src/java.base/share/classes/java/lang/foreign/Linker.java - master - niceSOFT/openjdk21

src/java.base/share/classes/java/lang/foreign/Linker.java
764 строки44 KB
Goetz Lindenmaier
8310643: Misformatted copyright messages in FFM
12 май 2025, 17:17
12 май 2025, 17:17d4f2f3a
/*
 * Copyright (c) 2020, 2023, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

package java.lang.foreign;

import jdk.internal.foreign.abi.AbstractLinker;
import jdk.internal.foreign.abi.LinkerOptions;
import jdk.internal.foreign.abi.CapturableState;
import jdk.internal.foreign.abi.SharedUtils;
import jdk.internal.javac.PreviewFeature;
import jdk.internal.reflect.CallerSensitive;
import jdk.internal.reflect.Reflection;

import java.lang.invoke.MethodHandle;
import java.nio.ByteOrder;
import java.util.Objects;
import java.util.Optional;
import java.util.Set;
import java.util.function.Consumer;
import java.util.stream.Collectors;
import java.util.stream.Stream;

/**
 * A linker provides access to foreign functions from Java code, and access to Java code from foreign functions.
 * <p>
 * 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.
 * <p>
 * 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:
 * <ul>
 * <li>A linker allows Java code to link against foreign functions, via
 * {@linkplain #downcallHandle(MemorySegment, FunctionDescriptor, Option...) downcall method handles}; and</li>
 * <li>A linker allows foreign functions to call Java method handles,
 * via the generation of {@linkplain #upcallStub(MethodHandle, FunctionDescriptor, Arena, Option...) upcall stubs}.</li>
 * </ul>
 * 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}.
 *
 * <h2 id="native-linker">Calling native functions</h2>
 *
 * The {@linkplain #nativeLinker() native linker} can be used to link against functions
 * defined in C libraries (native functions). Suppose we wish to downcall from Java to the {@code strlen} function
 * defined in the standard C library:
 * {@snippet lang = c:
 * size_t strlen(const char *s);
 * }
 * A downcall method handle that exposes {@code strlen} is obtained, using the native linker, as follows:
 *
 * {@snippet lang = java:
 * Linker linker = Linker.nativeLinker();
 * MethodHandle strlen = linker.downcallHandle(
 *     linker.defaultLookup().find("strlen").orElseThrow(),
 *     FunctionDescriptor.of(JAVA_LONG, ADDRESS)
 * );
 * }
 *
 * Note how the native linker also provides access, via its {@linkplain #defaultLookup() 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 {@code strlen} native function. That address is then passed, along with
 * a <em>platform-dependent description</em> of the signature of the function expressed as a
 * {@link FunctionDescriptor} (more on that below) to the native linker's
 * {@link #downcallHandle(MemorySegment, FunctionDescriptor, Option...)} method.
 * The obtained downcall method handle is then invoked as follows:
 *
 * {@snippet lang = java:
 * try (Arena arena = Arena.ofConfined()) {
 *     MemorySegment str = arena.allocateUtf8String("Hello");
 *     long len = (long) strlen.invokeExact(str);  // 5
 * }
 * }
 * <h3 id="describing-c-sigs">Describing C signatures</h3>
 *
 * 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 {@link FunctionDescriptor function descriptor},
 * defines the layouts associated with the parameter types and return type (if any) of the C function.
 * <p>
 * Scalar C types such as {@code bool}, {@code int} are modelled as {@linkplain ValueLayout value layouts}
 * of a suitable carrier. The mapping between a scalar type and its corresponding layout is dependent on the ABI
 * implemented by the native linker. For instance, the C type {@code 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. Similarly, the C type {@code size_t} maps to the layout constant {@link ValueLayout#JAVA_LONG}
 * on 64-bit platforms, but maps to the layout constant {@link ValueLayout#JAVA_INT} on 32-bit platforms.
 * <p>
 * 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.
 * <p>
 * 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}.
 * <p>
 * The following table shows some examples of how C types are modelled in Linux/x64:
 *
 * <blockquote><table class="plain">
 * <caption style="display:none">Mapping C types</caption>
 * <thead>
 * <tr>
 *     <th scope="col">C type</th>
 *     <th scope="col">Layout</th>
 *     <th scope="col">Java type</th>
 * </tr>
 * </thead>
 * <tbody>
 * <tr><th scope="row" style="font-weight:normal">{@code bool}</th>
 *     <td style="text-align:center;">{@link ValueLayout#JAVA_BOOLEAN}</td>
 *     <td style="text-align:center;">{@code boolean}</td>
 * <tr><th scope="row" style="font-weight:normal">{@code char}</th>
 *     <td style="text-align:center;">{@link ValueLayout#JAVA_BYTE}</td>
 *     <td style="text-align:center;">{@code byte}</td>
 * <tr><th scope="row" style="font-weight:normal">{@code short}</th>
 *     <td style="text-align:center;">{@link ValueLayout#JAVA_SHORT}</td>
