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8282191: Implementation of Foreign Function & Memory API (Preview)
Reviewed-by: erikj, jvernee, psandoz, dholmes, mchung
This commit is contained in:
Maurizio Cimadamore
parent
3be394e160
commit
2c5d136260
303 changed files with 33474 additions and 9186 deletions
238
src/java.base/share/classes/java/lang/foreign/package-info.java
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238
src/java.base/share/classes/java/lang/foreign/package-info.java
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/*
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* Copyright (c) 2019, 2022, Oracle and/or its affiliates. All rights reserved.
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* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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*
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* This code is free software; you can redistribute it and/or modify it
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* under the terms of the GNU General Public License version 2 only, as
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* published by the Free Software Foundation. Oracle designates this
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* particular file as subject to the "Classpath" exception as provided
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* by Oracle in the LICENSE file that accompanied this code.
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*
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* This code is distributed in the hope that it will be useful, but WITHOUT
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* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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* version 2 for more details (a copy is included in the LICENSE file that
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* accompanied this code).
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*
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* You should have received a copy of the GNU General Public License version
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* 2 along with this work; if not, write to the Free Software Foundation,
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* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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*
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* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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* or visit www.oracle.com if you need additional information or have any
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* questions.
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*
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*/
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/**
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* <p>Provides low-level access to memory and functions outside the Java runtime.
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*
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* <h2>Foreign memory access</h2>
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*
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* <p>
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* The main abstraction introduced to support foreign memory access is {@link java.lang.foreign.MemorySegment}, which
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* models a contiguous memory region, residing either inside or outside the Java heap. The contents of a memory
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* segment can be described using a {@link java.lang.foreign.MemoryLayout memory layout}, which provides
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* basic operations to query sizes, offsets and alignment constraints. Memory layouts also provide
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* an alternate, more abstract way, to <a href=MemorySegment.html#segment-deref>dereference memory segments</a>
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* using {@linkplain java.lang.foreign.MemoryLayout#varHandle(java.lang.foreign.MemoryLayout.PathElement...) access var handles},
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* which can be computed using <a href="MemoryLayout.html#layout-paths"><em>layout paths</em></a>.
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*
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* For example, to allocate an off-heap memory region big enough to hold 10 values of the primitive type {@code int}, and fill it with values
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* ranging from {@code 0} to {@code 9}, we can use the following code:
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*
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* {@snippet lang=java :
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* MemorySegment segment = MemorySegment.allocateNative(10 * 4, MemorySession.openImplicit());
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* for (int i = 0 ; i < 10 ; i++) {
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* segment.setAtIndex(ValueLayout.JAVA_INT, i, i);
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* }
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* }
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*
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* This code creates a <em>native</em> memory segment, that is, a memory segment backed by
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* off-heap memory; the size of the segment is 40 bytes, enough to store 10 values of the primitive type {@code int}.
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* Inside a loop, we then initialize the contents of the memory segment; note how the
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* {@linkplain java.lang.foreign.MemorySegment#setAtIndex(ValueLayout.OfInt, long, int) dereference method}
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* accepts a {@linkplain java.lang.foreign.ValueLayout value layout}, which specifies the size, alignment constraints,
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* byte order as well as the Java type ({@code int}, in this case) associated with the dereference operation. More specifically,
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* if we view the memory segment as a set of 10 adjacent slots, {@code s[i]}, where {@code 0 <= i < 10},
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* where the size of each slot is exactly 4 bytes, the initialization logic above will set each slot
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* so that {@code s[i] = i}, again where {@code 0 <= i < 10}.
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*
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* <h3><a id="deallocation"></a>Deterministic deallocation</h3>
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*
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* When writing code that manipulates memory segments, especially if backed by memory which resides outside the Java heap, it is
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* often crucial that the resources associated with a memory segment are released when the segment is no longer in use,
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* and in a timely fashion. For this reason, there might be cases where waiting for the garbage collector to determine that a segment
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* is <a href="../../../java/lang/ref/package.html#reachability">unreachable</a> is not optimal.
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* Clients that operate under these assumptions might want to programmatically release the memory associated
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* with a memory segment. This can be done, using the {@link java.lang.foreign.MemorySession} abstraction, as shown below:
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*
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* {@snippet lang=java :
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* try (MemorySession session = MemorySession.openConfined()) {
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* MemorySegment segment = MemorySegment.allocateNative(10 * 4, session);
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* for (int i = 0 ; i < 10 ; i++) {
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* segment.setAtIndex(ValueLayout.JAVA_INT, i, i);
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* }
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* }
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* }
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*
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* This example is almost identical to the prior one; this time we first create a so called <em>memory session</em>,
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* which is used to <em>bind</em> the life-cycle of the segment created immediately afterwards. Note the use of the
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* <em>try-with-resources</em> construct: this idiom ensures that all the memory resources associated with the segment will be released
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* at the end of the block, according to the semantics described in Section {@jls 14.20.3} of <cite>The Java Language Specification</cite>.
