text: /* Copyright (C) 2010-2015 The RetroArch team
   *
   * ---------------------------------------------------------------------------------------
   * The following license statement only applies to this libretro API header (libretro.h).
   * ---------------------------------------------------------------------------------------
   *
   * Permission is hereby granted, free of charge,
   * to any person obtaining a copy of this software and associated documentation files (the "Software"),
   * to deal in the Software without restriction, including without limitation the rights to
   * use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software,
   * and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
   *
   * The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
   *
   * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
   * INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
   * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
   * IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
   * WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
   * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
   */
  
  #ifndef LIBRETRO_H__
  #define LIBRETRO_H__
  
  #include <stdint.h>
  #include <stddef.h>
  #include <limits.h>
  
  #ifdef __cplusplus
  extern "C" {
  #endif
  
  #ifndef __cplusplus
  #if defined(_MSC_VER) && !defined(SN_TARGET_PS3)
  /* Hack applied for MSVC when compiling in C89 mode
   * as it isn't C99-compliant. */
  #define bool unsigned char
  #define true 1
  #define false 0
  #else
  #include <stdbool.h>
  #endif
  #endif
  
  #ifndef RETRO_CALLCONV
  #  if defined(__GNUC__) && defined(__i386__) && !defined(__x86_64__)
  #    define RETRO_CALLCONV __attribute__((cdecl))
  #  elif defined(_MSC_VER) && defined(_M_X86) && !defined(_M_X64)
  #    define RETRO_CALLCONV __cdecl
  #  else
  #    define RETRO_CALLCONV /* all other platforms only have one calling convention each */
  #  endif
  #endif
  
  #ifndef RETRO_API
  #  if defined(_WIN32) || defined(__CYGWIN__) || defined(__MINGW32__)
  #    ifdef RETRO_IMPORT_SYMBOLS
  #      ifdef __GNUC__
  #        define RETRO_API RETRO_CALLCONV __attribute__((dllimport))
  #      else
  #        define RETRO_API RETRO_CALLCONV __declspec(dllimport)
  #      endif
  #    else
  #      ifdef __GNUC__
  #        define RETRO_API RETRO_CALLCONV __attribute__((dllexport))
  #      else
  #        define RETRO_API RETRO_CALLCONV __declspec(dllexport)
  #      endif
  #    endif
  #  else
  #      if defined(__GNUC__) && __GNUC__ >= 4 && !defined(__CELLOS_LV2__)
  #        define RETRO_API RETRO_CALLCONV __attribute__((visibility("default")))
  #      else
  #        define RETRO_API RETRO_CALLCONV
  #      endif
  #  endif
  #endif
  
  /* Used for checking API/ABI mismatches that can break libretro 
   * implementations.
   * It is not incremented for compatible changes to the API.
   */
  #define RETRO_API_VERSION         1
  
  /*
   * Libretro's fundamental device abstractions.
   *
   * Libretro's input system consists of some standardized device types,
   * such as a joypad (with/without analog), mouse, keyboard, lightgun 
   * and a pointer.
   *
   * The functionality of these devices are fixed, and individual cores 
   * map their own concept of a controller to libretro's abstractions.
   * This makes it possible for frontends to map the abstract types to a 
   * real input device, and not having to worry about binding input 
   * correctly to arbitrary controller layouts.
   */
  
  #define RETRO_DEVICE_TYPE_SHIFT         8
  #define RETRO_DEVICE_MASK               ((1 << RETRO_DEVICE_TYPE_SHIFT) - 1)
  #define RETRO_DEVICE_SUBCLASS(base, id) (((id + 1) << RETRO_DEVICE_TYPE_SHIFT) | base)
  
  /* Input disabled. */
  #define RETRO_DEVICE_NONE         0
  
  /* The JOYPAD is called RetroPad. It is essentially a Super Nintendo 
   * controller, but with additional L2/R2/L3/R3 buttons, similar to a 
   * PS1 DualShock. */
  #define RETRO_DEVICE_JOYPAD       1
  
