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/OpenJDK 25

Interface SegmentAllocator

All Known Subinterfaces:
Arena
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
An object that may be used to allocate memory segments. 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 handlesRESTRICTED can accept an additional SegmentAllocator 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 arena instead, which, provides strong thread-safety, lifetime and non-overlapping guarantees.

Since:
22

Method Summary

Modifier and Type Method Description
default MemorySegment allocate(long byteSize)
Returns a new memory segment with the given byteSize.
MemorySegment allocate(long byteSize, long byteAlignment)
Returns a new memory segment with the given byteSize and byteAlignment.
default MemorySegment allocate(MemoryLayout layout)
Returns a new memory segment with the given layout.
default MemorySegment allocate(MemoryLayout elementLayout, long count)
Returns a new memory segment with the given elementLayout and count.
default MemorySegment allocateFrom(AddressLayout layout, MemorySegment value)
Returns a new memory segment initialized with the address of the provided memory segment.
default MemorySegment allocateFrom(ValueLayout.OfByte layout, byte value)
Returns a new memory segment initialized with the provided byte value.
default MemorySegment allocateFrom(ValueLayout.OfByte elementLayout, byte... elements)
Returns a new memory segment initialized with the elements in the provided byte array.
default MemorySegment allocateFrom(ValueLayout.OfChar layout, char value)
Returns a new memory segment initialized with the provided char value.
default MemorySegment allocateFrom(ValueLayout.OfChar elementLayout, char... elements)
Returns a new memory segment initialized with the elements in the provided char array.
default MemorySegment allocateFrom(ValueLayout.OfDouble layout, double value)
Returns a new memory segment initialized with the provided double value.
default MemorySegment allocateFrom(ValueLayout.OfDouble elementLayout, double... elements)
Returns a new memory segment initialized with the elements in the provided double array.
default MemorySegment allocateFrom(ValueLayout.OfFloat layout, float value)
Returns a new memory segment initialized with the provided float value.
default MemorySegment allocateFrom(ValueLayout.OfFloat elementLayout, float... elements)
Returns a new memory segment initialized with the elements in the provided float array.
default MemorySegment allocateFrom(ValueLayout.OfInt layout, int value)
Returns a new memory segment initialized with the provided int value.
default MemorySegment allocateFrom(ValueLayout.OfInt elementLayout, int... elements)
Returns a new memory segment initialized with the elements in the provided int array.
default MemorySegment allocateFrom(ValueLayout.OfLong layout, long value)
Returns a new memory segment initialized with the provided long value.
default MemorySegment allocateFrom(ValueLayout.OfLong elementLayout, long... elements)
Returns a new memory segment initialized with the elements in the provided long array.
default MemorySegment allocateFrom(ValueLayout.OfShort layout, short value)
Returns a new memory segment initialized with the provided short value.
default MemorySegment allocateFrom(ValueLayout.OfShort elementLayout, short... elements)
Returns a new memory segment initialized with the elements in the provided short array.
default MemorySegment allocateFrom(ValueLayout elementLayout, MemorySegment source, ValueLayout sourceElementLayout, long sourceOffset, long elementCount)
Returns a new memory segment initialized with the contents of the provided segment.
default MemorySegment allocateFrom(String str)
Converts a Java string into a null-terminated C string using the UTF-8 charset, storing the result into a memory segment.
default MemorySegment allocateFrom(String str, Charset charset)
Converts a Java string into a null-terminated C string using the provided charset, and storing the result into a memory segment.
static SegmentAllocator prefixAllocator(MemorySegment segment)
Returns a segment allocator that responds to allocation requests by recycling a single segment.
static SegmentAllocator slicingAllocator(MemorySegment segment)
Returns a segment allocator that responds to allocation requests by returning consecutive slices obtained from the provided segment.

Method Details

allocateFrom

default MemorySegment allocateFrom(String str)
Converts a Java string into a null-terminated C string using the UTF-8 charset, storing the result into a memory segment.

Calling this method is equivalent to the following code: 

 allocateFrom(str, StandardCharsets.UTF_8);
Parameters:
str - the Java string to be converted into a C string
Returns:
a new native segment containing the converted C string

allocateFrom

default MemorySegment allocateFrom(String str, Charset charset)
Converts a Java string into a null-terminated C string using the provided charset, and 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.getString(long), 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(B + N), where:
  • B is the size, in bytes, of the string encoded using the provided charset (e.g. str.getBytes(charset).length);
  • N is the size (in bytes) of the terminator char according to the provided charset. For instance, this is 1 for StandardCharsets.US_ASCII and 2 for StandardCharsets.UTF_16.
Parameters:
str - the Java string to be converted into a C string
charset - the charset used to encode the string bytes
Returns:
a new native segment containing the converted C string
Throws:
IllegalArgumentException - if charset is not a standard charset

allocateFrom

default MemorySegment allocateFrom(ValueLayout.OfByte layout, byte value)
Returns a new memory segment initialized with the provided byte value.

