Linker (Java SE 26 & JDK 26)
Description: declaration: module: java.base, package: java.lang.foreign, interface: Linker
Keywords: java.lang.foreign.Linker interface

# Interface Linker

public sealed interface Linker
A linker provides access to foreign functions from Java code, and access to Java code from foreign functions.

Foreign functions typically reside in libraries that can be loaded on demand. Each library conforms to a specific ABI (Application Binary Interface). An ABI is a set of calling conventions and data types associated with the compiler, OS, and processor where the library was built. For example, a C compiler on Linux/x64 usually builds libraries that conform to the SystemV ABI.

A linker has detailed knowledge of the calling conventions and data types used by a specific ABI. For any library that conforms to that ABI, the linker can mediate between Java code running in the JVM and foreign functions in the library. In particular:

- A linker allows Java code to link against foreign functions, via downcall method handlesRESTRICTED; and
- A linker allows foreign functions to call Java method handles, via the generation of upcall stubsRESTRICTED.

A linker provides a way to look up the canonical layouts associated with the data types used by the ABI. For example, a linker implementing the C ABI might choose to provide a canonical layout for the C `size_t` type. On 64-bit platforms, this canonical layout might be equal to `ValueLayout.JAVA_LONG`. The canonical layouts supported by a linker are exposed via the `canonicalLayouts()` method, which returns a map from type names to canonical layouts.

In addition, a linker provides a way to look up foreign functions in libraries that conform to the ABI. Each linker chooses a set of libraries that are commonly used on the OS and processor combination associated with the ABI. For example, a linker for Linux/x64 might choose two libraries: `libc` and `libm`. The functions in these libraries are exposed via a symbol lookup.

## Calling native functions

The native linker can be used to link against functions defined in C libraries (native functions). Suppose we wish to downcall from Java to the `strlen` function defined in the standard C library:

    size_t strlen(const char *s);

A downcall method handle that exposes `strlen` is obtained, using the native linker, as follows:

    Linker linker = Linker.nativeLinker();
    MethodHandle strlen = linker.downcallHandle(
        linker.defaultLookup().findOrThrow("strlen"),
        FunctionDescriptor.of(JAVA_LONG, ADDRESS)
    );

Note how the native linker also provides access, via its default lookup, to the native functions defined by the C libraries loaded with the Java runtime. Above, the default lookup is used to search the address of the `strlen` native function. That address is then passed, along with a platform-dependent description of the signature of the function expressed as a `FunctionDescriptor` (more on that below) to the native linker's `downcallHandle(MemorySegment, FunctionDescriptor, Option...)`RESTRICTED method. The obtained downcall method handle is then invoked as follows:

     try (Arena arena = Arena.ofConfined()) {
         MemorySegment str = arena.allocateFrom("Hello");
         long len = (long) strlen.invokeExact(str);  // 5
     }

### Describing C signatures

When interacting with the native linker, clients must provide a platform-dependent description of the signature of the C function they wish to link against. This description, a `function descriptor`, defines the layouts associated with the parameter types and return type (if any) of the C function.

Scalar C types such as `bool`, `int` are modeled as value layouts of a suitable carrier. The mapping between a scalar type and its corresponding canonical layout is dependent on the ABI implemented by the native linker (see below).

Composite types are modeled as group layouts. More specifically, a C `struct` type maps to a struct layout, whereas a C `union` type maps to a `union layout`. When defining a struct or union layout, clients must pay attention to the size and alignment constraint of the corresponding composite type definition in C. For instance, padding between two struct fields must be modeled explicitly, by adding an adequately sized padding layout member to the resulting struct layout.

Finally, pointer types such as `int**` and `int(*)(size_t*, size_t*)` are modeled as address layouts. When the spatial bounds of the pointer type are known statically, the address layout can be associated with a target layout. For instance, a pointer that is known to point to a C `int[2]` array can be modeled as an address layout whose target layout is a sequence layout whose element count is 2, and whose element type is `ValueLayout.JAVA_INT`.

