Module java.base
Package java.lang.foreign

Interface MemoryLayout

All Known Subinterfaces:
AddressLayout, GroupLayout, PaddingLayout, SequenceLayout, StructLayout, UnionLayout, ValueLayout, ValueLayout.OfBoolean, ValueLayout.OfByte, ValueLayout.OfChar, ValueLayout.OfDouble, ValueLayout.OfFloat, ValueLayout.OfInt, ValueLayout.OfLong, ValueLayout.OfShort
public sealed interface MemoryLayout permits SequenceLayout, GroupLayout, PaddingLayout, ValueLayout
A memory layout describes the contents of a memory segment.

There are two leaves in the layout hierarchy, value layouts, which are used to represent values of given size and kind (see and padding layouts which are used, as the name suggests, to represent a portion of a memory segment whose contents should be ignored, and which are primarily present for alignment reasons. Some common value layout constants, such as ValueLayout.JAVA_INT and ValueLayout.JAVA_FLOAT_UNALIGNED are defined in the ValueLayout class. A special kind of value layout, namely an address layout, is used to model values that denote the address of a region of memory.

More complex layouts can be derived from simpler ones: a sequence layout denotes a homogeneous repetition of zero or more occurrences of an element layout; a group layout denotes a heterogeneous aggregation of zero or more member layouts. Group layouts come in two flavors: struct layouts, where member layouts are laid out one after the other, and union layouts where member layouts are laid out at the same starting offset.

Layouts can be optionally associated with a name. A layout name can be referred to when constructing layout paths.

Consider the following struct declaration in C: 

typedef struct {
    char kind;
    int value;
} TaggedValues[5];
The above declaration can be modelled using a layout object, as follows: 
SequenceLayout taggedValues = MemoryLayout.sequenceLayout(5,
    MemoryLayout.structLayout(
        ValueLayout.JAVA_BYTE.withName("kind"),
        MemoryLayout.paddingLayout(3),
        ValueLayout.JAVA_INT.withName("value")
    )
).withName("TaggedValues");

Characteristics of memory layouts

All layouts have a size (expressed in bytes), which is defined as follows:
  • The size of a value layout is determined by the ValueLayout.carrier() associated with the value layout. That is, the constant ValueLayout.JAVA_INT has carrier int, and size of 4 bytes;
  • The size of an address layout is platform-dependent. That is, the constant ValueLayout.ADDRESS has size of 8 bytes on a 64-bit platform;
  • The size of a padding layout is always provided explicitly, on construction;
  • The size of a sequence layout whose element layout is E and element count is L, is the size of E, multiplied by L;
  • The size of a struct layout with member layouts M1, M2, ... Mn whose sizes are S1, S2, ... Sn, respectively, is S1 + S2 + ... + Sn;
  • The size of a union layout U with member layouts M1, M2, ... Mn whose sizes are S1, S2, ... Sn, respectively, is max(S1, S2, ... Sn).

Furthermore, all layouts have a natural alignment (expressed in bytes) which is defined as follows:

  • The natural alignment of a padding layout is 1;
  • The natural alignment of a value layout whose size is N is N;
  • The natural alignment of a sequence layout whose element layout is E is the alignment of E;
  • The natural alignment of a group layout with member layouts M1, M2, ... Mn whose alignments are A1, A2, ... An, respectively, is max(A1, A2 ... An).
A layout's alignment can be overridden if needed (see withByteAlignment(long)), which can be useful to describe layouts with weaker or stronger alignment constraints.

Layout paths

A layout path is used to unambiguously select a layout that is nested in some other layout. Layout paths are typically expressed as a sequence of one or more path elements. (A more formal definition of layout paths is provided below).

Layout paths can be used to:

  • obtain offsets of arbitrarily nested layouts;
  • obtain a var handle that can be used to access the value corresponding to the selected layout;
  • select an arbitrarily nested layout.