The size of the allocated memory segment is the size of the given layout. The given value is written into the segment according to the byte order and alignment constraint of the given layout.

Implementation Requirements:
The default implementation is equivalent to: 
 MemorySegment seg = allocate(Objects.requireNonNull(layout));
 seg.set(layout, 0, value);
 return seg;
Parameters:
layout - the layout of the block of memory to be allocated
value - the value to be set in the newly allocated memory segment
Returns:
a new memory segment initialized with the provided byte value

allocateFrom

default MemorySegment allocateFrom(ValueLayout.OfChar layout, char value)
Returns a new memory segment initialized with the provided char value.

The size of the allocated memory segment is the size of the given layout. The given value is written into the segment according to the byte order and alignment constraint of the given layout.

Implementation Requirements:
The default implementation is equivalent to: 
 MemorySegment seg = allocate(Objects.requireNonNull(layout));
 seg.set(layout, 0, value);
 return seg;
Parameters:
layout - the layout of the block of memory to be allocated
value - the value to be set in the newly allocated memory segment
Returns:
a new memory segment initialized with the provided char value

allocateFrom

default MemorySegment allocateFrom(ValueLayout.OfShort layout, short value)
Returns a new memory segment initialized with the provided short value.

The size of the allocated memory segment is the size of the given layout. The given value is written into the segment according to the byte order and alignment constraint of the given layout.

Implementation Requirements:
The default implementation is equivalent to: 
 MemorySegment seg = allocate(Objects.requireNonNull(layout));
 seg.set(layout, 0, value);
 return seg;
Parameters:
layout - the layout of the block of memory to be allocated
value - the value to be set in the newly allocated memory segment
Returns:
a new memory segment initialized with the provided short value

allocateFrom

default MemorySegment allocateFrom(ValueLayout.OfInt layout, int value)
Returns a new memory segment initialized with the provided int value.

The size of the allocated memory segment is the size of the given layout. The given value is written into the segment according to the byte order and alignment constraint of the given layout.

Implementation Requirements:
The default implementation is equivalent to: 
 MemorySegment seg = allocate(Objects.requireNonNull(layout));
 seg.set(layout, 0, value);
 return seg;
Parameters:
layout - the layout of the block of memory to be allocated
value - the value to be set in the newly allocated memory segment
Returns:
a new memory segment initialized with the provided int value

allocateFrom

default MemorySegment allocateFrom(ValueLayout.OfFloat layout, float value)
Returns a new memory segment initialized with the provided float value.

The size of the allocated memory segment is the size of the given layout. The given value is written into the segment according to the byte order and alignment constraint of the given layout.

Implementation Requirements:
The default implementation is equivalent to: 
 MemorySegment seg = allocate(Objects.requireNonNull(layout));
 seg.set(layout, 0, value);
 return seg;
Parameters:
layout - the layout of the block of memory to be allocated
value - the value to be set in the newly allocated memory segment
Returns:
a new memory segment initialized with the provided float value

allocateFrom

default MemorySegment allocateFrom(ValueLayout.OfLong layout, long value)
Returns a new memory segment initialized with the provided long value.

The size of the allocated memory segment is the size of the given layout. The given value is written into the segment according to the byte order and alignment constraint of the given layout.

Implementation Requirements:
The default implementation is equivalent to: 
 MemorySegment seg = allocate(Objects.requireNonNull(layout));
 seg.set(layout, 0, value);
 return seg;
Parameters:
layout - the layout of the block of memory to be allocated
value - the value to be set in the newly allocated memory segment
Returns:
a new memory segment initialized with the provided long value

allocateFrom

default MemorySegment allocateFrom(ValueLayout.OfDouble layout, double value)
Returns a new memory segment initialized with the provided double value.

The size of the allocated memory segment is the size of the given layout. The given value is written into the segment according to the byte order and alignment constraint of the given layout.

Implementation Requirements:
The default implementation is equivalent to: 
 MemorySegment seg = allocate(Objects.requireNonNull(layout));
 seg.set(layout, 0, value);
 return seg;
Parameters:
layout - the layout of the block of memory to be allocated
value - the value to be set in the newly allocated memory segment
Returns:
a new memory segment initialized with the provided double value

allocateFrom

default MemorySegment allocateFrom(AddressLayout layout, MemorySegment value)
Returns a new memory segment initialized with the address of the provided memory segment.