 *     <td style="text-align:center;">{@code short}</td>
 * <tr><th scope="row" style="font-weight:normal">{@code int}</th>
 *     <td style="text-align:center;">{@link ValueLayout#JAVA_INT}</td>
 *     <td style="text-align:center;">{@code int}</td>
 * <tr><th scope="row" style="font-weight:normal">{@code long}</th>
 *     <td style="text-align:center;">{@link ValueLayout#JAVA_LONG}</td>
 *     <td style="text-align:center;">{@code long}</td>
 * <tr><th scope="row" style="font-weight:normal">{@code long long}</th>
 *     <td style="text-align:center;">{@link ValueLayout#JAVA_LONG}</td>
 *     <td style="text-align:center;">{@code long}</td>
 * <tr><th scope="row" style="font-weight:normal">{@code float}</th>
 *     <td style="text-align:center;">{@link ValueLayout#JAVA_FLOAT}</td>
 *     <td style="text-align:center;">{@code float}</td>
 * <tr><th scope="row" style="font-weight:normal">{@code double}</th>
 *     <td style="text-align:center;">{@link ValueLayout#JAVA_DOUBLE}</td>
 *     <td style="text-align:center;">{@code double}</td>
 <tr><th scope="row" style="font-weight:normal">{@code size_t}</th>
 *     <td style="text-align:center;">{@link ValueLayout#JAVA_LONG}</td>
 *     <td style="text-align:center;">{@code long}</td>
 * <tr><th scope="row" style="font-weight:normal">{@code char*}, {@code int**}, {@code struct Point*}</th>
 *     <td style="text-align:center;">{@link ValueLayout#ADDRESS}</td>
 *     <td style="text-align:center;">{@link MemorySegment}</td>
 * <tr><th scope="row" style="font-weight:normal">{@code int (*ptr)[10]}</th>
 *     <td style="text-align:left;">
 * <pre>
 * ValueLayout.ADDRESS.withTargetLayout(
 *     MemoryLayout.sequenceLayout(10,
 *         ValueLayout.JAVA_INT)
 * );
 * </pre>
 *     <td style="text-align:center;">{@link MemorySegment}</td>
 * <tr><th scope="row" style="font-weight:normal"><code>struct Point { int x; long y; };</code></th>
 *     <td style="text-align:left;">
 * <pre>
 * MemoryLayout.structLayout(
 *     ValueLayout.JAVA_INT.withName("x"),
 *     MemoryLayout.paddingLayout(32),
 *     ValueLayout.JAVA_LONG.withName("y")
 * );
 * </pre>
 *     </td>
 *     <td style="text-align:center;">{@link MemorySegment}</td>
 * <tr><th scope="row" style="font-weight:normal"><code>union Choice { float a; int b; }</code></th>
 *     <td style="text-align:left;">
 * <pre>
 * MemoryLayout.unionLayout(
 *     ValueLayout.JAVA_FLOAT.withName("a"),
 *     ValueLayout.JAVA_INT.withName("b")
 * );
 * </pre>
 *     </td>
 *     <td style="text-align:center;">{@link MemorySegment}</td>
 * </tbody>
 * </table></blockquote>
 * <p>
 * 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:
 * <ul>
 * <li>{@code L} is a value layout {@code V} and {@code V.withoutName()} is {@linkplain MemoryLayout#equals(Object) equal}
 * to one of the following layout constants:
 * <ul>
 * <li>{@link ValueLayout#JAVA_BOOLEAN}</li>
 * <li>{@link ValueLayout#JAVA_BYTE}</li>
 * <li>{@link ValueLayout#JAVA_CHAR}</li>
 * <li>{@link ValueLayout#JAVA_SHORT}</li>
 * <li>{@link ValueLayout#JAVA_INT}</li>
 * <li>{@link ValueLayout#JAVA_LONG}</li>
 * <li>{@link ValueLayout#JAVA_FLOAT}</li>
 * <li>{@link ValueLayout#JAVA_DOUBLE}</li>
 * </ul></li>
 * <li>{@code L} is an address layout {@code A} and {@code A.withoutTargetLayout().withoutName()} is
 * {@linkplain MemoryLayout#equals(Object) equal} to {@link ValueLayout#ADDRESS}</li>
 * <li>{@code L} is a sequence layout {@code S} and all the following conditions hold:
 * <ol>
 * <li>the alignment constraint of {@code S} is set to its <a href="MemoryLayout.html#layout-align">natural alignment</a>, and</li>
 * <li>{@code S.elementLayout()} is a layout supported by {@code NL}.</li>
 * </ol>
 * </li>
 * <li>{@code L} is a group layout {@code G} and all the following conditions hold:
 * <ol>
 * <li>the alignment constraint of {@code G} is set to its <a href="MemoryLayout.html#layout-align">natural alignment</a>;</li>
 * <li>the size of {@code G} is a multiple of its alignment constraint;</li>
 * <li>each member layout in {@code G.memberLayouts()} is either a padding layout or a layout supported by {@code NL}, and</li>
 * <li>{@code G} does not contain padding other than what is strictly required to align its non-padding layout elements, or to satisfy (2).</li>
 * </ol>
 * </li>
 * </ul>
 *
 * A native linker only supports function descriptors whose argument/return layouts are layouts supported by that linker
 * and are not sequence layouts.
 *
 * <h3 id="function-pointers">Function pointers</h3>
 *
 * Sometimes, it is useful to pass Java code as a function pointer to some native function; this is achieved by using
 * an {@linkplain #upcallStub(MethodHandle, FunctionDescriptor, Arena, Option...) upcall stub}. To demonstrate this,
 * let's consider the following function from the C standard library:
 *
 * {@snippet lang = c:
 * void qsort(void *base, size_t nmemb, size_t size,
