# Interface Linker.Option

Enclosing interface:
  `Linker`

public static sealed interface Linker.Option
A linker option is used to provide additional parameters to a linkage request.

Since:
  22

- ## Method Summary

  Static Methods
  Modifier and Type
  Method
  Description
  `static Linker.Option`
  `captureCallState(String... capturedState)`
  Returns a linker option used to save portions of the execution state immediately after calling a foreign function associated with a downcall method handle, before it can be overwritten by the Java runtime, or read through conventional means.
  `static StructLayout`
  `captureStateLayout()`
  Returns a struct layout that represents the layout of the capture state segment that is passed to a downcall handle linked with `captureCallState(String...)`.
  `static Linker.Option`
  `critical(boolean allowHeapAccess)`
  Returns a linker option used to mark a foreign function as critical.
  `static Linker.Option`
  `firstVariadicArg(int index)`
  Returns a linker option used to denote the index indicating the start of the variadic arguments passed to the function described by the function descriptor associated with a downcall linkage request.

- ## Method Details

  - ### firstVariadicArg

    static Linker.Option firstVariadicArg(int index)
    Returns a linker option used to denote the index indicating the start of the variadic arguments passed to the function described by the function descriptor associated with a downcall linkage request.

    The `index` value must conform to `0 <= index <= N`, where `N` is the number of argument layouts of the function descriptor used in conjunction with this linker option. When the `index` is:

    - `0`, all arguments passed to the function are passed as variadic arguments
    - `N`, none of the arguments passed to the function are passed as variadic arguments
    - `n`, where `0 < m < N`, the arguments `m..N` are passed as variadic arguments
    It is important to always use this linker option when linking a variadic function, even if no variadic argument is passed (the second case in the list above), as this might still affect the calling convention on certain platforms.
    Implementation Note:
      The index value is validated when making a linkage request, which is when the function descriptor against which the index is validated is available.
    Parameters:
      `index` - the index of the first variadic argument layout in the function descriptor associated with a downcall linkage request
    Returns:
      a linker option used to denote the index indicating the start of the variadic arguments passed to the function described by the function descriptor associated with a downcall linkage request

  - ### captureCallState

    static Linker.Option captureCallState(String... capturedState)
    Returns a linker option used to save portions of the execution state immediately after calling a foreign function associated with a downcall method handle, before it can be overwritten by the Java runtime, or read through conventional means.

    Execution state is captured by a downcall method handle on invocation, by writing it to a native segment provided by the user to the downcall method handle. For this purpose, a downcall method handle linked with this option will feature an additional `MemorySegment` parameter directly following the target address, and optional `SegmentAllocator` parameters. This parameter, the capture state segment, represents the native segment into which the captured state is written.

    The capture state segment must have size and alignment compatible with the layout returned by captureStateLayout(). This layout is a struct layout which has a named field for each captured value.

    Captured state can be retrieved from the capture state segment by constructing var handles from the capture state layout.

    The following example demonstrates the use of this linker option:

        MemorySegment targetAddress = ...
        Linker.Option ccs = Linker.Option.captureCallState("errno");
        MethodHandle handle = Linker.nativeLinker().downcallHandle(targetAddress, FunctionDescriptor.ofVoid(), ccs);

        StructLayout capturedStateLayout = Linker.Option.captureStateLayout();
        VarHandle errnoHandle = capturedStateLayout.varHandle(PathElement.groupElement("errno"));
        try (Arena arena = Arena.ofConfined()) {
            MemorySegment capturedState = arena.allocate(capturedStateLayout);
            handle.invoke(capturedState);
            int errno = (int) errnoHandle.get(capturedState, 0L);
            // use errno
        }

    Parameters:
      `capturedState` - the names of the values to save
    Returns:
      a linker option used to save portions of the execution state immediately after calling a foreign function associated with a downcall method handle, before it can be overwritten by the Java runtime, or read through conventional means
    Throws:
      `IllegalArgumentException` - if at least one of the provided `capturedState` names is unsupported on the current platform
    See Also:
      - `captureStateLayout()`

  - ### captureStateLayout

    static StructLayout captureStateLayout()
    Returns a struct layout that represents the layout of the capture state segment that is passed to a downcall handle linked with `captureCallState(String...)`.

    The capture state layout is platform-dependent but is guaranteed to be a struct layout containing only value layouts and possibly padding layouts. As an example, on Windows, the returned layout might contain three value layouts named:

    - GetLastError
    - WSAGetLastError
    - errno

    Clients can obtain the names of the supported captured value layouts as follows:

           List<String> capturedNames = Linker.Option.captureStateLayout().memberLayouts().stream()
               .map(MemoryLayout::name)
               .flatMap(Optional::stream)
               .toList();

    Returns:
      a struct layout that represents the layout of the capture state segment that is passed to a downcall handle linked with `captureCallState(String...)`
    See Also:
      - `captureCallState(String...)`

  - ### critical

    static Linker.Option critical(boolean allowHeapAccess)
    Returns a linker option used to mark a foreign function as critical.

    A critical function is a function that has an extremely short running time in all cases (similar to calling an empty function), and does not call back into Java (e.g. using an upcall stub).

    Using this linker option is a hint that some implementations may use to apply optimizations that are only valid for critical functions.

    Using this linker option when linking non-critical functions is likely to have adverse effects, such as loss of performance or JVM crashes.

    Critical functions can optionally allow access to the Java heap. This allows clients to pass heap memory segments as addresses, where normally only off-heap memory segments would be allowed. The memory region inside the Java heap is exposed through a temporary native address that is valid for the duration of the function call. Use of this mechanism is therefore only recommended when a function needs to do short-lived access to Java heap memory, and copying the relevant data to an off-heap memory segment would be prohibitive in terms of performance.

    Parameters:
      `allowHeapAccess` - whether the linked function should allow access to the Java heap.
    Returns:
      a linker option used to mark a foreign function as critical