For instance, given the taggedValues sequence layout constructed above, we can obtain the offset, in bytes, of the member layout named value in the first sequence element, as follows: 

long valueOffset = taggedValues.byteOffset(PathElement.sequenceElement(0),
                                          PathElement.groupElement("value")); // yields 4
Similarly, we can select the member layout named value, as follows: 
MemoryLayout value = taggedValues.select(PathElement.sequenceElement(),
                                         PathElement.groupElement("value"));

Open path elements

Some layout path elements, said open path elements, can select multiple layouts at once. For instance, the open path elements MemoryLayout.PathElement.sequenceElement(), MemoryLayout.PathElement.sequenceElement(long, long) select an unspecified element in a sequence layout. A var handle derived from a layout path containing one or more open path element features additional coordinates of type long, which can be used by clients to bind the open elements in the path: 
VarHandle valueHandle = taggedValues.varHandle(PathElement.sequenceElement(),
                                               PathElement.groupElement("value"));
MemorySegment valuesSegment = ...
int val = (int) valueHandle.get(valuesSegment, 2); // reads the "value" field of the third struct in the array

Open path elements also affects the creation of offset-computing method handles. Each open path element becomes an additional long parameter in the obtained method handle. This parameter can be used to specify the index of the sequence element whose offset is to be computed: 

MethodHandle offsetHandle = taggedValues.byteOffsetHandle(PathElement.sequenceElement(),
                                                          PathElement.groupElement("kind"));
long offset1 = (long) offsetHandle.invokeExact(1L); // 8
long offset2 = (long) offsetHandle.invokeExact(2L); // 16

Dereference path elements

A special kind of path element, called dereference path element, allows var handles obtained from memory layouts to follow pointers. Consider the following layout: 
StructLayout RECTANGLE = MemoryLayout.structLayout(
        ValueLayout.ADDRESS.withTargetLayout(
                MemoryLayout.sequenceLayout(4,
                        MemoryLayout.structLayout(
                                ValueLayout.JAVA_INT.withName("x"),
                                ValueLayout.JAVA_INT.withName("y")
                        ).withName("point")
                 )
         ).withName("points")
);
This layout is a struct layout which describe a rectangle. It contains a single field, namely points, an address layout whose target layout is a sequence layout of four struct layouts. Each struct layout describes a two-dimensional point, and is defined as a pair or ValueLayout.JAVA_INT coordinates, with names x and y, respectively.

With dereference path elements, we can obtain a var handle which accesses the y coordinate of one of the point in the rectangle, as follows: 

VarHandle rectPointYs = RECTANGLE.varHandle(
        PathElement.groupElement("points"),
        PathElement.dereferenceElement(),
        PathElement.sequenceElement(),
        PathElement.groupElement("y")
);

MemorySegment rect = ...
int rect_y_4 = (int) rectPointYs.get(rect, 2); // rect.points[2]->y

Layout path well-formedness

A layout path is applied to a layout C_0, also called the initial layout. Each path element in a layout path can be thought of as a function which updates the current layout C_i-1 to some other layout C_i. That is, for each path element E1, E2, ... En, in a layout path P, we compute C_i = f_i(C_i-1), where f_i is the selection function associated with the path element under consideration, denoted as E_i. The final layout C_i is also called the selected layout.

A layout path P is considered well-formed for an initial layout C_0 if all its path elements E1, E2, ... En are well-formed for their corresponding input layouts C_0, C_1, ... C_n-1. A path element E is considered well-formed for a layout L if any of the following is true:

Any attempt to provide a layout path P that is not well-formed for an initial layout C_0 will result in an IllegalArgumentException.

Access mode restrictions

The var handles returned by varHandle(PathElement...) and ValueLayout.varHandle() feature certain access mode restrictions. A var handle is associated with an access size S, derived from the byte size of the receiver layout, and an alignment constraint B, derived from the alignment constraint of the receiver layout. We say that a memory access operation is fully aligned if it occurs at a memory address A which is compatible with both alignment constraints S and B. If access is fully aligned then following access modes are supported and are guaranteed to support atomic access:
  • read write access modes for all T, with the exception of access modes get and set for long and double on 32-bit platforms.
  • atomic update access modes for int, long, float, double or MemorySegment. (Future major platform releases of the JDK may support additional types for certain currently unsupported access modes.)
  • numeric atomic update access modes for int, long and MemorySegment. (Future major platform releases of the JDK may support additional numeric types for certain currently unsupported access modes.)
  • bitwise atomic update access modes for int, long and MemorySegment. (Future major platform releases of the JDK may support additional numeric types for certain currently unsupported access modes.)
If T is float, double or MemorySegment then atomic update access modes compare values using their bitwise representation (see Float.floatToRawIntBits(float), Double.doubleToRawLongBits(double) and MemorySegment.address(), respectively).

Alternatively, a memory access operation is partially aligned if it occurs at a memory address A which is only compatible with the alignment constraint B; in such cases, access for anything other than the get and set access modes will result in an IllegalStateException. If access is partially aligned, atomic access is only guaranteed when A is aligned according to S.