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*
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* <h3><a id="safety"></a>Safety</h3>
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*
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* This API provides strong safety guarantees when it comes to memory access. First, when dereferencing a memory segment,
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* the access coordinates are validated (upon access), to make sure that access does not occur at any address which resides
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* <em>outside</em> the boundaries of the memory segment used by the dereference operation. We call this guarantee <em>spatial safety</em>;
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* in other words, access to memory segments is bounds-checked, in the same way as array access is, as described in
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* Section {@jls 15.10.4} of <cite>The Java Language Specification</cite>.
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* <p>
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* Since memory segments can be closed (see above), segments are also validated (upon access) to make sure that
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* the memory session associated with the segment being accessed has not been closed prematurely.
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* We call this guarantee <em>temporal safety</em>. Together, spatial and temporal safety ensure that each memory access
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* operation either succeeds - and accesses a valid memory location - or fails.
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*
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* <h2>Foreign function access</h2>
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* The key abstractions introduced to support foreign function access are {@link java.lang.foreign.SymbolLookup},
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* {@link java.lang.foreign.FunctionDescriptor} and {@link java.lang.foreign.Linker}. The first is used to look up symbols
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* inside libraries; the second is used to model the signature of foreign functions, while the third provides
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* linking capabilities which allows modelling foreign functions as {@link java.lang.invoke.MethodHandle} instances,
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* so that clients can perform foreign function calls directly in Java, without the need for intermediate layers of C/C++
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* code (as is the case with the <a href="{@docRoot}/../specs/jni/index.html">Java Native Interface (JNI)</a>).
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* <p>
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* For example, to compute the length of a string using the C standard library function {@code strlen} on a Linux x64 platform,
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* we can use the following code:
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*
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* {@snippet lang=java :
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* var linker = Linker.nativeLinker();
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* MethodHandle strlen = linker.downcallHandle(
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* linker.lookup("strlen").get(),
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* FunctionDescriptor.of(ValueLayout.JAVA_LONG, ValueLayout.ADDRESS)
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* );
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*
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* try (var session = MemorySession.openConfined()) {
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* var cString = MemorySegment.allocateNative(5 + 1, session);
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* cString.setUtf8String("Hello");
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* long len = (long)strlen.invoke(cString); // 5
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* }
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* }
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*
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* Here, we obtain a {@linkplain java.lang.foreign.Linker#nativeLinker() native linker} and we use it
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* to {@linkplain java.lang.foreign.SymbolLookup#lookup(java.lang.String) look up} the {@code strlen} symbol in the
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* standard C library; a <em>downcall method handle</em> targeting said symbol is subsequently
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* {@linkplain java.lang.foreign.Linker#downcallHandle(java.lang.foreign.FunctionDescriptor) obtained}.
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* To complete the linking successfully, we must provide a {@link java.lang.foreign.FunctionDescriptor} instance,
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* describing the signature of the {@code strlen} function.
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* From this information, the linker will uniquely determine the sequence of steps which will turn
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* the method handle invocation (here performed using {@link java.lang.invoke.MethodHandle#invoke(java.lang.Object...)})
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* into a foreign function call, according to the rules specified by the ABI of the underlying platform.
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* The {@link java.lang.foreign.MemorySegment} class also provides many useful methods for
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* interacting with foreign code, such as converting Java strings
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* {@linkplain java.lang.foreign.MemorySegment#setUtf8String(long, java.lang.String) into} zero-terminated, UTF-8 strings and
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* {@linkplain java.lang.foreign.MemorySegment#getUtf8String(long) back}, as demonstrated in the above example.
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*
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* <h3>Foreign addresses</h3>
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*
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* When a memory segment is created from Java code, the segment properties (spatial bounds, temporal bounds and confinement)
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* are fully known at segment creation. But when interacting with foreign functions, clients will often receive <em>raw</em> pointers.
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* Such pointers have no spatial bounds. For example, the C type {@code char*} can refer to a single {@code char} value,
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* or an array of {@code char} values, of given size. Nor do said pointers have any notion of temporal bounds or thread-confinement.
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* <p>
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* Raw pointers are modelled using the {@link java.lang.foreign.MemoryAddress} class. When clients receive a
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* memory address instance from a foreign function call, they can perform memory dereference on it directly,
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* using one of the many <em>unsafe</em>
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* {@linkplain java.lang.foreign.MemoryAddress#get(java.lang.foreign.ValueLayout.OfInt, long) dereference methods}
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* provided:
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*
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* {@snippet lang=java :
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* MemoryAddress addr = ... // obtain address from foreign function call
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* int x = addr.get(ValueLayout.JAVA_INT, 0);
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* }
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*
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* Alternatively, the client can
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* {@linkplain java.lang.foreign.MemorySegment#ofAddress(java.lang.foreign.MemoryAddress, long, java.lang.foreign.MemorySession) create}
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* a memory segment <em>unsafely</em>. This allows the client to inject extra knowledge about spatial bounds which might,
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* for instance, be available in the documentation of the foreign function which produced the native address.
