source/modules/juce_core/files/juce_MemoryMappedFile.h

   1 /*
   2   ==============================================================================
   3
   4    This file is part of the JUCE library.
   5    Copyright (c) 2022 - Raw Material Software Limited
   6
   7    JUCE is an open source library subject to commercial or open-source
   8    licensing.
   9
  10    The code included in this file is provided under the terms of the ISC license
  11    http://www.isc.org/downloads/software-support-policy/isc-license. Permission
  12    To use, copy, modify, and/or distribute this software for any purpose with or
  13    without fee is hereby granted provided that the above copyright notice and
  14    this permission notice appear in all copies.
  15
  16    JUCE IS PROVIDED "AS IS" WITHOUT ANY WARRANTY, AND ALL WARRANTIES, WHETHER
  17    EXPRESSED OR IMPLIED, INCLUDING MERCHANTABILITY AND FITNESS FOR PURPOSE, ARE
  18    DISCLAIMED.
  19
  20   ==============================================================================
  21 */
  22
  23 namespace juce
  24 {
  25
  26 //==============================================================================
  27 /**
  28     Maps a file into virtual memory for easy reading and/or writing.
  29
  30     @tags{Core}
  31 */
  32 class JUCE_API  MemoryMappedFile
  33 {
  34 public:
  35     /** The read/write flags used when opening a memory mapped file. */
  36     enum AccessMode
  37     {
  38         readOnly,   /**< Indicates that the memory can only be read. */
  39         readWrite   /**< Indicates that the memory can be read and written to - changes that are
  40                          made will be flushed back to disk at the whim of the OS. */
  41     };
  42
  43     /** Opens a file and maps it to an area of virtual memory.
  44
  45         The file should already exist, and should already be the size that you want to work with
  46         when you call this. If the file is resized after being opened, the behaviour is undefined.
  47
  48         If the file exists and the operation succeeds, the getData() and getSize() methods will
  49         return the location and size of the data that can be read or written. Note that the entire
  50         file is not read into memory immediately - the OS simply creates a virtual mapping, which
  51         will lazily pull the data into memory when blocks are accessed.
  52
  53         If the file can't be opened for some reason, the getData() method will return a null pointer.
  54
  55         If exclusive is false then other apps can also open the same memory mapped file and use this
  56         mapping as an effective way of communicating. If exclusive is true then the mapped file will
  57         be opened exclusively - preventing other apps to access the file which may improve the
  58         performance of accessing the file.
  59     */
  60     MemoryMappedFile (const File& file, AccessMode mode, bool exclusive = false);
  61
  62     /** Opens a section of a file and maps it to an area of virtual memory.
  63
  64         The file should already exist, and should already be the size that you want to work with
  65         when you call this. If the file is resized after being opened, the behaviour is undefined.
  66
  67         If the file exists and the operation succeeds, the getData() and getSize() methods will
  68         return the location and size of the data that can be read or written. Note that the entire
  69         file is not read into memory immediately - the OS simply creates a virtual mapping, which
  70         will lazily pull the data into memory when blocks are accessed.
  71
  72         If the file can't be opened for some reason, the getData() method will return a null pointer.
  73
  74         NOTE: The start of the actual range used may be rounded-down to a multiple of the OS's page-size,
  75         so do not assume that the mapped memory will begin at exactly the position you requested - always
  76         use getRange() to check the actual range that is being used.
  77     */
  78     MemoryMappedFile (const File& file,
  79                       const Range<int64>& fileRange,
  80                       AccessMode mode,
  81                       bool exclusive = false);
  82
  83     /** Destructor. */
  84     ~MemoryMappedFile();
  85
  86     /** Returns the address at which this file has been mapped, or a null pointer if
  87         the file couldn't be successfully mapped.
  88     */
  89     void* getData() const noexcept              { return address; }
  90
  91     /** Returns the number of bytes of data that are available for reading or writing.
  92         This will normally be the size of the file.
  93     */
  94     size_t getSize() const noexcept             { return (size_t) range.getLength(); }
  95
  96     /** Returns the section of the file at which the mapped memory represents. */
  97     Range<int64> getRange() const noexcept      { return range; }
  98
  99 private:
 100     //==============================================================================
 101     void* address = nullptr;
 102     Range<int64> range;
 103
 104    #if JUCE_WINDOWS
 105     void* fileHandle = nullptr;
 106    #else
 107     int fileHandle = 0;
 108    #endif
 109
 110     void openInternal (const File&, AccessMode, bool);
 111
 112     JUCE_DECLARE_NON_COPYABLE_WITH_LEAK_DETECTOR (MemoryMappedFile)
 113 };
 114
 115 } // namespace juce