- Enclosing interface:
Linker
- Since:
- 22
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Method Summary
Modifier and TypeMethodDescriptionstatic 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
Returns a struct layout that represents the layout of the capture state segment that is passed to a downcall handle linked withcaptureCallState(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.
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Method Details
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firstVariadicArg
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 to0 <= index <= N
, whereN
is the number of argument layouts of the function descriptor used in conjunction with this linker option. When theindex
is:0
, all arguments passed to the function are passed as variadic argumentsN
, none of the arguments passed to the function are passed as variadic argumentsn
, where0 < m < N
, the argumentsm..N
are passed as variadic arguments
- 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
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captureCallState
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 optionalSegmentAllocator
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); // use errno }
This linker option can not be combined with
critical(boolean)
.- 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 providedcapturedState
names is unsupported on the current platform- See Also:
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captureStateLayout
Returns a struct layout that represents the layout of the capture state segment that is passed to a downcall handle linked withcaptureCallState(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:
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critical
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
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