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/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

#include "nsISupports.idl"

/**
 *
 * nsIMemory: interface to allocate and deallocate memory. Also provides
 * for notifications in low-memory situations.
 *
 * The frozen exported symbols moz_xmalloc, moz_xrealloc, and free
 * provide a more efficient way to access XPCOM memory allocation. Using
 * those symbols is preferred to using the methods on this interface.
 *
 * A client that wishes to be notified of low memory situations (for
 * example, because the client maintains a large memory cache that
 * could be released when memory is tight) should register with the
 * observer service (see nsIObserverService) using the topic 
 * "memory-pressure".  There are specific types of notications 
 * that can occur.  These types will be passed as the |aData| 
 * parameter of the of the "memory-pressure" notification: 
 * 
 * "low-memory"
 * This will be passed as the extra data when the pressure 
 * observer is being asked to flush for low-memory conditions.
 *
 * "low-memory-ongoing"
 * This will be passed when we continue to be in a low-memory
 * condition and we want to flush caches and do other cheap
 * forms of memory minimization, but heavy handed approaches like
 * a GC are unlikely to succeed.
 *
 * "-no-forward"
 * This is appended to the above two parameters when the resulting
 * notification should not be forwarded to the child processes.
 *
 * "heap-minimize"
 * This will be passed as the extra data when the pressure 
 * observer is being asked to flush because of a heap minimize 
 * call.
 *
 * "alloc-failure"
 * This will be passed as the extra data when the pressure 
 * observer has been asked to flush because a malloc() or 
 * realloc() has failed.
 *
 * "lowering-priority"
 * This will be passed as the extra data when the priority of a child
 * process is lowered. The pressure observers could take the chance to
 * clear caches that could be easily regenerated. This type of
 * notification only appears in child processes.
 */

[scriptable, uuid(1e004834-6d8f-425a-bc9c-a2812ed43bb7)]
interface nsIMemory : nsISupports
{
    /**
     * Attempts to shrink the heap.
     * @param immediate - if true, heap minimization will occur
     *   immediately if the call was made on the main thread. If
     *   false, the flush will be scheduled to happen when the app is
     *   idle.
     * @throws NS_ERROR_FAILURE if 'immediate' is set an the call
     *   was not on the application's main thread.
     */
    void heapMinimize(in boolean immediate);

    /**
     * This predicate can be used to determine if we're in a low-memory
     * situation (what constitutes low-memory is platform dependent). This
     * can be used to trigger the memory pressure observers.
     *
     * DEPRECATED - Always returns false.  See bug 592308.
     */
    boolean isLowMemory();

    /**
     * This predicate can be used to determine if the platform is a "low-memory"
     * platform. Callers may use this to dynamically tune their behaviour
     * to favour reduced memory usage at the expense of performance. The value
     * returned by this function will not change over the lifetime of the process.
     */
    boolean isLowMemoryPlatform();
};