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#include "nsISupports.idl"

/**
 *
 * nsIMemory: interface to allocate and deallocate memory. Also provides
 * for notifications in low-memory situations.
 *
 * The frozen exported symbols NS_Alloc, NS_Realloc, and NS_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 three 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.
 *
 * "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.
 *
 * @status FROZEN
 */

[scriptable, uuid(59e7e77a-38e4-11d4-8cf5-0060b0fc14a3)]
interface nsIMemory : nsISupports
{
    /**
     * Allocates a block of memory of a particular size. If the memory 
     * cannot be allocated (because of an out-of-memory condition), null
     * is returned.
     *
     * @param size - the size of the block to allocate
     * @result the block of memory
     */
    [noscript, notxpcom] voidPtr alloc(in size_t size);

    /**
     * Reallocates a block of memory to a new size.
     *
     * @param ptr - the block of memory to reallocate
     * @param size - the new size
     * @result the reallocated block of memory
     *
     * If ptr is null, this function behaves like malloc.
     * If s is the size of the block to which ptr points, the first
     * min(s, size) bytes of ptr's block are copied to the new block.
     * If the allocation succeeds, ptr is freed and a pointer to the 
     * new block returned.  If the allocation fails, ptr is not freed
     * and null is returned. The returned value may be the same as ptr.
     */
    [noscript, notxpcom] voidPtr realloc(in voidPtr ptr,
                                         in size_t newSize);

    /**
     * Frees a block of memory. Null is a permissible value, in which case
     * nothing happens. 
     *
     * @param ptr - the block of memory to free
     */
    [noscript, notxpcom] void free(in voidPtr ptr);

    /**
     * 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.
     * @return 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.
     */
    boolean isLowMemory();
};