name and size.
int	describeContents() Describe the kinds of special objects contained in this Parcelable instance's marshaled representation.
static SharedMemory	fromFileDescriptor(ParcelFileDescriptor fd) Creates an instance from existing shared memory passed as ParcelFileDescriptor.
int	getSize()
ByteBuffer	map(int prot, int offset, int length) Creates an mmap of the SharedMemory with the specified prot, offset, and length.
ByteBuffer	mapReadOnly() Creates a read-only mapping of the entire shared memory region.
ByteBuffer	mapReadWrite() Creates a read/write mapping of the entire shared memory region.
boolean	setProtect(int prot) Sets the protection on the shared memory to the combination specified in prot, which is either a bitwise-or'd combination of OsConstants.PROT_READ, OsConstants.PROT_WRITE, OsConstants.PROT_EXEC from OsConstants, or OsConstants.PROT_NONE, to remove all further access.
static void	unmap(ByteBuffer buffer) Unmaps a buffer previously returned by map(int, int, int).
void	writeToParcel(Parcel dest, int flags) Flatten this object in to a Parcel.

Inherited methods
From class java.lang.Object Object clone() Creates and returns a copy of this object. boolean equals(Object obj) Indicates whether some other object is "equal to" this one. void finalize() Called by the garbage collector on an object when garbage collection determines that there are no more references to the object. final Class<?> getClass() Returns the runtime class of this Object. int hashCode() Returns a hash code value for the object. final void notify() Wakes up a single thread that is waiting on this object's monitor. final void notifyAll() Wakes up all threads that are waiting on this object's monitor. String toString() Returns a string representation of the object. final void wait(long timeoutMillis, int nanos) Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed. final void wait(long timeoutMillis) Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed. final void wait() Causes the current thread to wait until it is awakened, typically by being notified or interrupted.
From interface java.io.Closeable abstract void close() Closes this stream and releases any system resources associated with it.
From interface android.os.Parcelable abstract int describeContents() Describe the kinds of special objects contained in this Parcelable instance's marshaled representation. abstract void writeToParcel(Parcel dest, int flags) Flatten this object in to a Parcel.
From interface java.lang.AutoCloseable abstract void close() Closes this resource, relinquishing any underlying resources.

Fields

CREATOR

Added in API level 27

public static final Creator<SharedMemory> CREATOR

Public methods

close

Added in API level 27

public void close ()

Close the backing FileDescriptor of this SharedMemory instance. Note that all open mappings of the shared memory will remain valid and may continue to be used. The shared memory will not be freed until all file descriptor handles are closed and all memory mappings are unmapped.

create

Added in API level 27

public static SharedMemory create (String name, 
                int size)

Creates an anonymous SharedMemory instance with the provided debug name and size. The name is only used for debugging purposes and can help identify what the shared memory is used for when inspecting memory maps for the processes that have mapped this SharedMemory instance.

Parameters
name	String: The debug name to use for this SharedMemory instance. This can be null, however a debug name is recommended to help identify memory usage when using tools such as lsof or examining /proc/[pid]/maps
size	int: The size of the shared memory to create. Must be greater than 0.

Returns
SharedMemory	A SharedMemory instance of the requested size This value cannot be null.

Throws
ErrnoException	if the requested allocation fails.

describeContents

Added in API level 27

public int describeContents ()

Describe the kinds of special objects contained in this Parcelable instance's marshaled representation. For example, if the object will include a file descriptor in the output of writeToParcel(android.os.Parcel, int), the return value of this method must include the CONTENTS_FILE_DESCRIPTOR bit.

Returns
int	a bitmask indicating the set of special object types marshaled by this Parcelable object instance. Value is either 0 or CONTENTS_FILE_DESCRIPTOR

fromFileDescriptor

Added in API level 33

public static SharedMemory fromFileDescriptor (ParcelFileDescriptor fd)

Creates an instance from existing shared memory passed as ParcelFileDescriptor.

The fd should be a shared memory created from SharedMemory or ASharedMemory. This can be useful when shared memory is passed as file descriptor through JNI or binder service implemented in cpp.

Note that newly created SharedMemory takes ownership of passed fd and the original fd becomes detached (Check ParcelFileDescriptor.detachFd()). If the caller wants to use the file descriptor after the call, the caller should duplicate the file descriptor (Check ParcelFileDescriptor.dup()) and pass the duped version instead.