  /* The mouse is a simple mouse, similar to Super Nintendo's mouse.
   * X and Y coordinates are reported relatively to last poll (poll callback).
   * It is up to the libretro implementation to keep track of where the mouse 
   * pointer is supposed to be on the screen.
   * The frontend must make sure not to interfere with its own hardware 
   * mouse pointer.
   */
  #define RETRO_DEVICE_MOUSE        2
  
  /* KEYBOARD device lets one poll for raw key pressed.
   * It is poll based, so input callback will return with the current 
   * pressed state.
   * For event/text based keyboard input, see
   * RETRO_ENVIRONMENT_SET_KEYBOARD_CALLBACK.
   */
  #define RETRO_DEVICE_KEYBOARD     3
  
  /* Lightgun X/Y coordinates are reported relatively to last poll,
   * similar to mouse. */
  #define RETRO_DEVICE_LIGHTGUN     4
  
  /* The ANALOG device is an extension to JOYPAD (RetroPad).
   * Similar to DualShock it adds two analog sticks.
   * This is treated as a separate device type as it returns values in the 
   * full analog range of [-0x8000, 0x7fff]. Positive X axis is right.
   * Positive Y axis is down.
   * Only use ANALOG type when polling for analog values of the axes.
   */
  #define RETRO_DEVICE_ANALOG       5
  
  /* Abstracts the concept of a pointing mechanism, e.g. touch.
   * This allows libretro to query in absolute coordinates where on the 
   * screen a mouse (or something similar) is being placed.
   * For a touch centric device, coordinates reported are the coordinates
   * of the press.
   *
   * Coordinates in X and Y are reported as:
   * [-0x7fff, 0x7fff]: -0x7fff corresponds to the far left/top of the screen,
   * and 0x7fff corresponds to the far right/bottom of the screen.
   * The "screen" is here defined as area that is passed to the frontend and 
   * later displayed on the monitor.
   *
   * The frontend is free to scale/resize this screen as it sees fit, however,
   * (X, Y) = (-0x7fff, -0x7fff) will correspond to the top-left pixel of the 
   * game image, etc.
   *
   * To check if the pointer coordinates are valid (e.g. a touch display 
   * actually being touched), PRESSED returns 1 or 0.
   *
   * If using a mouse on a desktop, PRESSED will usually correspond to the 
   * left mouse button, but this is a frontend decision.
   * PRESSED will only return 1 if the pointer is inside the game screen.
   *
   * For multi-touch, the index variable can be used to successively query 
   * more presses.
   * If index = 0 returns true for _PRESSED, coordinates can be extracted
   * with _X, _Y for index = 0. One can then query _PRESSED, _X, _Y with 
   * index = 1, and so on.
   * Eventually _PRESSED will return false for an index. No further presses 
   * are registered at this point. */
  #define RETRO_DEVICE_POINTER      6
  
  /* Buttons for the RetroPad (JOYPAD).
   * The placement of these is equivalent to placements on the 
   * Super Nintendo controller.
   * L2/R2/L3/R3 buttons correspond to the PS1 DualShock. */
  #define RETRO_DEVICE_ID_JOYPAD_B        0
  #define RETRO_DEVICE_ID_JOYPAD_Y        1
  #define RETRO_DEVICE_ID_JOYPAD_SELECT   2
  #define RETRO_DEVICE_ID_JOYPAD_START    3
  #define RETRO_DEVICE_ID_JOYPAD_UP       4
  #define RETRO_DEVICE_ID_JOYPAD_DOWN     5
  #define RETRO_DEVICE_ID_JOYPAD_LEFT     6
  #define RETRO_DEVICE_ID_JOYPAD_RIGHT    7
  #define RETRO_DEVICE_ID_JOYPAD_A        8
  #define RETRO_DEVICE_ID_JOYPAD_X        9
  #define RETRO_DEVICE_ID_JOYPAD_L       10
  #define RETRO_DEVICE_ID_JOYPAD_R       11
  #define RETRO_DEVICE_ID_JOYPAD_L2      12
  #define RETRO_DEVICE_ID_JOYPAD_R2      13
  #define RETRO_DEVICE_ID_JOYPAD_L3      14
  #define RETRO_DEVICE_ID_JOYPAD_R3      15
  