 *            int (*compar)(const void *, const void *));
 * }
 *
 * The {@code 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 {@code compar} parameter). To be able to call the {@code qsort} function from Java,
 * we must first create a downcall method handle for it, as follows:
 *
 * {@snippet lang = java:
 * Linker linker = Linker.nativeLinker();
 * MethodHandle qsort = linker.downcallHandle(
 *     linker.defaultLookup().find("qsort").orElseThrow(),
 *         FunctionDescriptor.ofVoid(ADDRESS, JAVA_LONG, JAVA_LONG, ADDRESS)
 * );
 * }
 *
 * As before, we use {@link ValueLayout#JAVA_LONG} to map the C type {@code size_t} type, and {@link ValueLayout#ADDRESS}
 * for both the first pointer parameter (the array pointer) and the last parameter (the function pointer).
 * <p>
 * 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.allocateArray(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.
 *
 * <h3 id="by-ref">Functions returning pointers</h3>
 *
 * 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:
 *
 * {@snippet lang = c:
 * void *malloc(size_t size);
 * }
 *
 * The {@code malloc} function allocates a region of memory of given size,
 * and returns a pointer to that region of memory, which is later deallocated using another function from
 * the C standard library:
 *
 * {@snippet lang = c:
 * void free(void *ptr);
 * }
 *
 * The {@code 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 <em>safe</em> allocation
 * API (the approach outlined below can of course be generalized to allocation functions other than {@code malloc}
 * and {@code free}).
 * <p>
 * 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
 * <a href="MemorySegment.html#wrapping-addresses">zero</a>. Moreover, the scope of the
 * returned segment is a fresh scope that is always alive. 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 <em>cleanup action</em> 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 <em>try-with-resources</em> block.
 *
 * <h3 id="variadic-funcs">Variadic functions</h3>
 *
 * Variadic functions are C functions which can accept a variable number and type of arguments. They are declared:
 * <ol>
 * <li>With a trailing ellipsis ({@code ...}) at the end of the formal parameter list, such as: {@code void foo(int x, ...);}</li>
 * <li>With an empty formal parameter list, called a prototype-less function, such as: {@code void foo();}</li>
 * </ol>
 * The arguments passed in place of the ellipsis, or the arguments passed to a prototype-less function are called
 * <em>variadic arguments</em>. Variadic functions are, essentially, templates that can be <em>specialized</em> into multiple
 * non-variadic functions by replacing the {@code ...} or empty formal parameter list with a list of <em>variadic parameters</em>
 * of a fixed number and type.
 * <p>
 * It should be noted that values passed as variadic arguments undergo default argument promotion in C. For instance, the
 * following argument promotions are applied:
 * <ul>
 * <li>{@code _Bool} -> {@code unsigned int}</li>
 * <li>{@code [signed] char} -> {@code [signed] int}</li>
 * <li>{@code [signed] short} -> {@code [signed] int}</li>
 * <li>{@code float} -> {@code double}</li>
 * </ul>
 * 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
 * specialized form of a variadic function, as the variadic parameters of the specialized form will always have a promoted
 * type.
 * <p>
 * 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 <em>variadic argument layouts</em>. For a prototype-less function, the index passed to
 * {@link Linker.Option#firstVariadicArg(int)} should always be {@code 0}.
 * <p>
 * The native linker will reject an attempt to link a specialized function descriptor with any variadic argument layouts
 * corresponding to a C type that would be subject to default argument promotion (as described above). Exactly which layouts
 * will be rejected is platform specific, but as an example: on Linux/x64 the layouts {@link ValueLayout#JAVA_BOOLEAN},
 * {@link ValueLayout#JAVA_BYTE}, {@link ValueLayout#JAVA_CHAR}, {@link ValueLayout#JAVA_SHORT}, and
 * {@link ValueLayout#JAVA_FLOAT} will be rejected.
 * <p>
 * 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.allocateUtf8String("%d plus %d equals %d"), 2, 2, 4); //prints "2 plus 2 equals 4"
 * }
 * }
 *
 * <h2 id="safety">Safety considerations</h2>
 *
 * 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.
 * <p>
 * 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 <a href="{@docRoot}/java.base/java/lang/doc-files/ValueBased.html">value-based</a>.
 *
 * @since 19
 */
@PreviewFeature(feature=PreviewFeature.Feature.FOREIGN)
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.
     * @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}.
     *
     * @throws UnsupportedOperationException if the underlying native platform is not supported.
     */
    static Linker nativeLinker() {
        return SharedUtils.getSystemLinker();
    }

