* 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: *
* 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: {@code libc} and {@code libm}. The functions in these * libraries are exposed via a {@linkplain #defaultLookup() symbol lookup}. * *
* Scalar C types such as {@code bool}, {@code int} are modelled as {@linkplain ValueLayout value layouts} * of a suitable carrier. The {@linkplain #canonicalLayouts() 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 modelled as {@linkplain GroupLayout group layouts}. More specifically, a C {@code struct} type * maps to a {@linkplain StructLayout struct layout}, whereas a C {@code union} type maps to a {@link UnionLayout 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 modelled explicitly, by adding an adequately sized {@linkplain PaddingLayout padding layout} member * to the resulting struct layout. *
* Finally, pointer types such as {@code int**} and {@code int(*)(size_t*, size_t*)} are modelled as * {@linkplain AddressLayout address layouts}. When the spatial bounds of the pointer type are known statically, * the address layout can be associated with a {@linkplain AddressLayout#targetLayout() target layout}. For instance, * a pointer that is known to point to a C {@code int[2]} array can be modelled as an address layout whose * target layout is a sequence layout whose element count is 2, and whose element type is {@link ValueLayout#JAVA_INT}. *
* All native linker implementations are guaranteed to provide canonical layouts for the following set of types: *
* A native linker typically does not provide canonical layouts for C's unsigned integral types. Instead, they are * modelled using the canonical layouts associated with their corresponding signed integral types. For instance, * the C type {@code unsigned long} maps to the layout constant {@link ValueLayout#JAVA_LONG} on Linux/x64, but maps to * the layout constant {@link ValueLayout#JAVA_INT} on Windows/x64. *
* The following table shows some examples of how C types are modelled in Linux/x64 according to the * "System V Application Binary Interface" (all the examples provided here will assume these platform-dependent mappings): * *
** * *
* * * *C type *Layout *Java type *{@code bool} *{@link ValueLayout#JAVA_BOOLEAN} *{@code boolean} *{@code char} *
{@code unsigned char}{@link ValueLayout#JAVA_BYTE} *{@code byte} *{@code short} *
{@code unsigned short}{@link ValueLayout#JAVA_SHORT} *{@code short} *{@code int} *
{@code unsigned int}{@link ValueLayout#JAVA_INT} *{@code int} *{@code long} *
{@code unsigned long}{@link ValueLayout#JAVA_LONG} *{@code long} *{@code long long} *
{@code unsigned long long}{@link ValueLayout#JAVA_LONG} *{@code long} *{@code float} *{@link ValueLayout#JAVA_FLOAT} *{@code float} *{@code double} *{@link ValueLayout#JAVA_DOUBLE} *{@code double} {@code size_t} *{@link ValueLayout#JAVA_LONG} *{@code long} *{@code char*}, {@code int**}, {@code struct Point*} *{@link ValueLayout#ADDRESS} *{@link MemorySegment} *{@code int (*ptr)[10]} ** * ValueLayout.ADDRESS.withTargetLayout( * MemoryLayout.sequenceLayout(10, * ValueLayout.JAVA_INT) * ); **{@link MemorySegment} ** struct Point { int x; long y; };* ** MemoryLayout.structLayout( * ValueLayout.JAVA_INT.withName("x"), * MemoryLayout.paddingLayout(32), * ValueLayout.JAVA_LONG.withName("y") * ); **{@link MemorySegment} ** * union Choice { float a; int b; }* ** MemoryLayout.unionLayout( * ValueLayout.JAVA_FLOAT.withName("a"), * ValueLayout.JAVA_INT.withName("b") * ); **{@link MemorySegment} *
* All native linker implementations operate on a subset of memory layouts. More formally, a layout {@code L} * is supported by a native linker {@code NL} if: *
* To invoke the {@code 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 {@linkplain MemorySegment memory segments}): * * {@snippet lang = java: * 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: * * {@snippet lang = java: * 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 {@code int[]} array, we can specify {@link 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 {@linkplain FunctionDescriptor#toMethodType() turn} * that function descriptor into a suitable {@linkplain java.lang.invoke.MethodType method type} which we then use to look up * the comparator method handle. We can now create an upcall stub which points to that method, and pass it, as a function * pointer, to the {@code qsort} downcall handle, as follows: * * {@snippet lang = java: * 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 * {@code 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. * *
* First, we need to create the downcall method handles for {@code malloc} and {@code free}, as follows: * * {@snippet lang = java: * Linker linker = Linker.nativeLinker(); * * MethodHandle malloc = linker.downcallHandle( * linker.defaultLookup().find("malloc").orElseThrow(), * FunctionDescriptor.of(ADDRESS, JAVA_LONG) * ); * * MethodHandle free = linker.downcallHandle( * linker.defaultLookup().find("free").orElseThrow(), * FunctionDescriptor.ofVoid(ADDRESS) * ); * } * * When a native function returning a pointer (such as {@code 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: * * {@snippet lang = java: * MemorySegment segment = (MemorySegment)malloc.invokeExact(100); * } * * The size of the segment returned by the {@code 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 {@linkplain Arena 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 {@link MemorySegment#reinterpret(long, Arena, Consumer)}, * as follows: * * {@snippet lang = java: * 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 {@code allocateMemory} method defined above accepts two parameters: a size and an arena. The method calls the * {@code malloc} downcall method handle, and unsafely reinterprets the returned segment, by giving it a new size * (the size passed to the {@code 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 {@code free} downcall method handle, to deallocate the underlying * region of memory. We can use the {@code allocateMemory} method as follows: * * {@snippet lang = java: * try (Arena arena = Arena.ofConfined()) { * MemorySegment segment = allocateMemory(100, arena); * } // 'free' called here * } * * Note how the segment obtained from {@code 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 which created * the confined arena), and its lifetime is tied to the surrounding try-with-resources block. * *
