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/OpenJDK 21

Interface SegmentAllocator

All Known Subinterfaces:
ArenaPREVIEW
Functional Interface:
This is a functional interface and can therefore be used as the assignment target for a lambda expression or method reference.
@FunctionalInterface public interface SegmentAllocator
SegmentAllocator is a preview API of the Java platform.
Programs can only use SegmentAllocator when preview features are enabled.
Preview features may be removed in a future release, or upgraded to permanent features of the Java platform.
An object that may be used to allocate memory segmentsPREVIEW. Clients implementing this interface must implement the allocate(long, long) method. A segment allocator defines several methods which can be useful to create segments from several kinds of Java values such as primitives and arrays.

SegmentAllocator is a functional interface. Clients can easily obtain a new segment allocator by using either a lambda expression or a method reference: 

SegmentAllocator autoAllocator = (byteSize, byteAlignment) -> Arena.ofAuto().allocate(byteSize, byteAlignment);

This interface defines factories for commonly used allocators:

Passing a segment allocator to an API can be especially useful in circumstances where a client wants to communicate where the results of a certain operation (performed by the API) should be stored, as a memory segment. For instance, downcall method handlesPREVIEW can accept an additional SegmentAllocatorPREVIEW parameter if the underlying foreign function is known to return a struct by-value. Effectively, the allocator parameter tells the linker where to store the return value of the foreign function.

API Note:
Unless otherwise specified, the allocate(long, long) method is not thread-safe. Furthermore, memory segments allocated by a segment allocator can be associated with different lifetimes, and can even be backed by overlapping regions of memory. For these reasons, clients should generally only interact with a segment allocator they own.

Clients should consider using an arenaPREVIEW instead, which, provides strong thread-safety, lifetime and non-overlapping guarantees.

Method Summary

Modifier and Type Method Description
default MemorySegmentPREVIEW allocate(long byteSize)
Allocates a memory segment with the given size.
MemorySegmentPREVIEW allocate(long byteSize, long byteAlignment)
Allocates a memory segment with the given size and alignment constraint.
default MemorySegmentPREVIEW allocate(AddressLayoutPREVIEW layout, MemorySegmentPREVIEW value)
Allocates a memory segment with the given layout and initializes it with the given address value.
default MemorySegmentPREVIEW allocate(MemoryLayoutPREVIEW layout)
Allocates a memory segment with the given layout.
default MemorySegmentPREVIEW allocate(ValueLayout.OfBytePREVIEW layout, byte value)
Allocates a memory segment with the given layout and initializes it with the given byte value.
default MemorySegmentPREVIEW allocate(ValueLayout.OfCharPREVIEW layout, char value)
Allocates a memory segment with the given layout and initializes it with the given char value.
default MemorySegmentPREVIEW allocate(ValueLayout.OfDoublePREVIEW layout, double value)
Allocates a memory segment with the given layout and initializes it with the given double value.
default MemorySegmentPREVIEW allocate(ValueLayout.OfFloatPREVIEW layout, float value)
Allocates a memory segment with the given layout and initializes it with the given float value.
default MemorySegmentPREVIEW allocate(ValueLayout.OfIntPREVIEW layout, int value)
Allocates a memory segment with the given layout and initializes it with the given int value.
default MemorySegmentPREVIEW allocate(ValueLayout.OfLongPREVIEW layout, long value)
Allocates a memory segment with the given layout and initializes it with the given long value.
default MemorySegmentPREVIEW allocate(ValueLayout.OfShortPREVIEW layout, short value)
Allocates a memory segment with the given layout and initializes it with the given short value.
default MemorySegmentPREVIEW allocateArray(MemoryLayoutPREVIEW elementLayout, long count)
Allocates a memory segment with the given element layout and size.
default MemorySegmentPREVIEW allocateArray(ValueLayout.OfBytePREVIEW elementLayout, byte... elements)
Allocates a memory segment with the given layout and initializes it with the given byte elements.
default MemorySegmentPREVIEW allocateArray(ValueLayout.OfCharPREVIEW elementLayout, char... elements)
Allocates a memory segment with the given layout and initializes it with the given char elements.
default MemorySegmentPREVIEW allocateArray(ValueLayout.OfDoublePREVIEW elementLayout, double... elements)
Allocates a memory segment with the given layout and initializes it with the given double elements.
default MemorySegmentPREVIEW allocateArray(ValueLayout.OfFloatPREVIEW elementLayout, float... elements)
Allocates a memory segment with the given layout and initializes it with the given float elements.
default MemorySegmentPREVIEW allocateArray(ValueLayout.OfIntPREVIEW elementLayout, int... elements)
Allocates a memory segment with the given layout and initializes it with the given int elements.
default MemorySegmentPREVIEW allocateArray(ValueLayout.OfLongPREVIEW elementLayout, long... elements)
Allocates a memory segment with the given layout and initializes it with the given long elements.
default MemorySegmentPREVIEW allocateArray(ValueLayout.OfShortPREVIEW elementLayout, short... elements)
Allocates a memory segment with the given layout and initializes it with the given short elements.
default MemorySegmentPREVIEW allocateUtf8String(String str)
Converts a Java string into a UTF-8 encoded, null-terminated C string, storing the result into a memory segment.
static SegmentAllocatorPREVIEW prefixAllocator(MemorySegmentPREVIEW segment)
Returns a segment allocator which responds to allocation requests by recycling a single segment.
static SegmentAllocatorPREVIEW slicingAllocator(MemorySegmentPREVIEW segment)
Returns a segment allocator which responds to allocation requests by returning consecutive slices obtained from the provided segment.

