Linker
is a preview API of the Java platform.
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 which 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 handles; and
- A linker allows foreign functions to call Java method handles, via the generation of upcall stubs.
libc
and libm
. The functions in these
libraries are exposed via a symbol lookup.
The nativeLinker()
method provides a linker for the ABI associated with the OS and processor where the Java runtime
is currently executing. This linker also provides access, via its default lookup,
to the native libraries loaded with the Java runtime.
Downcall method handles
Linking a foreign function is a process which requires a function descriptor, a set of memory layouts which, together, specify the signature of the foreign function to be linked, and returns, when complete, a downcall method handle, that is, a method handle that can be used to invoke the target foreign function.The Java method type associated with the returned method handle is derivedPREVIEW from the argument and return layouts in the function descriptor. The downcall method handle type, might then be decorated by additional leading parameters, in the given order if both are present:
- If the downcall method handle is created without specifying a target address,
the downcall method handle type features a leading parameter of type
MemorySegment
PREVIEW, from which the address of the target foreign function can be derived. - If the function descriptor's return layout is a group layout, the resulting downcall method handle accepts
an additional leading parameter of type
SegmentAllocator
PREVIEW, which is used by the linker runtime to allocate the memory region associated with the struct returned by the downcall method handle.
Upcall stubs
Creating an upcall stub requires a method handle and a function descriptor; in this case, the set of memory layouts in the function descriptor specify the signature of the function pointer associated with the upcall stub.The type of the provided method handle's type has to match the method type associated with the upcall stub, which is derivedPREVIEW from the provided function descriptor.
Upcall stubs are modelled by instances of type MemorySegment
PREVIEW; upcall stubs can be passed by reference to other
downcall method handles and, they are released via their associated scopePREVIEW.
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. On downcall handle invocation, the linker runtime guarantees the following for any argumentA
of type MemorySegment
PREVIEW whose corresponding
layout is ValueLayout.ADDRESS
PREVIEW:
- The scope of
A
is alivePREVIEW. Otherwise, the invocation throwsIllegalStateException
; - The invocation occurs in a thread
T
such thatA.scope().isAccessibleBy(T) == true
. Otherwise, the invocation throwsWrongThreadException
; and - The scope of
A
is kept alivePREVIEW during the invocation.
0
.
However, if the return layout is an unboundedPREVIEW address layout,
then the size of the returned segment is Long.MAX_VALUE
.
When creating upcall stubs the linker runtime validates the type of the target method handle against the provided function descriptor and report an error if any mismatch is detected. As for downcalls, JVM crashes might occur, if the foreign code casts the function pointer associated with an upcall stub to a type that is incompatible with the provided function descriptor. Moreover, if the target method handle associated with an upcall stub returns a memory segmentPREVIEW, clients must ensure that this address cannot become invalid after the upcall completes. 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.
An upcall stub argument whose corresponding layout is an address layoutPREVIEW
is a native segment associated with the global scopePREVIEW.
Under normal conditions, the size of this segment argument is 0
. However, if the layout associated with
the upcall stub argument is an unboundedPREVIEW address layout,
then the size of the segment argument is Long.MAX_VALUE
.
- Implementation Requirements:
- Implementations of this interface are immutable, thread-safe and value-based.
- Since:
- 19
-
Nested Class Summary
Modifier and TypeInterfaceDescriptionstatic interface
Preview.A linker option is used to indicate additional linking requirements to the linker, besides what is described by a function descriptor. -
Method Summary
Modifier and TypeMethodDescriptionReturns a symbol lookup for symbols in a set of commonly used libraries.downcallHandle
(FunctionDescriptorPREVIEW function, Linker.OptionPREVIEW... options) Creates a method handle which can be used to call a foreign function with the given signature.default MethodHandle
downcallHandle
(MemorySegmentPREVIEW symbol, FunctionDescriptorPREVIEW function, Linker.OptionPREVIEW... options) Creates a method handle which can be used to call a foreign function with the given signature and address.Returns a linker for the ABI associated with the underlying native platform.upcallStub
(MethodHandle target, FunctionDescriptorPREVIEW function, SegmentScopePREVIEW scope) Creates a stub which can be passed to other foreign functions as a function pointer, associated with the given scope.
-
Method Details
-
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.When interacting with the returned linker, clients must describe the signature of a foreign function using a
function descriptor
PREVIEW whose argument and return layouts are specified as follows:- Scalar types are modelled by a value layoutPREVIEW instance of a suitable carrier. Example
of scalar types in C are
int
,long
,size_t
, etc. The mapping between a scalar type and its corresponding layout is dependent on the ABI of the returned linker; - Composite types are modelled by a group layoutPREVIEW. Depending on the ABI of the
returned linker, additional paddingPREVIEW member layouts might be required to conform
to the size and alignment constraint of a composite type definition in C (e.g. using
struct
orunion
); and - Pointer types are modelled by a value layoutPREVIEW instance with carrier
MemorySegment
PREVIEW. Examples of pointer types in C areint**
andint(*)(size_t*, size_t*)
;
Any layout not listed above is unsupported; function descriptors containing unsupported layouts will cause an
IllegalArgumentException
to be thrown, when used to create adowncall method handle
or an upcall stub.Variadic functions (e.g. a C function declared with a trailing ellipses
...