  /* Index / Id values for ANALOG device. */
  #define RETRO_DEVICE_INDEX_ANALOG_LEFT   0
  #define RETRO_DEVICE_INDEX_ANALOG_RIGHT  1
  #define RETRO_DEVICE_ID_ANALOG_X         0
  #define RETRO_DEVICE_ID_ANALOG_Y         1
  
  /* Id values for MOUSE. */
  #define RETRO_DEVICE_ID_MOUSE_X                0
  #define RETRO_DEVICE_ID_MOUSE_Y                1
  #define RETRO_DEVICE_ID_MOUSE_LEFT             2
  #define RETRO_DEVICE_ID_MOUSE_RIGHT            3
  #define RETRO_DEVICE_ID_MOUSE_WHEELUP          4
  #define RETRO_DEVICE_ID_MOUSE_WHEELDOWN        5
  #define RETRO_DEVICE_ID_MOUSE_MIDDLE           6
  #define RETRO_DEVICE_ID_MOUSE_HORIZ_WHEELUP    7
  #define RETRO_DEVICE_ID_MOUSE_HORIZ_WHEELDOWN  8
  
  /* Id values for LIGHTGUN types. */
  #define RETRO_DEVICE_ID_LIGHTGUN_X        0
  #define RETRO_DEVICE_ID_LIGHTGUN_Y        1
  #define RETRO_DEVICE_ID_LIGHTGUN_TRIGGER  2
  #define RETRO_DEVICE_ID_LIGHTGUN_CURSOR   3
  #define RETRO_DEVICE_ID_LIGHTGUN_TURBO    4
  #define RETRO_DEVICE_ID_LIGHTGUN_PAUSE    5
  #define RETRO_DEVICE_ID_LIGHTGUN_START    6
  
  /* Id values for POINTER. */
  #define RETRO_DEVICE_ID_POINTER_X         0
  #define RETRO_DEVICE_ID_POINTER_Y         1
  #define RETRO_DEVICE_ID_POINTER_PRESSED   2
  
  /* Returned from retro_get_region(). */
  #define RETRO_REGION_NTSC  0
  #define RETRO_REGION_PAL   1
  
  /* Id values for LANGUAGE */
  enum retro_language
  {
      RETRO_LANGUAGE_ENGLISH             =  0,
      RETRO_LANGUAGE_JAPANESE            =  1,
      RETRO_LANGUAGE_FRENCH              =  2,
      RETRO_LANGUAGE_SPANISH             =  3,
      RETRO_LANGUAGE_GERMAN              =  4,
      RETRO_LANGUAGE_ITALIAN             =  5,
      RETRO_LANGUAGE_DUTCH               =  6,
      RETRO_LANGUAGE_PORTUGUESE          =  7,
      RETRO_LANGUAGE_RUSSIAN             =  8,
      RETRO_LANGUAGE_KOREAN              =  9,
      RETRO_LANGUAGE_CHINESE_TRADITIONAL = 10,
      RETRO_LANGUAGE_CHINESE_SIMPLIFIED  = 11,
      RETRO_LANGUAGE_ESPERANTO           = 12,
      RETRO_LANGUAGE_POLISH              = 13,
      RETRO_LANGUAGE_LAST,
  
      /* Ensure sizeof(enum) == sizeof(int) */
              RETRO_LANGUAGE_DUMMY          = INT_MAX
  };
  
  /* Passed to retro_get_memory_data/size().
   * If the memory type doesn't apply to the 
   * implementation NULL/0 can be returned.
   */
  #define RETRO_MEMORY_MASK        0xff
  
  /* Regular save RAM. This RAM is usually found on a game cartridge,
   * backed up by a battery.
   * If save game data is too complex for a single memory buffer,
   * the SAVE_DIRECTORY (preferably) or SYSTEM_DIRECTORY environment
   * callback can be used. */
  #define RETRO_MEMORY_SAVE_RAM    0
  