Method Details

allocateUtf8String

default MemorySegmentPREVIEW allocateUtf8String(String str)
Converts a Java string into a UTF-8 encoded, null-terminated C string, storing the result into a memory segment.

This method always replaces malformed-input and unmappable-character sequences with this charset's default replacement byte array. The CharsetEncoder class should be used when more control over the encoding process is required.

If the given string contains any '\0' characters, they will be copied as well. This means that, depending on the method used to read the string, such as MemorySegment.getUtf8String(long)PREVIEW, the string will appear truncated when read again.

Implementation Requirements:
the default implementation for this method copies the contents of the provided Java string into a new memory segment obtained by calling this.allocate(str.length() + 1).
Parameters:
str - the Java string to be converted into a C string.
Returns:
a new native segment containing the converted C string.

allocate

default MemorySegmentPREVIEW allocate(ValueLayout.OfBytePREVIEW layout, byte value)
Allocates a memory segment with the given layout and initializes it with the given byte value.
Implementation Requirements:
the default implementation for this method calls this.allocate(layout).
Parameters:
layout - the layout of the block of memory to be allocated.
value - the value to be set on the newly allocated memory block.
Returns:
a segment for the newly allocated memory block.

allocate

default MemorySegmentPREVIEW allocate(ValueLayout.OfCharPREVIEW layout, char value)
Allocates a memory segment with the given layout and initializes it with the given char value.
Implementation Requirements:
the default implementation for this method calls this.allocate(layout).
Parameters:
layout - the layout of the block of memory to be allocated.
value - the value to be set on the newly allocated memory block.
Returns:
a segment for the newly allocated memory block.

allocate

default MemorySegmentPREVIEW allocate(ValueLayout.OfShortPREVIEW layout, short value)
Allocates a memory segment with the given layout and initializes it with the given short value.
Implementation Requirements:
the default implementation for this method calls this.allocate(layout).
Parameters:
layout - the layout of the block of memory to be allocated.
value - the value to be set on the newly allocated memory block.
Returns:
a segment for the newly allocated memory block.

allocate

default MemorySegmentPREVIEW allocate(ValueLayout.OfIntPREVIEW layout, int value)
Allocates a memory segment with the given layout and initializes it with the given int value.
Implementation Requirements:
the default implementation for this method calls this.allocate(layout).
Parameters:
layout - the layout of the block of memory to be allocated.
value - the value to be set on the newly allocated memory block.
Returns:
a segment for the newly allocated memory block.

allocate

default MemorySegmentPREVIEW allocate(ValueLayout.OfFloatPREVIEW layout, float value)
Allocates a memory segment with the given layout and initializes it with the given float value.
Implementation Requirements:
the default implementation for this method calls this.allocate(layout).
Parameters:
layout - the layout of the block of memory to be allocated.
value - the value to be set on the newly allocated memory block.
Returns:
a segment for the newly allocated memory block.