at the end of the formal parameter list or with an empty formal parameter list) are not supported directly. However, it is possible to link a variadic function by using a linker optionPREVIEW to indicate the start of the list of variadic arguments, together with a specialized function descriptor describing a given variable arity callsite. Alternatively, where the foreign library allows it, clients might be able to interact with variadic functions by passing a trailing parameter of typeVaList
PREVIEW (e.g. as invsprintf
).This method is restricted. Restricted methods are unsafe, and, if used incorrectly, their use might crash the JVM or, worse, silently result in memory corruption. Thus, clients should refrain from depending on restricted methods, and use safe and supported functionalities, where possible.
- API Note:
- It is not currently possible to obtain a linker for a different combination of OS and processor.
- 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
andlibdl
. - Returns:
- a linker for the ABI associated with the OS and processor where the Java runtime is currently executing.
- Throws:
UnsupportedOperationException
- if the underlying native platform is not supported.IllegalCallerException
- If the caller is in a module that does not have native access enabled.
- Scalar types are modelled by a value layoutPREVIEW instance of a suitable carrier. Example
of scalar types in C are
-
downcallHandle
default MethodHandle downcallHandle(MemorySegmentPREVIEW symbol, FunctionDescriptorPREVIEW function, Linker.OptionPREVIEW... options) Creates a method handle which can be used to call a foreign function with the given signature and address.If the provided method type's return type is
MemorySegment
, then the resulting method handle features an additional prefix parameter, of typeSegmentAllocator
PREVIEW, which will be used by the linker to allocate structs returned by-value.Calling this method is equivalent to the following code:
linker.downcallHandle(function).bindTo(symbol);
- Parameters:
symbol
- the address of the target function.function
- the function descriptor of the target function.options
- any linker options.- Returns:
- a downcall method handle. The method handle type is inferred
- Throws:
IllegalArgumentException
- if the provided function descriptor is not supported by this linker. or if the symbol isMemorySegment.NULL
PREVIEWIllegalArgumentException
- if an invalid combination of linker options is given.
-
downcallHandle
Creates a method handle which can be used to call a foreign function with the given signature. The resulting method handle features a prefix parameter (as the first parameter) corresponding to the foreign function entry point, of typeMemorySegment
PREVIEW, which is used to specify the address of the target function to be called.If the provided function descriptor's return layout is a
GroupLayout
PREVIEW, then the resulting method handle features an additional prefix parameter (inserted immediately after the address parameter), of typeSegmentAllocator
PREVIEW), which will be used by the linker to allocate structs returned by-value.The returned method handle will throw an
IllegalArgumentException
if theMemorySegment
PREVIEW parameter passed to it is associated with theMemorySegment.NULL
PREVIEW address, or aNullPointerException
if that parameter isnull
.- Parameters:
function
- the function descriptor of the target function.options
- any linker options.- Returns:
- a downcall method handle. The method handle type is inferred from the provided function descriptor.
- Throws:
IllegalArgumentException
- if the provided function descriptor is not supported by this linker.IllegalArgumentException
- if an invalid combination of linker options is given.
-
upcallStub
MemorySegmentPREVIEW upcallStub(MethodHandle target, FunctionDescriptorPREVIEW function, SegmentScopePREVIEW scope) Creates a stub which can be passed to other foreign functions as a function pointer, associated with the given scope. 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 scope. As such, the corresponding upcall stub will be deallocated when the scope becomes not alivePREVIEW.
The target method handle should not throw any exceptions. If the target method handle does throw an exception, the VM will exit with a non-zero exit code. To avoid the VM aborting due to an uncaught exception, clients could wrap all code in the target method handle in a try/catch block that catches any
Throwable
, for instance by using theMethodHandles.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.scope
- the scope associated with the returned upcall stub segment.- 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 it is determined that the target method handle can throw an exception, or if the target method handle has a type that does not match the upcall stub inferred type.IllegalStateException
- ifscope
is not alivePREVIEW.WrongThreadException
- if this method is called from a threadT
, such thatscope.isAccessibleBy(T) == false
.
-
defaultLookup
SymbolLookupPREVIEW defaultLookup()Returns a symbol lookup for symbols in a set of commonly used libraries.Each
Linker
PREVIEW is responsible for choosing libraries that are widely recognized as useful on the OS and processor combination supported by theLinker
PREVIEW. Accordingly, the precise set of symbols exposed by the symbol lookup is unspecified; it varies from oneLinker
PREVIEW to another.- Implementation Note:
- It is strongly recommended that the result of
defaultLookup()
exposes a set of symbols that is stable over time. Clients ofdefaultLookup()
are likely to fail if a symbol that was previously exposed by the symbol lookup is no longer exposed.If an implementer provides
Linker
PREVIEW implementations for multiple OS and processor combinations, then it is strongly recommended that the result ofdefaultLookup()
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.
-
Linker
when preview features are enabled.