In all other cases, we say that a memory access operation is misaligned; in such cases an IllegalStateException is thrown, irrespective of the access mode being used.

Finally, if T is MemorySegment all write access modes throw IllegalArgumentException unless the value to be written is a native memory segment.

Since:
22
Implementation Requirements:
Implementations of this interface are immutable, thread-safe and value-based.
Sealed Graph:

  • Method Details

    • byteSize

      long byteSize()
      Returns the layout size, in bytes.
      Returns:
      the layout size, in bytes

    • name

      Optional<String> name()
      Returns the name (if any) associated with this layout.
      Returns:
      the name (if any) associated with this layout
      See Also:

    • withName

      MemoryLayout withName(String name)
      Returns a memory layout with the same characteristics as this layout, but with the given name.
      Parameters:
      name - the layout name.
      Returns:
      a memory layout with the same characteristics as this layout, but with the given name
      See Also:

    • withoutName

      MemoryLayout withoutName()
      Returns a memory layout with the same characteristics as this layout, but with no name.
      Returns:
      a memory layout with the same characteristics as this layout, but with no name
      See Also:
      API Note:
      This can be useful to compare two layouts that have different names, but are otherwise equal.

    • byteAlignment

      long byteAlignment()
      Returns the alignment constraint associated with this layout, expressed in bytes. Layout alignment defines a power of two A which is the byte-wise alignment of the layout, where A is the number of bytes that must be aligned for any pointer that correctly points to this layout. Thus:
      • A=1 means unaligned (in the usual sense), which is common in packets.
      • A=8 means word aligned (on LP64), A=4 int aligned, A=2 short aligned, etc.
      • A=64 is the most strict alignment required by the x86/SV ABI (for AVX-512 data).
      If no explicit alignment constraint was set on this layout (see withByteAlignment(long)), then this method returns the natural alignment constraint (in bytes) associated with this layout.
      Returns:
      the alignment constraint associated with this layout, expressed in bytes

    • withByteAlignment

      MemoryLayout withByteAlignment(long byteAlignment)
      Returns a memory layout with the same characteristics as this layout, but with the given alignment constraint (in bytes).
      Parameters:
      byteAlignment - the layout alignment constraint, expressed in bytes.
      Returns:
      a memory layout with the same characteristics as this layout, but with the given alignment constraint (in bytes)
      Throws:
      IllegalArgumentException - if byteAlignment is not a power of two.

    • scale

      default long scale(long offset, long index)
      Returns offset + (byteSize() * index).
      Parameters:
      offset - the base offset
      index - the index to be scaled by the byte size of this layout
      Returns:
      offset + (byteSize() * index)
      Throws:
      IllegalArgumentException - if offset or index is negative
      ArithmeticException - if either the addition or multiplication overflows

    • scaleHandle

      default MethodHandle scaleHandle()
      Returns a method handle that can be used to invoke scale(long, long) on this layout.
      Returns:
      a method handle that can be used to invoke scale(long, long) on this layout

    • byteOffset

      default long byteOffset(MemoryLayout.PathElement... elements)
      Computes the offset, in bytes, of the layout selected by the given layout path, where the initial layout in the path is this layout.
      Parameters:
      elements - the layout path elements.
      Returns:
      The offset, in bytes, of the layout selected by the layout path in elements.
      Throws:
      IllegalArgumentException - if the layout path is not well-formed for this layout.
      IllegalArgumentException - if the layout path contains one or more open path elements.
      IllegalArgumentException - if the layout path contains one or more dereference path elements.

    • byteOffsetHandle

      default MethodHandle byteOffsetHandle(MemoryLayout.PathElement... elements)
      Creates a method handle that computes the offset, in bytes, of the layout selected by the given layout path, where the initial layout in the path is this layout.

      The returned method handle has the following characteristics:

      • its return type is long;
      • it has one leading long parameter representing the base offset;
      • it has as zero or more trailing parameters of type long, one for each open path element in the provided layout path. The order of these parameters corresponds to the order in which the open path elements occur in the provided layout path.