The address value might be narrowed according to the platform address size (see ValueLayout.ADDRESS).

The size of the allocated memory segment is the size of the given layout. The given value is written into the segment according to the byte order and alignment constraint of the given layout.

Implementation Requirements:
The default implementation is equivalent to: 
 Objects.requireNonNull(value);
 MemorySegment seg = allocate(Objects.requireNonNull(layout));
 seg.set(layout, 0, value);
 return seg;
Parameters:
layout - the layout of the block of memory to be allocated
value - the value to be set in the newly allocated memory segment
Returns:
a new memory segment initialized with the address of the provided memory segment
Throws:
IllegalArgumentException - if value is not a native segment

allocateFrom

default MemorySegment allocateFrom(ValueLayout elementLayout, MemorySegment source, ValueLayout sourceElementLayout, long sourceOffset, long elementCount)
Returns a new memory segment initialized with the contents of the provided segment.

The size of the allocated memory segment is the elementLayout.byteSize() * elementCount. The contents of the source segment is copied into the result segment element by element, according to the byte order and alignment constraint of the given element layout.

Implementation Requirements:
The default implementation for this method is equivalent to the following code: 
MemorySegment dest = this.allocate(elementLayout, elementCount);
MemorySegment.copy(source, sourceElementLayout, sourceOffset, dest, elementLayout, 0, elementCount);
return dest;
Parameters:
elementLayout - the element layout of the allocated array
source - the source segment
sourceElementLayout - the element layout of the source segment
sourceOffset - the starting offset, in bytes, of the source segment
elementCount - the number of elements in the source segment to be copied
Returns:
a new memory segment initialized with the contents of the provided segment
Throws:
IllegalArgumentException - if elementLayout.byteSize() != sourceElementLayout.byteSize()
IllegalArgumentException - if the source segment/offset are incompatible with the alignment constraint in the source element layout
IllegalArgumentException - if elementLayout.byteAlignment() > elementLayout.byteSize()
IllegalArgumentException - if sourceElementLayout.byteAlignment() > sourceElementLayout.byteSize()
IllegalStateException - if the scope associated with source is not alive
WrongThreadException - if this method is called from a thread T, such that source.isAccessibleBy(T) == false
IllegalArgumentException - if elementCount * sourceElementLayout.byteSize() overflows
IllegalArgumentException - if elementCount < 0
IndexOutOfBoundsException - if sourceOffset > source.byteSize() - (elementCount * sourceElementLayout.byteSize())
IndexOutOfBoundsException - if sourceOffset < 0

allocateFrom

default MemorySegment allocateFrom(ValueLayout.OfByte elementLayout, byte... elements)
Returns a new memory segment initialized with the elements in the provided byte array.

The size of the allocated memory segment is elementLayout.byteSize() * elements.length. The contents of the source array is copied into the result segment element by element, according to the byte order and alignment constraint of the given element layout.

Implementation Requirements:
The default implementation for this method is equivalent to the following code: 
 this.allocateFrom(layout, MemorySegment.ofArray(array),
                   ValueLayout.JAVA_BYTE, 0, 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 new memory segment initialized with the elements in the provided byte array
Throws:
IllegalArgumentException - if elementLayout.byteAlignment() > elementLayout.byteSize()

allocateFrom

default MemorySegment allocateFrom(ValueLayout.OfShort elementLayout, short... elements)
Returns a new memory segment initialized with the elements in the provided short array.

The size of the allocated memory segment is elementLayout.byteSize() * elements.length. The contents of the source array are copied into the result segment element by element, according to the byte order and alignment constraint of the given element layout.

Implementation Requirements:
The default implementation for this method is equivalent to the following code: 
 this.allocateFrom(layout, MemorySegment.ofArray(array),
                   ValueLayout.JAVA_SHORT, 0, 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 new memory segment initialized with the elements in the provided short array
Throws:
IllegalArgumentException - if elementLayout.byteAlignment() > elementLayout.byteSize()

allocateFrom

default MemorySegment allocateFrom(ValueLayout.OfChar elementLayout, char... elements)
Returns a new memory segment initialized with the elements in the provided char array.

The size of the allocated memory segment is elementLayout.byteSize() * elements.length. The contents of the source array is copied into the result segment element by element, according to the byte order and alignment constraint of the given element layout.