    /**
     * Creates a method handle which is used to call a foreign function with the given signature and address.
     * <p>
     * Calling this method is equivalent to the following code:
     * {@snippet lang=java :
     * linker.downcallHandle(function).bindTo(symbol);
     * }
     * <p>
     * This method is <a href="package-summary.html#restricted"><em>restricted</em></a>.
     * 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.
     *
     * @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
    MethodHandle downcallHandle(MemorySegment address, FunctionDescriptor function, Option... options);

    /**
     * Creates a method handle which is used to call a foreign function with the given signature.
     * <p>
     * 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.
     * <p>
     * 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}:
     * <ul>
     *     <li>{@code A.scope().isAlive() == true}. Otherwise, the invocation throws {@link IllegalStateException};</li>
     *     <li>The invocation occurs in a thread {@code T} such that {@code A.isAccessibleBy(T) == true}.
     *     Otherwise, the invocation throws {@link WrongThreadException}; and</li>
     *     <li>{@code A} is kept alive during the invocation. For instance, if {@code A} has been obtained using a
     *     {@linkplain Arena#ofShared() shared arena}, any attempt to {@linkplain Arena#close() close}
     *     the arena while the downcall method handle is still executing will result in an {@link IllegalStateException}.</li>
     *</ul>
     * <p>
     * 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
     * a fresh scope that is always alive. 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()}.
     * <p>
     * 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.
     * The returned method handle will additionally throw {@link NullPointerException} if any argument passed to it is {@code null}.
     * <p>
     * This method is <a href="package-summary.html#restricted"><em>restricted</em></a>.
     * 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.
     *
     * @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
    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.
     * <p>
     * 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}.
     * <p>
     * An upcall stub argument whose corresponding layout is an {@linkplain AddressLayout address layout}
     * is a native segment associated with a fresh scope that is always alive.
     * 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()}.
     * <p>
     * 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.
     * <p>
     * This method is <a href="package-summary.html#restricted"><em>restricted</em></a>.
     * 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.
     *
     * @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
    MemorySegment upcallStub(MethodHandle target, FunctionDescriptor function, Arena arena, Linker.Option... options);