      The final offset returned by the method handle is computed as follows: 

      
 offset = b + c_1 + c_2 + ... + c_m + (x_1 * s_1) + (x_2 * s_2) + ... + (x_n * s_n)
      where b represents the base offset provided as a dynamic long argument, x_1, x_2, ... x_n represent indices into sequences provided as dynamic long arguments, whereas s_1, s_2, ... s_n are static stride constants derived from the size of the element layout of a sequence, and c_1, c_2, ... c_m are other static offset constants (such as field offsets) which are derived from the layout path.
      Parameters:
      elements - the layout path elements.
      Returns:
      a method handle that computes the offset, in bytes, of the layout selected by the given layout path.
      Throws:
      IllegalArgumentException - if the layout path is not well-formed for this layout.
      IllegalArgumentException - if the layout path contains one or more dereference path elements.
      API Note:
      The returned method handle can be used to compute a layout offset, similarly to byteOffset(PathElement...), but more flexibly, as some indices can be specified when invoking the method handle.

    • varHandle

      default VarHandle varHandle(MemoryLayout.PathElement... elements)
      Creates a var handle that accesses a memory segment at the offset selected by the given layout path, where the initial layout in the path is this layout.

      The returned var handle has the following characteristics:

      • its type is derived from the carrier of the selected value layout;
      • it has a leading parameter of type MemorySegment representing the accessed segment
      • a following long parameter, corresponding to the base offset, denoted as B;
      • it has zero or more trailing access coordinates of type long, one for each open path element in the provided layout path, denoted as I1, I2, ... In, respectively. The order of these access coordinates corresponds to the order in which the open path elements occur in the provided layout path.

      If the provided layout path P contains no dereference elements, then the offset of the access operation is computed as follows: 

      offset = this.offsetHandle(P).invokeExact(B, I1, I2, ... In);
      The physical address corresponding to the accessed offset must be aligned according to the alignment constraint of the root layout (this layout). Note that this can be more strict (but not less) than the alignment constraint of the selected value layout.

      If the selected layout is an address layout, calling VarHandle.get(Object...) on the returned var handle will return a new memory segment. The segment is associated with a fresh scope that is always alive. Moreover, the size of the segment depends on whether the address layout has a target layout. More specifically:

      • If the address layout has a target layout T, then the size of the returned segment is T.byteSize();
      • Otherwise, the address layout has no target layout, and the size of the returned segment is zero.

      If the provided layout path has size m and contains a dereference path element in position k (where k <= m) then two layout paths P and P' are derived, where P contains all the path elements from 0 to k - 1 and P' contains all the path elements from k + 1 to m (if any). Then, the returned var handle is computed as follows: 

      VarHandle baseHandle = this.varHandle(P);
MemoryLayout target = ((AddressLayout)this.select(P)).targetLayout().get();
VarHandle targetHandle = target.varHandle(P');
targetHandle = MethodHandles.insertCoordinates(targetHandle, 1, 0L); // always access nested targets at offset 0
targetHandle = MethodHandles.collectCoordinates(targetHandle, 0,
        baseHandle.toMethodHandle(VarHandle.AccessMode.GET));
      (The above can be trivially generalized to cases where the provided layout path contains more than one dereference path elements).

      As an example, consider the memory layout expressed by a GroupLayout instance constructed as follows: 

          GroupLayout grp = java.lang.foreign.MemoryLayout.structLayout(
            MemoryLayout.paddingLayout(4),
            ValueLayout.JAVA_INT.withOrder(ByteOrder.BIG_ENDIAN).withName("value")
    );
      To access the member layout named value, we can construct a var handle as follows: 
          VarHandle handle = grp.varHandle(PathElement.groupElement("value")); //(MemorySegment, long) -> int
      Parameters:
      elements - the layout path elements.
      Returns:
      a var handle that accesses a memory segment at the offset selected by the given layout path.
      Throws:
      IllegalArgumentException - if the layout path is not well-formed for this layout.
      IllegalArgumentException - if the layout selected by the provided path is not a value layout.
      API Note:
      The resulting var handle features certain access mode restrictions, which are common to all memory access var handles derived from memory layouts.

    • sliceHandle

      default MethodHandle sliceHandle(MemoryLayout.PathElement... elements)
      Creates a method handle which, given a memory segment, returns a slice corresponding to the layout selected by the given layout path, where the initial layout in the path is this layout.

      The returned method handle has the following characteristics:

      • its return type is MemorySegment;
      • it has a leading parameter of type MemorySegment corresponding to the memory segment to be sliced
      • a following long parameter, corresponding to the base offset
      • it has as zero or more trailing parameters of type long, one for each open path element in the provided layout path. The order of these parameters corresponds to the order in which the open path elements occur in the provided layout path.

      The offset of the returned segment is computed as if by a call to a byte offset handle constructed using the given path elements.