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* Here is how an unsafe segment can be created from a memory address:
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*
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* {@snippet lang=java :
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* MemorySession session = ... // initialize a memory session object
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* MemoryAddress addr = ... // obtain address from foreign function call
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* MemorySegment segment = MemorySegment.ofAddress(addr, 4, session); // segment is 4 bytes long
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* int x = segment.get(ValueLayout.JAVA_INT, 0);
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* }
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*
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* <h3>Upcalls</h3>
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* The {@link java.lang.foreign.Linker} interface also allows clients to turn an existing method handle (which might point
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* to a Java method) into a memory address, so that Java code can effectively be passed to other foreign functions.
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* For instance, we can write a method that compares two integer values, as follows:
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*
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* {@snippet lang=java :
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* class IntComparator {
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* static int intCompare(MemoryAddress addr1, MemoryAddress addr2) {
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* return addr1.get(ValueLayout.JAVA_INT, 0) - addr2.get(ValueLayout.JAVA_INT, 0);
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* }
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* }
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* }
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*
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* The above method dereferences two memory addresses containing an integer value, and performs a simple comparison
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* by returning the difference between such values. We can then obtain a method handle which targets the above static
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* method, as follows:
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*
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* {@snippet lang=java :
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* FunctionDescriptor intCompareDescriptor = FunctionDescriptor.of(ValueLayout.JAVA_INT, ValueLayout.ADDRESS, ValueLayout.ADDRESS);
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* MethodHandle intCompareHandle = MethodHandles.lookup().findStatic(IntComparator.class,
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* "intCompare",
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* CLinker.upcallType(comparFunction));
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* }
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*
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* As before, we need to create a {@link java.lang.foreign.FunctionDescriptor} instance, this time describing the signature
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* of the function pointer we want to create. The descriptor can be used to
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* {@linkplain java.lang.foreign.Linker#upcallType(java.lang.foreign.FunctionDescriptor) derive} a method type
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* that can be used to look up the method handle for {@code IntComparator.intCompare}.
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* <p>
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* Now that we have a method handle instance, we can turn it into a fresh function pointer,
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* using the {@link java.lang.foreign.Linker} interface, as follows:
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*
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* {@snippet lang=java :
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* MemorySession session = ...
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* Addressable comparFunc = CLinker.nativeLinker().upcallStub(
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* intCompareHandle, intCompareDescriptor, session);
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* );
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* }
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*
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* The {@link java.lang.foreign.FunctionDescriptor} instance created in the previous step is then used to
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* {@linkplain java.lang.foreign.Linker#upcallStub(java.lang.invoke.MethodHandle, java.lang.foreign.FunctionDescriptor, java.lang.foreign.MemorySession) create}
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* a new upcall stub; the layouts in the function descriptors allow the linker to determine the sequence of steps which
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* allow foreign code to call the stub for {@code intCompareHandle} according to the rules specified by the ABI of the
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* underlying platform.
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* The lifecycle of the upcall stub is tied to the {@linkplain java.lang.foreign.MemorySession memory session}
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* provided when the upcall stub is created. This same session is made available by the {@link java.lang.foreign.MemorySegment}
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* instance returned by that method.
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*
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* <a id="restricted"></a>
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* <h2>Restricted methods</h2>
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* Some methods in this package are considered <em>restricted</em>. Restricted methods are typically used to bind native
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* foreign data and/or functions to first-class Java API elements which can then be used directly by clients. For instance
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* the restricted method {@link java.lang.foreign.MemorySegment#ofAddress(MemoryAddress, long, MemorySession)}
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* can be used to create a fresh segment with the given spatial bounds out of a native address.
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* <p>
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* Binding foreign data and/or functions is generally unsafe and, if done incorrectly, can result in VM crashes, or memory corruption when the bound Java API element is accessed.
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* For instance, in the case of {@link java.lang.foreign.MemorySegment#ofAddress(MemoryAddress, long, MemorySession)},
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* if the provided spatial bounds are incorrect, a client of the segment returned by that method might crash the VM, or corrupt
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* memory when attempting to dereference said segment. For these reasons, it is crucial for code that calls a restricted method
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* to never pass arguments that might cause incorrect binding of foreign data and/or functions to a Java API.
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* <p>
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* Access to restricted methods can be controlled using the command line option {@code --enable-native-access=M1,M2, ... Mn},
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* where {@code M1}, {@code M2}, {@code ... Mn} are module names (for the unnamed module, the special value {@code ALL-UNNAMED}
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* can be used). If this option is specified, access to restricted methods is only granted to the modules listed by that
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* option. If this option is not specified, access to restricted methods is enabled for all modules, but
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* access to restricted methods will result in runtime warnings.
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* <p>
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* For every class in this package, unless specified otherwise, any method arguments of reference
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* type must not be null, and any null argument will elicit a {@code NullPointerException}. This fact is not individually
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* documented for methods of this API.
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*/
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package java.lang.foreign;
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