allocate

default MemorySegmentPREVIEW allocate(ValueLayout.OfLongPREVIEW layout, long value)
Allocates a memory segment with the given layout and initializes it with the given long value.
Implementation Requirements:
the default implementation for this method calls this.allocate(layout).
Parameters:
layout - the layout of the block of memory to be allocated.
value - the value to be set on the newly allocated memory block.
Returns:
a segment for the newly allocated memory block.

allocate

default MemorySegmentPREVIEW allocate(ValueLayout.OfDoublePREVIEW layout, double value)
Allocates a memory segment with the given layout and initializes it with the given double value.
Implementation Requirements:
the default implementation for this method calls this.allocate(layout).
Parameters:
layout - the layout of the block of memory to be allocated.
value - the value to be set on the newly allocated memory block.
Returns:
a segment for the newly allocated memory block.

allocate

default MemorySegmentPREVIEW allocate(AddressLayoutPREVIEW layout, MemorySegmentPREVIEW value)
Allocates a memory segment with the given layout and initializes it with the given address value. The address value might be narrowed according to the platform address size (see ValueLayout.ADDRESSPREVIEW).
Implementation Requirements:
the default implementation for this method calls this.allocate(layout).
Parameters:
layout - the layout of the block of memory to be allocated.
value - the value to be set on the newly allocated memory block.
Returns:
a segment for the newly allocated memory block.

allocateArray

default MemorySegmentPREVIEW allocateArray(ValueLayout.OfBytePREVIEW elementLayout, byte... elements)
Allocates a memory segment with the given layout and initializes it with the given byte elements.
Implementation Requirements:
the default implementation for this method calls this.allocateArray(layout, array.length).
Parameters:
elementLayout - the element layout of the array to be allocated.
elements - the byte elements to be copied to the newly allocated memory block.
Returns:
a segment for the newly allocated memory block.

allocateArray

default MemorySegmentPREVIEW allocateArray(ValueLayout.OfShortPREVIEW elementLayout, short... elements)
Allocates a memory segment with the given layout and initializes it with the given short elements.
Implementation Requirements:
the default implementation for this method calls this.allocateArray(layout, array.length).
Parameters:
elementLayout - the element layout of the array to be allocated.
elements - the short elements to be copied to the newly allocated memory block.
Returns:
a segment for the newly allocated memory block.

allocateArray

default MemorySegmentPREVIEW allocateArray(ValueLayout.OfCharPREVIEW elementLayout, char... elements)
Allocates a memory segment with the given layout and initializes it with the given char elements.
Implementation Requirements:
the default implementation for this method calls this.allocateArray(layout, array.length).
Parameters:
elementLayout - the element layout of the array to be allocated.
elements - the char elements to be copied to the newly allocated memory block.
Returns:
a segment for the newly allocated memory block.

allocateArray

default MemorySegmentPREVIEW allocateArray(ValueLayout.OfIntPREVIEW elementLayout, int... elements)
Allocates a memory segment with the given layout and initializes it with the given int elements.
Implementation Requirements:
the default implementation for this method calls this.allocateArray(layout, array.length).
Parameters:
elementLayout - the element layout of the array to be allocated.
elements - the int elements to be copied to the newly allocated memory block.
Returns:
a segment for the newly allocated memory block.

allocateArray

default MemorySegmentPREVIEW allocateArray(ValueLayout.OfFloatPREVIEW elementLayout, float... elements)
Allocates a memory segment with the given layout and initializes it with the given float elements.
Implementation Requirements:
the default implementation for this method calls this.allocateArray(layout, array.length).
Parameters:
elementLayout - the element layout of the array to be allocated.
elements - the float elements to be copied to the newly allocated memory block.
Returns:
a segment for the newly allocated memory block.

allocateArray

default MemorySegmentPREVIEW allocateArray(ValueLayout.OfLongPREVIEW elementLayout, long... elements)
Allocates a memory segment with the given layout and initializes it with the given long elements.
Implementation Requirements:
the default implementation for this method calls this.allocateArray(layout, array.length).
Parameters:
elementLayout - the element layout of the array to be allocated.
elements - the long elements to be copied to the newly allocated memory block.
Returns:
a segment for the newly allocated memory block.