Parameters
fd	ParcelFileDescriptor: File descriptor of shared memory passed as ParcelFileDescriptor. This value cannot be null.

Returns
SharedMemory	This value cannot be null.

getSize

Added in API level 27

public int getSize ()

Returns
int	The size of the SharedMemory region.

map

Added in API level 27

public ByteBuffer map (int prot, 
                int offset, 
                int length)

Creates an mmap of the SharedMemory with the specified prot, offset, and length. This will always produce a new ByteBuffer window to the backing shared memory region. Every call to map() may be paired with a call to unmap(java.nio.ByteBuffer) when the ByteBuffer returned by map() is no longer needed.

Parameters
prot	int: A bitwise-or'd combination of PROT_READ, PROT_WRITE, PROT_EXEC, or PROT_NONE.
offset	int: The offset into the shared memory to begin mapping. Must be >= 0 and less than getSize().
length	int: The length of the region to map. Must be > 0 and offset + length must not exceed getSize().

Returns
ByteBuffer	A ByteBuffer mapping. This value cannot be null.

Throws
ErrnoException	if the mmap call failed.

mapReadOnly

Added in API level 27

public ByteBuffer mapReadOnly ()

Creates a read-only mapping of the entire shared memory region. This requires the the protection level of the shared memory is at least PROT_READ or the map will fail. Use map(int, int, int) to have more control over the mapping if desired. This is equivalent to map(OsConstants.PROT_READ, 0, getSize())

Returns
ByteBuffer	A ByteBuffer mapping This value cannot be null.

Throws
ErrnoException	if the mmap call failed.

mapReadWrite

Added in API level 27

public ByteBuffer mapReadWrite ()

Creates a read/write mapping of the entire shared memory region. This requires the the protection level of the shared memory is at least PROT_READ|PROT_WRITE or the map will fail. Use map(int, int, int) to have more control over the mapping if desired. This is equivalent to map(OsConstants.PROT_READ | OsConstants.PROT_WRITE, 0, getSize())

Returns
ByteBuffer	A ByteBuffer mapping This value cannot be null.

Throws
ErrnoException	if the mmap call failed.

setProtect

Added in API level 27

public boolean setProtect (int prot)

Sets the protection on the shared memory to the combination specified in prot, which is either a bitwise-or'd combination of OsConstants.PROT_READ, OsConstants.PROT_WRITE, OsConstants.PROT_EXEC from OsConstants, or OsConstants.PROT_NONE, to remove all further access. Note that protection can only ever be removed, not added. By default shared memory is created with protection set to PROT_READ | PROT_WRITE | PROT_EXEC. The protection passed here also only applies to any mappings created after calling this method. Existing mmaps of the shared memory retain whatever protection they had when they were created. A common usage of this is to share a read-only copy of the data with something else. To do that first create the read/write mapping with PROT_READ | PROT_WRITE, then call setProtect(PROT_READ) to remove write capability, then send the SharedMemory to another process. That process will only be able to mmap with PROT_READ.

Parameters
prot	int: Any bitwise-or'ed combination of OsConstants.PROT_READ, OsConstants.PROT_WRITE, and OsConstants.PROT_EXEC; or OsConstants.PROT_NONE

Returns
boolean	Whether or not the requested protection was applied. Returns true on success, false if the requested protection was broader than the existing protection.

unmap

Added in API level 27

public static void unmap (ByteBuffer buffer)

Unmaps a buffer previously returned by map(int, int, int). This will immediately release the backing memory of the ByteBuffer, invalidating all references to it. Only call this method if there are no duplicates of the ByteBuffer in use and don't access the ByteBuffer after calling this method.

Parameters
buffer	ByteBuffer: The buffer to unmap This value cannot be null.

writeToParcel

Added in API level 27

public void writeToParcel (Parcel dest, 
                int flags)

Flatten this object in to a Parcel.

Parameters
dest	Parcel: This value cannot be null.
flags	int: Additional flags about how the object should be written. May be 0 or Parcelable.PARCELABLE_WRITE_RETURN_VALUE. Value is either 0 or a combination of Parcelable.PARCELABLE_WRITE_RETURN_VALUE, and android.os.Parcelable.PARCELABLE_ELIDE_DUPLICATES