All native linker implementations are guaranteed to provide canonical layouts for the following set of types:

- `bool`
- `char`
- `short`
- `int`
- `long`
- `long long`
- `float`
- `double`
- `size_t`
- `wchar_t`
- `void*`

As noted above, the specific canonical layout associated with each type can vary, depending on the data model supported by a given ABI. For instance, the C type `long` maps to the layout constant `ValueLayout.JAVA_LONG` on Linux/x64, but maps to the layout constant `ValueLayout.JAVA_INT` on Windows/x64. Similarly, the C type `size_t` maps to the layout constant `ValueLayout.JAVA_LONG` on 64-bit platforms, but maps to the layout constant `ValueLayout.JAVA_INT` on 32-bit platforms.

A native linker typically does not provide canonical layouts for C's unsigned integral types. Instead, they are modeled using the canonical layouts associated with their corresponding signed integral types. For instance, the C type `unsigned long` maps to the layout constant `ValueLayout.JAVA_LONG` on Linux/x64, but maps to the layout constant `ValueLayout.JAVA_INT` on Windows/x64.

The following table shows some examples of how C types are modeled in Linux/x64 according to the "System V Application Binary Interface" (all the examples provided here will assume these platform-dependent mappings):

> Mapping C types
> C type | Layout | Java type
> --- | --- | ---
> `bool` | `ValueLayout.JAVA_BOOLEAN` | `boolean`
> `char` `unsigned char` | `ValueLayout.JAVA_BYTE` | `byte`
> `short` `unsigned short` | `ValueLayout.JAVA_SHORT` | `short`
> `int` `unsigned int` | `ValueLayout.JAVA_INT` | `int`
> `long` `unsigned long` | `ValueLayout.JAVA_LONG` | `long`
> `long long` `unsigned long long` | `ValueLayout.JAVA_LONG` | `long`
> `float` | `ValueLayout.JAVA_FLOAT` | `float`
> `double` | `ValueLayout.JAVA_DOUBLE` | `double`
> `size_t` | `ValueLayout.JAVA_LONG` | `long`
> `char*`, `int**`, `struct Point*` | `ValueLayout.ADDRESS` | `MemorySegment`
> `int (*ptr)[10]` |
>      ValueLayout.ADDRESS.withTargetLayout(
>          MemoryLayout.sequenceLayout(10,
>              ValueLayout.JAVA_INT)
>      );
> | `MemorySegment`
> `struct Point { int x; long y; };` |
>      MemoryLayout.structLayout(
>          ValueLayout.JAVA_INT.withName("x"),
>          MemoryLayout.paddingLayout(4),
>          ValueLayout.JAVA_LONG.withName("y")
>      );
> | `MemorySegment`
> `union Choice { float a; int b; }` |
>      MemoryLayout.unionLayout(
>          ValueLayout.JAVA_FLOAT.withName("a"),
>          ValueLayout.JAVA_INT.withName("b")
>      );
> | `MemorySegment`

A native linker only supports function descriptors whose argument/return layouts are well-formed layouts. More formally, a layout `L` is well-formed if:

- `L` is a value layout and `L` is derived from a canonical layout `C` such that `L.byteAlignment() <= C.byteAlignment()`
- `L` is a sequence layout `S` and all the following conditions hold:
  1. `L.byteAlignment()` is equal to the sequence layout's natural alignment , and
  2. `S.elementLayout()` is a well-formed layout.
- `L` is a group layout `G` and all the following conditions hold:
  1. `G.byteAlignment()` is equal to the group layout's natural alignment
  2. `G.byteSize()` is a multiple of `G.byteAlignment()`
  3. Each member layout in `G.memberLayouts()` is either a padding layout or a well-formed layout
  4. Each non-padding member layout `E` in `G.memberLayouts()` follows an optional padding member layout, whose size is the minimum size required to align `E`
  5. `G` contains an optional trailing padding member layout, whose size is the minimum size that satisfies (2)

A function descriptor is well-formed if its argument and return layouts are well-formed and are not sequence layouts. A native linker is guaranteed to reject function descriptors that are not well-formed. However, a native linker can still reject well-formed function descriptors, according to platform-specific rules. For example, some native linkers may reject packed struct layouts -- struct layouts whose member layouts feature relaxed alignment constraints, to avoid the insertion of additional padding.