allocateArray

default MemorySegmentPREVIEW allocateArray(ValueLayout.OfDoublePREVIEW elementLayout, double... elements)
Allocates a memory segment with the given layout and initializes it with the given double elements.
Implementation Requirements:
the default implementation for this method calls this.allocateArray(layout, array.length).
Parameters:
elementLayout - the element layout of the array to be allocated.
elements - the double elements to be copied to the newly allocated memory block.
Returns:
a segment for the newly allocated memory block.

allocate

default MemorySegmentPREVIEW allocate(MemoryLayoutPREVIEW layout)
Allocates a memory segment with the given layout.
Implementation Requirements:
the default implementation for this method calls this.allocate(layout.byteSize(), layout.byteAlignment()).
Parameters:
layout - the layout of the block of memory to be allocated.
Returns:
a segment for the newly allocated memory block.

allocateArray

default MemorySegmentPREVIEW allocateArray(MemoryLayoutPREVIEW elementLayout, long count)
Allocates a memory segment with the given element layout and size.
Implementation Requirements:
the default implementation for this method calls this.allocate(MemoryLayout.sequenceLayout(count, elementLayout)).
Parameters:
elementLayout - the array element layout.
count - the array element count.
Returns:
a segment for the newly allocated memory block.
Throws:
IllegalArgumentException - if elementLayout.byteSize() * count overflows.
IllegalArgumentException - if count < 0.

allocate

default MemorySegmentPREVIEW allocate(long byteSize)
Allocates a memory segment with the given size.
Implementation Requirements:
the default implementation for this method calls this.allocate(byteSize, 1).
Parameters:
byteSize - the size (in bytes) of the block of memory to be allocated.
Returns:
a segment for the newly allocated memory block.
Throws:
IllegalArgumentException - if byteSize < 0

allocate

MemorySegmentPREVIEW allocate(long byteSize, long byteAlignment)
Allocates a memory segment with the given size and alignment constraint.
Parameters:
byteSize - the size (in bytes) of the block of memory to be allocated.
byteAlignment - the alignment (in bytes) of the block of memory to be allocated.
Returns:
a segment for the newly allocated memory block.
Throws:
IllegalArgumentException - if byteSize < 0, byteAlignment <= 0, or if byteAlignment is not a power of 2.

slicingAllocator

static SegmentAllocatorPREVIEW slicingAllocator(MemorySegmentPREVIEW segment)
Returns a segment allocator which responds to allocation requests by returning consecutive slices obtained from the provided segment. Each new allocation request will return a new slice starting at the current offset (modulo additional padding to satisfy alignment constraint), with given size.

The returned allocator throws IndexOutOfBoundsException when a slice of the provided segment with the requested size and alignment cannot be found.

Implementation Note:
A slicing allocator is not thread-safe.
Parameters:
segment - the segment which the returned allocator should slice from.
Returns:
a new slicing allocator

prefixAllocator

static SegmentAllocatorPREVIEW prefixAllocator(MemorySegmentPREVIEW segment)
Returns a segment allocator which responds to allocation requests by recycling a single segment. Each new allocation request will return a new slice starting at the segment offset 0, hence the name prefix allocator. Equivalent to (but likely more efficient than) the following code: 
MemorySegment segment = ...
SegmentAllocator prefixAllocator = (size, align) -> segment.asSlice(0, size, align);
The returned allocator throws IndexOutOfBoundsException when a slice of the provided segment with the requested size and alignment cannot be found.
API Note:
A prefix allocator can be useful to limit allocation requests in case a client knows that they have fully processed the contents of the allocated segment before the subsequent allocation request takes place.
Implementation Note:
While a prefix allocator is thread-safe, concurrent access on the same recycling allocator might cause a thread to overwrite contents written to the underlying segment by a different thread.
Parameters:
segment - the memory segment to be recycled by the returned allocator.
Returns:
an allocator which recycles an existing segment upon each new allocation request.

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https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/foreign/SegmentAllocator.html