* It should be noted that values passed as variadic arguments undergo default argument promotion in C. For instance, the * following argument promotions are applied: *
* 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 * {@link 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 {@code 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 {@code _Bool}, * {@code (unsigned) char}, {@code (unsigned) short}, and {@code float} (among others), will be rejected by the linker. * The {@link #canonicalLayouts()} method can be used to find which layout corresponds to a particular C type. *
* A well-known variadic function is the {@code printf} function, defined in the C standard library: * * {@snippet lang = c: * 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: * * {@snippet lang = c: * 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 {@code (char*, int, int, int)} as the format string accepts three integer parameters. We then need to use * a {@linkplain Linker.Option#firstVariadicArg(int) 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: * * {@snippet lang = java: * Linker linker = Linker.nativeLinker(); * MethodHandle printf = linker.downcallHandle( * linker.defaultLookup().find("printf").orElseThrow(), * 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: * * {@snippet lang = java: * try (Arena arena = Arena.ofConfined()) { * int res = (int)printf.invokeExact(arena.allocateFrom("%d plus %d equals %d"), 2, 2, 4); //prints "2 plus 2 equals 4" * } *} * *
* 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 {@linkplain MemorySegment memory segment}, 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. * * @implSpec * Implementations of this interface are immutable, thread-safe and value-based. * * @since 22 */ public sealed interface Linker permits AbstractLinker { /** * {@return 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. * * @apiNote It is not currently possible to obtain a linker for a different combination of OS and processor. * @implSpec A native linker implementation is guaranteed to provide canonical layouts for * basic C types. * @implNote The libraries exposed by the {@linkplain #defaultLookup() 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 {@code libc}, {@code libm} and {@code libdl}. */ static Linker nativeLinker() { return SharedUtils.getSystemLinker(); } /** * Creates a method handle which is used to call a foreign function with the given signature and address. *
* Calling this method is equivalent to the following code: * {@snippet lang=java : * linker.downcallHandle(function).bindTo(symbol); * } * * @param address the native memory segment whose {@linkplain MemorySegment#address() base address} is the * address of the target foreign function. * @param function the function descriptor of the target foreign function. * @param options the linker options associated with this linkage request. * @return a downcall method handle. * @throws IllegalArgumentException if the provided function descriptor is not supported by this linker * @throws IllegalArgumentException if {@code !address.isNative()}, or if {@code address.equals(MemorySegment.NULL)} * @throws IllegalArgumentException if an invalid combination of linker options is given * @throws IllegalCallerException If the caller is in a module that does not have native access enabled * * @see SymbolLookup */ @CallerSensitive @Restricted MethodHandle downcallHandle(MemorySegment address, FunctionDescriptor function, Option... options); /** * Creates a method handle which is used to call a foreign function with the given signature. *
* The Java {@linkplain java.lang.invoke.MethodType method type} associated with the returned method handle is * {@linkplain FunctionDescriptor#toMethodType() derived} from the argument and return layouts in the function descriptor, * but features an additional leading parameter of type {@link 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 {@link 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 * {@code A} of type {@link MemorySegment} whose corresponding layout is an {@linkplain AddressLayout address layout}: *
* Moreover, if the provided function descriptor's return layout is an {@linkplain AddressLayout 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 {@code 0}. * However, if the function descriptor's return layout has a {@linkplain AddressLayout#targetLayout() target layout} * {@code T}, then the size of the returned segment is set to {@code T.byteSize()}. *
* The returned method handle will throw an {@link IllegalArgumentException} if the {@link MemorySegment} * representing the target address of the foreign function is the {@link MemorySegment#NULL} address. If an argument * is a {@link MemorySegment}, whose corresponding layout is a {@linkplain GroupLayout group layout}, the linker * might attempt to access the contents of the segment. As such, one of the exceptions specified by the * {@link MemorySegment#get(ValueLayout.OfByte, long)} or the * {@link MemorySegment#copy(MemorySegment, long, MemorySegment, long, long)} methods may be thrown. * The returned method handle will additionally throw {@link NullPointerException} if any argument * passed to it is {@code null}. * * @param function the function descriptor of the target foreign function. * @param options the linker options associated with this linkage request. * @return a downcall method handle. * @throws IllegalArgumentException if the provided function descriptor is not supported by this linker * @throws IllegalArgumentException if an invalid combination of linker options is given * @throws IllegalCallerException If the caller is in a module that does not have native access enabled */ @CallerSensitive @Restricted MethodHandle downcallHandle(FunctionDescriptor function, Option... options); /** * 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 {@linkplain Arena#close() closed}. *