### Function pointers

Sometimes, it is useful to pass Java code as a function pointer to some native function; this is achieved by using an upcall stubRESTRICTED. To demonstrate this, let's consider the following function from the C standard library:

    void qsort(void *base, size_t nmemb, size_t size,
               int (*compar)(const void *, const void *));

The `qsort` function can be used to sort the contents of an array, using a custom comparator function which is passed as a function pointer (the `compar` parameter). To be able to call the `qsort` function from Java, we must first create a downcall method handle for it, as follows:

    Linker linker = Linker.nativeLinker();
    MethodHandle qsort = linker.downcallHandle(
        linker.defaultLookup().findOrThrow("qsort"),
            FunctionDescriptor.ofVoid(ADDRESS, JAVA_LONG, JAVA_LONG, ADDRESS)
    );

As before, we use `ValueLayout.JAVA_LONG` to map the C type `size_t` type, and `ValueLayout.ADDRESS` for both the first pointer parameter (the array pointer) and the last parameter (the function pointer).

To invoke the `qsort` downcall handle obtained above, we need a function pointer to be passed as the last parameter. That is, we need to create a function pointer out of an existing method handle. First, let's write a Java method that can compare two int elements passed as pointers (i.e. as memory segments):

    class Qsort {
        static int qsortCompare(MemorySegment elem1, MemorySegment elem2) {
            return Integer.compare(elem1.get(JAVA_INT, 0), elem2.get(JAVA_INT, 0));
        }
    }

Now let's create a method handle for the comparator method defined above:

    FunctionDescriptor comparDesc = FunctionDescriptor.of(JAVA_INT,
                                                          ADDRESS.withTargetLayout(JAVA_INT),
                                                          ADDRESS.withTargetLayout(JAVA_INT));
    MethodHandle comparHandle = MethodHandles.lookup()
                                             .findStatic(Qsort.class, "qsortCompare",
                                                         comparDesc.toMethodType());

First, we create a function descriptor for the function pointer type. Since we know that the parameters passed to the comparator method will be pointers to elements of a C `int[]` array, we can specify `ValueLayout.JAVA_INT` as the target layout for the address layouts of both parameters. This will allow the comparator method to access the contents of the array elements to be compared. We then turn that function descriptor into a suitable method type which we then use to look up the comparator method handle. We can now create an upcall stub that points to that method, and pass it, as a function pointer, to the `qsort` downcall handle, as follows:

    try (Arena arena = Arena.ofConfined()) {
        MemorySegment comparFunc = linker.upcallStub(comparHandle, comparDesc, arena);
        MemorySegment array = arena.allocateFrom(JAVA_INT, 0, 9, 3, 4, 6, 5, 1, 8, 2, 7);
        qsort.invokeExact(array, 10L, 4L, comparFunc);
        int[] sorted = array.toArray(JAVA_INT); // [ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 ]
    }

This code creates an off-heap array, copies the contents of a Java array into it, and then passes the array to the `qsort` method handle along with the comparator function we obtained from the native linker. After the invocation, the contents of the off-heap array will be sorted according to our comparator function, written in Java. We then extract a new Java array from the segment, which contains the sorted elements.

### Functions returning pointers

When interacting with native functions, it is common for those functions to allocate a region of memory and return a pointer to that region. Let's consider the following function from the C standard library:

    void *malloc(size_t size);

The `malloc` function allocates a region of memory with the given size, and returns a pointer to that region of memory, which is later deallocated using another function from the C standard library:

    void free(void *ptr);

The `free` function takes a pointer to a region of memory and deallocates that region. In this section we will show how to interact with these native functions, with the aim of providing a safe allocation API (the approach outlined below can of course be generalized to allocation functions other than `malloc` and `free`).