Implementation Requirements:
The default implementation for this method is equivalent to the following code: 
 this.allocateFrom(layout, MemorySegment.ofArray(array),
                   ValueLayout.JAVA_CHAR, 0, 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 new memory segment initialized with the elements in the provided char array
Throws:
IllegalArgumentException - if elementLayout.byteAlignment() > elementLayout.byteSize()

allocateFrom

default MemorySegment allocateFrom(ValueLayout.OfInt elementLayout, int... elements)
Returns a new memory segment initialized with the elements in the provided int array.

The size of the allocated memory segment is elementLayout.byteSize() * elements.length. The contents of the source array is copied into the result segment element by element, according to the byte order and alignment constraint of the given element layout.

Implementation Requirements:
The default implementation for this method is equivalent to the following code: 
 this.allocateFrom(layout, MemorySegment.ofArray(array),
                   ValueLayout.JAVA_INT, 0, 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 new memory segment initialized with the elements in the provided int array
Throws:
IllegalArgumentException - if elementLayout.byteAlignment() > elementLayout.byteSize()

allocateFrom

default MemorySegment allocateFrom(ValueLayout.OfFloat elementLayout, float... elements)
Returns a new memory segment initialized with the elements in the provided float array.

The size of the allocated memory segment is elementLayout.byteSize() * elements.length. The contents of the source array is copied into the result segment element by element, according to the byte order and alignment constraint of the given element layout.

Implementation Requirements:
The default implementation for this method is equivalent to the following code: 
 this.allocateFrom(layout, MemorySegment.ofArray(array),
                   ValueLayout.JAVA_FLOAT, 0, 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 new memory segment initialized with the elements in the provided float array
Throws:
IllegalArgumentException - if elementLayout.byteAlignment() > elementLayout.byteSize()

allocateFrom

default MemorySegment allocateFrom(ValueLayout.OfLong elementLayout, long... elements)
Returns a new memory segment initialized with the elements in the provided long array.

The size of the allocated memory segment is elementLayout.byteSize() * elements.length. The contents of the source array is copied into the result segment element by element, according to the byte order and alignment constraint of the given element layout.

Implementation Requirements:
The default implementation for this method is equivalent to the following code: 
 this.allocateFrom(layout, MemorySegment.ofArray(array),
                   ValueLayout.JAVA_LONG, 0, 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 new memory segment initialized with the elements in the provided long array
Throws:
IllegalArgumentException - if elementLayout.byteAlignment() > elementLayout.byteSize()

allocateFrom

default MemorySegment allocateFrom(ValueLayout.OfDouble elementLayout, double... elements)
Returns a new memory segment initialized with the elements in the provided double array.

The size of the allocated memory segment is elementLayout.byteSize() * elements.length. The contents of the source array is copied into the result segment element by element, according to the byte order and alignment constraint of the given element layout.

Implementation Requirements:
The default implementation for this method is equivalent to the following code: 
 this.allocateFrom(layout, MemorySegment.ofArray(array),
                   ValueLayout.JAVA_DOUBLE, 0, 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 new memory segment initialized with the elements in the provided double array
Throws:
IllegalArgumentException - if elementLayout.byteAlignment() > elementLayout.byteSize()

allocate

default MemorySegment allocate(MemoryLayout layout)
Returns a new 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 new memory segment with the given layout

allocate

default MemorySegment allocate(MemoryLayout elementLayout, long count)
Returns a new memory segment with the given elementLayout and count.
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 new memory segment with the given elementLayout and count
Throws:
IllegalArgumentException - if elementLayout.byteSize() * count overflows
IllegalArgumentException - if count < 0

allocate

default MemorySegment allocate(long byteSize)
Returns a new memory segment with the given byteSize.
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 new memory segment with the given byteSize
Throws:
IllegalArgumentException - if byteSize < 0

allocate

MemorySegment allocate(long byteSize, long byteAlignment)
Returns a new memory segment with the given byteSize and byteAlignment.
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 new memory segment with the given byteSize and byteAlignment
Throws:
IllegalArgumentException - if byteSize < 0, byteAlignment <= 0, or if byteAlignment is not a power of 2

slicingAllocator

static SegmentAllocator slicingAllocator(MemorySegment segment)
Returns a segment allocator that 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 from which the returned allocator should slice from
Returns:
a new slicing allocator
Throws:
IllegalArgumentException - if the segment is read-only

prefixAllocator

static SegmentAllocator prefixAllocator(MemorySegment segment)
Returns a segment allocator that 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 that recycles an existing segment upon each new allocation request
Throws:
IllegalArgumentException - if the segment is read-only

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https://docs.oracle.com/en/java/javase/25/docs/api/java.base/java/lang/foreign/SegmentAllocator.html