* An upcall stub argument whose corresponding layout is an {@linkplain AddressLayout address layout} * is a native segment associated with the global scope. * Under normal conditions, the size of this segment argument is {@code 0}. * However, if the address layout has a {@linkplain AddressLayout#targetLayout() target layout} {@code T}, then the size of the * segment argument is set to {@code 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 * {@link java.lang.invoke.MethodHandles#catchException(MethodHandle, Class, MethodHandle)} method handle combinator, * and handle exceptions as desired in the corresponding catch block. * * @param target the target method handle. * @param function the upcall stub function descriptor. * @param arena the arena associated with the returned upcall stub segment. * @param options the linker options associated with this linkage request. * @return 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 * @throws IllegalArgumentException if the type of {@code target} is incompatible with the * type {@linkplain FunctionDescriptor#toMethodType() derived} from {@code function} * @throws IllegalArgumentException if it is determined that the target method handle can throw an exception * @throws IllegalStateException if {@code arena.scope().isAlive() == false} * @throws WrongThreadException if {@code arena} is a confined arena, and this method is called from a * thread {@code T}, other than the arena's owner thread * @throws IllegalCallerException If the caller is in a module that does not have native access enabled */ @CallerSensitive @Restricted MemorySegment upcallStub(MethodHandle target, FunctionDescriptor function, Arena arena, Linker.Option... options); /** * Returns a symbol lookup for symbols in a set of commonly used libraries. *
* Each {@link Linker} is responsible for choosing libraries that are widely recognized as useful on the OS * and processor combination supported by the {@link Linker}. Accordingly, the precise set of symbols exposed by the * symbol lookup is unspecified; it varies from one {@link Linker} to another. * @implNote It is strongly recommended that the result of {@link #defaultLookup} exposes a set of symbols that is stable over time. * Clients of {@link #defaultLookup()} are likely to fail if a symbol that was previously exposed by the symbol lookup is no longer exposed. *
If an implementer provides {@link Linker} implementations for multiple OS and processor combinations, then it is strongly * recommended that the result of {@link #defaultLookup()} exposes, as much as possible, a consistent set of symbols * across all the OS and processor combinations. * @return a symbol lookup for symbols in a set of commonly used libraries. */ SymbolLookup defaultLookup(); /** * {@return an unmodifiable mapping between the names of data types used by the ABI implemented by this linker and their * canonical layouts} *
* Each {@link Linker} is responsible for choosing the data types that are widely recognized as useful on the OS * and processor combination supported by the {@link Linker}. Accordingly, the precise set of data type names * and canonical layouts exposed by the linker is unspecified; it varies from one {@link Linker} to another. * @implNote It is strongly recommended that the result of {@link #canonicalLayouts()} exposes a set of symbols that is stable over time. * Clients of {@link #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 {@link Linker} implementations for multiple OS and processor combinations, then it is strongly
* recommended that the result of {@link #canonicalLayouts()} exposes, as much as possible, a consistent set of symbols
* across all the OS and processor combinations.
*/
Map
* The {@code index} value must conform to {@code 0 <= index <= N}, where {@code N} is the number of argument
* layouts of the function descriptor used in conjunction with this linker option. When the {@code index} is:
*
* 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 {@link MemorySegment} parameter directly
* following the target address, and optional {@link 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
* {@linkplain #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 {@linkplain #captureStateLayout capture state layout}.
*
* The following example demonstrates the use of this linker option:
* {@snippet lang = "java":
* 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 {@link #critical}.
*
* @param capturedState the names of the values to save.
* @throws IllegalArgumentException if at least one of the provided {@code capturedState} names
* is unsupported on the current platform
* @see #captureStateLayout()
*/
static Option captureCallState(String... capturedState) {
Set
* The capture state layout is platform-dependent but is guaranteed to be
* a {@linkplain StructLayout struct layout} containing only {@linkplain ValueLayout value layouts}
* and possibly {@linkplain PaddingLayout padding layouts}.
* As an example, on Windows, the returned layout might contain three value layouts named:
*
* Clients can obtain the names of the supported captured value layouts as follows:
* {@snippet lang = java:
* List
* 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 which 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.
*/
static Option critical() {
return LinkerOptions.Critical.INSTANCE;
}
}
}
*
* 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.
*
* @implNote The index value is validated when making a linkage request, which is when the function descriptor
* against which the index is validated is available.
*
* @param index the index of the first variadic argument layout in the function descriptor associated
* with a downcall linkage request.
*/
static Option firstVariadicArg(int index) {
return new LinkerOptions.FirstVariadicArg(index);
}
/**
* {@return 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}
*
*
*