First, we need to create the downcall method handles for `malloc` and `free`, as follows:

    Linker linker = Linker.nativeLinker();

    MethodHandle malloc = linker.downcallHandle(
        linker.defaultLookup().findOrThrow("malloc"),
        FunctionDescriptor.of(ADDRESS, JAVA_LONG)
    );

    MethodHandle free = linker.downcallHandle(
        linker.defaultLookup().findOrThrow("free"),
        FunctionDescriptor.ofVoid(ADDRESS)
    );

When a native function returning a pointer (such as `malloc`) is invoked using a downcall method handle, the Java runtime has no insight into the size or the lifetime of the returned pointer. Consider the following code:

    MemorySegment segment = (MemorySegment)malloc.invokeExact(100);

The size of the segment returned by the `malloc` downcall method handle is zero. Moreover, the scope of the returned segment is the global scope. To provide safe access to the segment, we must, unsafely, resize the segment to the desired size (100, in this case). It might also be desirable to attach the segment to some existing arena, so that the lifetime of the region of memory backing the segment can be managed automatically, as for any other native segment created directly from Java code. Both of these operations are accomplished using the restricted method `MemorySegment.reinterpret(long, Arena, Consumer)`RESTRICTED, as follows:

    MemorySegment allocateMemory(long byteSize, Arena arena) throws Throwable {
        MemorySegment segment = (MemorySegment) malloc.invokeExact(byteSize); // size = 0, scope = always alive
        return segment.reinterpret(byteSize, arena, s -> {
            try {
                free.invokeExact(s);
            } catch (Throwable e) {
                throw new RuntimeException(e);
            }
        });  // size = byteSize, scope = arena.scope()
    }

The `allocateMemory` method defined above accepts two parameters: a size and an arena. The method calls the `malloc` downcall method handle, and unsafely reinterprets the returned segment, by giving it a new size (the size passed to the `allocateMemory` method) and a new scope (the scope of the provided arena). The method also specifies a cleanup action to be executed when the provided arena is closed. Unsurprisingly, the cleanup action passes the segment to the `free` downcall method handle, to deallocate the underlying region of memory. We can use the `allocateMemory` method as follows:

    try (Arena arena = Arena.ofConfined()) {
        MemorySegment segment = allocateMemory(100, arena);
    } // 'free' called here

Note how the segment obtained from `allocateMemory` acts as any other segment managed by the confined arena. More specifically, the obtained segment has the desired size, can only be accessed by a single thread (the thread that created the confined arena), and its lifetime is tied to the surrounding try-with-resources block.

### Variadic functions

Variadic functions are C functions that can accept a variable number and type of arguments. They are declared with a trailing ellipsis (`...`) at the end of the formal parameter list, such as: `void foo(int x, ...);` The arguments passed in place of the ellipsis are called variadic arguments. Variadic functions are, essentially, templates that can be specialized into multiple non-variadic functions by replacing the `...` with a list of variadic parameters of a fixed number and type.

It should be noted that values passed as variadic arguments undergo default argument promotion in C. For instance, the following argument promotions are applied:

- `_Bool` -> `unsigned int`
- `[signed] char` -> `[signed] int`
- `[signed] short` -> `[signed] int`
- `float` -> `double`

whereby the signed-ness of the source type corresponds to the signed-ness of the promoted type. The complete process of default argument promotion is described in the C specification. In effect, these promotions place limits on the types that can be used to replace the `...`, as the variadic parameters of the specialized form of a variadic function will always have a promoted type.

The native linker only supports linking the specialized form of a variadic function. A variadic function in its specialized form can be linked using a function descriptor describing the specialized form. Additionally, the `Linker.Option.firstVariadicArg(int)` linker option must be provided to indicate the first variadic parameter in the parameter list. The corresponding argument layout (if any), and all following argument layouts in the specialized function descriptor, are called variadic argument layouts.

The native linker does not automatically perform default argument promotions. However, since passing an argument of a non-promoted type as a variadic argument is not supported in C, the native linker will reject an attempt to link a specialized function descriptor with any variadic argument value layouts corresponding to a non-promoted C type. Since the size of the C `int` type is platform-specific, exactly which layouts will be rejected is platform-specific as well. As an example: on Linux/x64 the layouts corresponding to the C types `_Bool`, `(unsigned) char`, `(unsigned) short`, and `float` (among others), will be rejected by the linker. The `canonicalLayouts()` method can be used to find which layout corresponds to a particular C type.