      The segment to be sliced must be aligned according to the alignment constraint of the root layout (this layout). Note that this can be more strict (but not less) than the alignment constraint of the selected value layout.

      Parameters:
      elements - the layout path elements.
      Returns:
      a method handle which is used to slice a memory segment at the offset selected by the given layout path.
      Throws:
      IllegalArgumentException - if the layout path is not well-formed for this layout.
      IllegalArgumentException - if the layout path contains one or more dereference path elements.
      API Note:
      The returned method handle can be used to obtain a memory segment slice, similarly to MemorySegment.asSlice(long, long), but more flexibly, as some indices can be specified when invoking the method handle.

    • select

      default MemoryLayout select(MemoryLayout.PathElement... elements)
      Returns the layout selected from the provided path, where the initial layout in the path is this layout.
      Parameters:
      elements - the layout path elements.
      Returns:
      the layout selected by the layout path in elements.
      Throws:
      IllegalArgumentException - if the layout path is not well-formed for this layout.
      IllegalArgumentException - if the layout path contains one or more dereference path elements.
      IllegalArgumentException - if the layout path contains one or more path elements that select one or more sequence element indices, such as MemoryLayout.PathElement.sequenceElement(long) and MemoryLayout.PathElement.sequenceElement(long, long)).

    • equals

      boolean equals(Object other)
      Compares the specified object with this layout for equality. Returns true if and only if the specified object is also a layout, and it is equal to this layout. Two layouts are considered equal if they are of the same kind, have the same size, name and alignment constraint. Furthermore, depending on the layout kind, additional conditions must be satisfied:
      Overrides:
      equals in class Object
      Parameters:
      other - the object to be compared for equality with this layout.
      Returns:
      true if the specified object is equal to this layout.
      See Also:

    • hashCode

      int hashCode()
      Returns the hash code value for this layout.
      Overrides:
      hashCode in class Object
      Returns:
      the hash code value for this layout
      See Also:

    • toString

      String toString()
      Returns the string representation of this layout.
      Overrides:
      toString in class Object
      Returns:
      the string representation of this layout

    • paddingLayout

      static PaddingLayout paddingLayout(long byteSize)
      Creates a padding layout with the given byte size. The alignment constraint of the returned layout is 1. As such, regardless of its size, in the absence of an explicit alignment constraint, a padding layout does not affect the natural alignment of the group or sequence layout it is nested into.
      Parameters:
      byteSize - the padding size (expressed in bytes).
      Returns:
      the new selector layout.
      Throws:
      IllegalArgumentException - if byteSize <= 0.

    • sequenceLayout

      static SequenceLayout sequenceLayout(long elementCount, MemoryLayout elementLayout)
      Creates a sequence layout with the given element layout and element count.
      Parameters:
      elementCount - the sequence element count.
      elementLayout - the sequence element layout.
      Returns:
      the new sequence layout with the given element layout and size.
      Throws:
      IllegalArgumentException - if elementCount is negative.
      IllegalArgumentException - if elementLayout.byteSize() * elementCount overflows.
      IllegalArgumentException - if elementLayout.byteSize() % elementLayout.byteAlignment() != 0.

    • structLayout

      static StructLayout structLayout(MemoryLayout... elements)
      Creates a struct layout with the given member layouts.
      Parameters:
      elements - The member layouts of the struct layout.
      Returns:
      a struct layout with the given member layouts.
      Throws:
      IllegalArgumentException - if the sum of the byte sizes of the member layouts overflows.
      IllegalArgumentException - if a member layout in elements occurs at an offset (relative to the start of the struct layout) which is not compatible with its alignment constraint.
      API Note:
      This factory does not automatically align element layouts, by inserting additional padding layout elements. As such, the following struct layout creation will fail with an exception: 
      structLayout(JAVA_SHORT, JAVA_INT);
      To avoid the exception, clients can either insert additional padding layout elements: 
      structLayout(JAVA_SHORT, MemoryLayout.paddingLayout(2), JAVA_INT);
      Or, alternatively, they can use a member layout which features a smaller alignment constraint. This will result in a packed struct layout: 
      structLayout(JAVA_SHORT, JAVA_INT.withByteAlignment(2));

    • unionLayout

      static UnionLayout unionLayout(MemoryLayout... elements)
      Creates a union layout with the given member layouts.
      Parameters:
      elements - The member layouts of the union layout.
      Returns:
      a union layout with the given member layouts.