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A library for invoking native functions from java

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/*
 * Copyright (C) 2008-2010 Wayne Meissner
 *
 * This file is part of the JNR project.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *    http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package jnr.ffi.annotations;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

/**
 * Indicates that the parameter is an OUT parameter.
 * 
 * When a java object is passed to a native function as a pointer
 * (for example {@link jnr.ffi.Pointer}, {@link jnr.ffi.Struct}, {@link java.nio.ByteBuffer}),
 * then a temporary native memory block is allocated, the java data is copied to
 * the temporary memory and the address of the temporary memory is passed to the function.
 * After the function returns, the java data is automatically updated from the
 * contents of the native memory.
 *
 * 
As this extra copying can be expensive, for native functions which only
 * write to the passed in memory block and do not use the existing contents, 
 * parameters can be annotated with {@code @Out} so there is only copied {@code OUT} 
 * from native memory to java memory after the call, and the unneccessary copy {@code IN}
 * from java to native memory before the call can be avoided.
 *
 * Parameters with neither a {@code @In} nor a {@code @Out} annotation will copy both ways.
 *
 * @see In
 * @see Clear
 */
@Retention(RetentionPolicy.RUNTIME)
@Target({ ElementType.PARAMETER, ElementType.METHOD, ElementType.ANNOTATION_TYPE })
public @interface Out {

}