A well-known variadic function is the `printf` function, defined in the C standard library:

    int printf(const char *format, ...);

This function takes a format string, and a number of additional arguments (the number of such arguments is dictated by the format string). Consider the following variadic call:

    printf("%d plus %d equals %d", 2, 2, 4);

To perform an equivalent call using a downcall method handle we must create a function descriptor which describes the specialized signature of the C function we want to call. This descriptor must include an additional layout for each variadic argument we intend to provide. In this case, the specialized signature of the C function is `(char*, int, int, int)` as the format string accepts three integer parameters. We then need to use a linker option to specify the position of the first variadic layout in the provided function descriptor (starting from 0). In this case, since the first parameter is the format string (a non-variadic argument), the first variadic index needs to be set to 1, as follows:

    Linker linker = Linker.nativeLinker();
    MethodHandle printf = linker.downcallHandle(
        linker.defaultLookup().findOrThrow("printf"),
            FunctionDescriptor.of(JAVA_INT, ADDRESS, JAVA_INT, JAVA_INT, JAVA_INT),
            Linker.Option.firstVariadicArg(1) // first int is variadic
    );

We can then call the specialized downcall handle as usual:

     try (Arena arena = Arena.ofConfined()) {
         //prints "2 plus 2 equals 4"
         int res = (int)printf.invokeExact(arena.allocateFrom("%d plus %d equals %d"), 2, 2, 4);
     }

## Safety considerations

Creating a downcall method handle is intrinsically unsafe. A symbol in a foreign library does not, in general, contain enough signature information (e.g. arity and types of foreign function parameters). As a consequence, the linker runtime cannot validate linkage requests. When a client interacts with a downcall method handle obtained through an invalid linkage request (e.g. by specifying a function descriptor featuring too many argument layouts), the result of such interaction is unspecified and can lead to JVM crashes.

When an upcall stub is passed to a foreign function, a JVM crash might occur, if the foreign code casts the function pointer associated with the upcall stub to a type that is incompatible with the type of the upcall stub, and then attempts to invoke the function through the resulting function pointer. Moreover, if the method handle associated with an upcall stub returns a memory segment, clients must ensure that this address cannot become invalid after the upcall is completed. This can lead to unspecified behavior, and even JVM crashes, since an upcall is typically executed in the context of a downcall method handle invocation.

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

- ## Nested Class Summary

  Nested Classes
  Modifier and Type
  Interface
  Description
  `static interface`
  `Linker.Option`
  A linker option is used to provide additional parameters to a linkage request.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `Map<String, MemoryLayout>`
  `canonicalLayouts()`
  Returns an unmodifiable mapping between the names of data types used by the ABI implemented by this linker and their canonical layouts.
  `SymbolLookup`
  `defaultLookup()`
  Returns a symbol lookup for symbols in a set of commonly used libraries.
  `MethodHandle`
  `downcallHandle(FunctionDescriptor function, Linker.Option... options)`
  Restricted.
  Creates a method handle that is used to call a foreign function with the given signature.
  `MethodHandle`
  `downcallHandle(MemorySegment address, FunctionDescriptor function, Linker.Option... options)`
  Restricted.
  Creates a method handle that is used to call a foreign function with the given signature and address.
  `static Linker`
  `nativeLinker()`
  Returns a linker for the ABI associated with the underlying native platform.
  `MemorySegment`
  `upcallStub(MethodHandle target, FunctionDescriptor function, Arena arena, Linker.Option... options)`
  Restricted.
  Creates an upcall stub which can be passed to other foreign functions as a function pointer, associated with the given arena.

- ## Method Details

  - ### nativeLinker

    static Linker nativeLinker()
    Returns a linker for the ABI associated with the underlying native platform.

    The underlying native platform is the combination of OS and processor where the Java runtime is currently executing.