    /**
     * Returns a symbol lookup for symbols in a set of commonly used libraries.
     * <p>
     * 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.
     * <p>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();

    /**
     * A linker option is used to provide additional parameters to a linkage request.
     * @since 20
     */
    @PreviewFeature(feature=PreviewFeature.Feature.FOREIGN)
    sealed interface Option
            permits LinkerOptions.LinkerOptionImpl {

        /**
         * {@return 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}
         * <p>
         * 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:
         * <ul>
         * <li>{@code 0}, all arguments passed to the function are passed as variadic arguments</li>
         * <li>{@code N}, none of the arguments passed to the function are passed as variadic arguments</li>
         * <li>{@code n}, where {@code 0 < m < N}, the arguments {@code m..N} are passed as variadic arguments</li>
         * </ul>
         * It is important to always use this linker option when linking a <a href=Linker.html#variadic-funcs>variadic
         * function</a>, 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}
         * <p>
         * 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 <em>capture state segment</em>, represents the native segment into which
         * the captured state is written.
         * <p>
         * 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.
         * <p>
         * Captured state can be retrieved from the capture state segment by constructing var handles
         * from the {@linkplain #captureStateLayout capture state layout}.
         * <p>
         * 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
         * }
         * }
         *
         * @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<CapturableState> set = Stream.of(Objects.requireNonNull(capturedState))
                    .map(Objects::requireNonNull)
                    .map(CapturableState::forName)
                    .collect(Collectors.toSet());
            return new LinkerOptions.CaptureCallState(set);
        }

         /**
         * {@return a struct layout that represents the layout of the capture state segment that is passed
         *          to a downcall handle linked with {@link #captureCallState(String...)}}
         * <p>
         * The capture state layout is <em>platform-dependent</em> 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:
         * <ul>
         *     <li>GetLastError</li>
         *     <li>WSAGetLastError</li>
         *     <li>errno</li>
         * </ul>
         * <p>
         * Clients can obtain the names of the supported captured value layouts as follows:
         * {@snippet lang = java:
         *    List<String> capturedNames = Linker.Option.captureStateLayout().memberLayouts().stream()
         *        .map(MemoryLayout::name)
         *        .flatMap(Optional::stream)
         *        .toList();
         * }
         *
         * @see #captureCallState(String...)
         */
        static StructLayout captureStateLayout() {
            return CapturableState.LAYOUT;
        }

        /**
         * {@return a linker option used to mark a foreign function as <em>trivial</em>}
         * <p>
         * A trivial 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).
         * <p>
         * Using this linker option is a hint which some implementations may use to apply
         * optimizations that are only valid for trivial functions.
         * <p>
         * Using this linker option when linking non trivial functions is likely to have adverse effects,
         * such as loss of performance, or JVM crashes.
         */
        static Option isTrivial() {
            return LinkerOptions.IsTrivial.INSTANCE;
        }
    }
}