    API Note:
      It is not currently possible to obtain a linker for a different combination of OS and processor.
    Implementation Requirements:
      A native linker implementation is guaranteed to provide canonical layouts for basic C types.
    Implementation Note:
      The libraries exposed by the default lookup associated with the returned linker are the native libraries loaded in the process where the Java runtime is currently executing. For example, on Linux, these libraries typically include `libc`, `libm` and `libdl`.
    Returns:
      a linker for the ABI associated with the underlying native platform

  - ### downcallHandle

    MethodHandle downcallHandle(MemorySegment address, FunctionDescriptor function, Linker.Option... options)
    `downcallHandle` is a restricted method of the Java platform.
    Programs can only use `downcallHandle` when access to restricted methods is enabled.
    Restricted methods are unsafe, and, if used incorrectly, might crash the JVM or result in memory corruption.
    Creates a method handle that is used to call a foreign function with the given signature and address.

    Calling this method is equivalent to the following code:

        linker.downcallHandle(function, options).bindTo(address);

    Parameters:
      `address` - the native memory segment whose base address is the address of the target foreign function
      `function` - the function descriptor of the target foreign function
      `options` - the linker options associated with this linkage request
    Returns:
      a downcall method handle
    Throws:
      `IllegalArgumentException` - if the provided function descriptor is not supported by this linker
      `IllegalArgumentException` - if `!address.isNative()`, or if `address.equals(MemorySegment.NULL)`
      `IllegalArgumentException` - if an invalid combination of linker options is given
      `IllegalCallerException` - if the caller is in a module that does not have native access enabled
    See Also:
      - `SymbolLookup`

  - ### downcallHandle

    MethodHandle downcallHandle(FunctionDescriptor function, Linker.Option... options)
    `downcallHandle` is a restricted method of the Java platform.
    Programs can only use `downcallHandle` when access to restricted methods is enabled.
    Restricted methods are unsafe, and, if used incorrectly, might crash the JVM or result in memory corruption.
    Creates a method handle that is used to call a foreign function with the given signature.

    The Java method type associated with the returned method handle is derived from the argument and return layouts in the function descriptor, but features an additional leading parameter of type `MemorySegment`, from which the address of the target foreign function is derived. Moreover, if the function descriptor's return layout is a group layout, the resulting downcall method handle accepts an additional leading parameter of type `SegmentAllocator`, which is used by the linker runtime to allocate the memory region associated with the struct returned by the downcall method handle.

    Upon invoking a downcall method handle, the linker provides the following guarantees for any argument `A` of type `MemorySegment` whose corresponding layout is an address layout:

    - `A.scope().isAlive() == true`. Otherwise, the invocation throws `IllegalStateException`;
    - The invocation occurs in a thread `T` such that `A.isAccessibleBy(T) == true`. Otherwise, the invocation throws `WrongThreadException`; and
    - `A` is kept alive during the invocation. For instance, if `A` has been obtained using a shared arena, any attempt to close the arena while the downcall method handle is still executing will result in an `IllegalStateException`.

    Moreover, if the provided function descriptor's return layout is an address layout, invoking the returned method handle will return a native segment associated with the global scope. Under normal conditions, the size of the returned segment is `0`. However, if the function descriptor's return layout has a target layout `T`, then the size of the returned segment is set to `T.byteSize()`.

    The returned method handle will throw an `IllegalArgumentException` if the `MemorySegment` representing the target address of the foreign function is the `MemorySegment.NULL` address. If an argument is a `MemorySegment`, whose corresponding layout is a group layout, the linker might attempt to access the contents of the segment. As such, one of the exceptions specified by the `MemorySegment.get(ValueLayout.OfByte, long)` or the `MemorySegment.copy(MemorySegment, long, MemorySegment, long, long)` methods may be thrown. If an argument is a `MemorySegment` whose corresponding layout is an address layout, the linker will throw an `IllegalArgumentException` if the segment is a heap memory segment, unless heap memory segments are explicitly allowed through the `Linker.Option.critical(boolean)` linker option. The returned method handle will additionally throw `NullPointerException` if any argument passed to it is `null`.

    Parameters:
      `function` - the function descriptor of the target foreign function
      `options` - the linker options associated with this linkage request
    Returns:
      a downcall method handle
    Throws:
      `IllegalArgumentException` - if the provided function descriptor is not supported by this linker
      `IllegalArgumentException` - if an invalid combination of linker options is given
      `IllegalCallerException` - if the caller is in a module that does not have native access enabled

  - ### upcallStub

    MemorySegment upcallStub(MethodHandle target, FunctionDescriptor function, Arena arena, Linker.Option... options)
    `upcallStub` is a restricted method of the Java platform.
    Programs can only use `upcallStub` when access to restricted methods is enabled.
    Restricted methods are unsafe, and, if used incorrectly, might crash the JVM or result in memory corruption.
    Creates an upcall stub which can be passed to other foreign functions as a function pointer, associated with the given arena. Calling such a function pointer from foreign code will result in the execution of the provided method handle.

    The returned memory segment's address points to the newly allocated upcall stub, and is associated with the provided arena. As such, the lifetime of the returned upcall stub segment is controlled by the provided arena. For instance, if the provided arena is a confined arena, the returned upcall stub segment will be deallocated when the provided confined arena is closed.

    An upcall stub argument whose corresponding layout is an address layout is a native segment associated with the global scope. Under normal conditions, the size of this segment argument is `0`. However, if the address layout has a target layout `T`, then the size of the segment argument is set to `T.byteSize()`.

    The target method handle should not throw any exceptions. If the target method handle does throw an exception, the JVM will terminate abruptly. To avoid this, clients should wrap the code in the target method handle in a try/catch block to catch any unexpected exceptions. This can be done using the `MethodHandles.catchException(MethodHandle, Class, MethodHandle)` method handle combinator, and handle exceptions as desired in the corresponding catch block.

    Parameters:
      `target` - the target method handle
      `function` - the upcall stub function descriptor
      `arena` - the arena associated with the returned upcall stub segment
      `options` - the linker options associated with this linkage request
    Returns:
      a zero-length segment whose address is the address of the upcall stub
    Throws:
      `IllegalArgumentException` - if the provided function descriptor is not supported by this linker
      `IllegalArgumentException` - if the type of `target` is incompatible with the type derived from `function`
      `IllegalArgumentException` - if it is determined that the target method handle can throw an exception
      `IllegalStateException` - if `arena.scope().isAlive() == false`
      `WrongThreadException` - if `arena` is a confined arena, and this method is called from a thread `T`, other than the arena's owner thread
      `IllegalCallerException` - if the caller is in a module that does not have native access enabled

  - ### defaultLookup

    SymbolLookup defaultLookup()
    Returns a symbol lookup for symbols in a set of commonly used libraries.

    Each `Linker` is responsible for choosing libraries that are widely recognized as useful on the OS and processor combination supported by the `Linker`. Accordingly, the precise set of symbols exposed by the symbol lookup is unspecified; it varies from one `Linker` to another.

    Implementation Note:
      It is strongly recommended that the result of `defaultLookup()` exposes a set of symbols that is stable over time. Clients of `defaultLookup()` are likely to fail if a symbol that was previously exposed by the symbol lookup is no longer exposed.

      If an implementer provides `Linker` implementations for multiple OS and processor combinations, then it is strongly recommended that the result of `defaultLookup()` exposes, as much as possible, a consistent set of symbols across all the OS and processor combinations.

    Returns:
      a symbol lookup for symbols in a set of commonly used libraries

  - ### canonicalLayouts

    Map<String, MemoryLayout> canonicalLayouts()
    Returns an unmodifiable mapping between the names of data types used by the ABI implemented by this linker and their canonical layouts.

    Each `Linker` is responsible for choosing the data types that are widely recognized as useful on the OS and processor combination supported by the `Linker`. Accordingly, the precise set of data type names and canonical layouts exposed by the linker are unspecified; they vary from one `Linker` to another.

    Implementation Note:
      It is strongly recommended that the result of `canonicalLayouts()` exposes a set of symbols that is stable over time. Clients of `canonicalLayouts()` are likely to fail if a data type that was previously exposed by the linker is no longer exposed, or if its canonical layout is updated.

      If an implementer provides `Linker` implementations for multiple OS and processor combinations, then it is strongly recommended that the result of `canonicalLayouts()` exposes, as much as possible, a consistent set of symbols across all the OS and processor combinations.

    Returns:
      an unmodifiable mapping between the names of data types used by the ABI implemented by this linker and their canonical layouts