  /* Some games have a built-in clock to keep track of time.
   * This memory is usually just a couple of bytes to keep track of time.
   */
  #define RETRO_MEMORY_RTC         1
  
  /* System ram lets a frontend peek into a game systems main RAM. */
  #define RETRO_MEMORY_SYSTEM_RAM  2
  
  /* Video ram lets a frontend peek into a game systems video RAM (VRAM). */
  #define RETRO_MEMORY_VIDEO_RAM   3
  
  /* Keysyms used for ID in input state callback when polling RETRO_KEYBOARD. */
  enum retro_key
  {
      RETROK_UNKNOWN        = 0,
      RETROK_FIRST          = 0,
      RETROK_BACKSPACE      = 8,
      RETROK_TAB            = 9,
      RETROK_CLEAR          = 12,
      RETROK_RETURN         = 13,
      RETROK_PAUSE          = 19,
      RETROK_ESCAPE         = 27,
      RETROK_SPACE          = 32,
      RETROK_EXCLAIM        = 33,
      RETROK_QUOTEDBL       = 34,
      RETROK_HASH           = 35,
      RETROK_DOLLAR         = 36,
      RETROK_AMPERSAND      = 38,
      RETROK_QUOTE          = 39,
      RETROK_LEFTPAREN      = 40,
      RETROK_RIGHTPAREN     = 41,
      RETROK_ASTERISK       = 42,
      RETROK_PLUS           = 43,
      RETROK_COMMA          = 44,
      RETROK_MINUS          = 45,
      RETROK_PERIOD         = 46,
      RETROK_SLASH          = 47,
      RETROK_0              = 48,
      RETROK_1              = 49,
      RETROK_2              = 50,
      RETROK_3              = 51,
      RETROK_4              = 52,
      RETROK_5              = 53,
      RETROK_6              = 54,
      RETROK_7              = 55,
      RETROK_8              = 56,
      RETROK_9              = 57,
      RETROK_COLON          = 58,
      RETROK_SEMICOLON      = 59,
      RETROK_LESS           = 60,
      RETROK_EQUALS         = 61,
      RETROK_GREATER        = 62,
      RETROK_QUESTION       = 63,
      RETROK_AT             = 64,
      RETROK_LEFTBRACKET    = 91,
      RETROK_BACKSLASH      = 92,
      RETROK_RIGHTBRACKET   = 93,
      RETROK_CARET          = 94,
      RETROK_UNDERSCORE     = 95,
      RETROK_BACKQUOTE      = 96,
      RETROK_a              = 97,
      RETROK_b              = 98,
      RETROK_c              = 99,
      RETROK_d              = 100,
      RETROK_e              = 101,
      RETROK_f              = 102,
      RETROK_g              = 103,
      RETROK_h              = 104,
      RETROK_i              = 105,
      RETROK_j              = 106,
      RETROK_k              = 107,
      RETROK_l              = 108,
      RETROK_m              = 109,
      RETROK_n              = 110,
      RETROK_o              = 111,
      RETROK_p              = 112,
      RETROK_q              = 113,
      RETROK_r              = 114,
      RETROK_s              = 115,
      RETROK_t              = 116,
      RETROK_u              = 117,
      RETROK_v              = 118,
      RETROK_w              = 119,
      RETROK_x              = 120,
      RETROK_y              = 121,
      RETROK_z              = 122,
      RETROK_DELETE         = 127,
  
      RETROK_KP0            = 256,
      RETROK_KP1            = 257,
      RETROK_KP2            = 258,
      RETROK_KP3            = 259,
      RETROK_KP4            = 260,
      RETROK_KP5            = 261,
      RETROK_KP6            = 262,
      RETROK_KP7            = 263,
      RETROK_KP8            = 264,
      RETROK_KP9            = 265,
      RETROK_KP_PERIOD      = 266,
      RETROK_KP_DIVIDE      = 267,
      RETROK_KP_MULTIPLY    = 268,
      RETROK_KP_MINUS       = 269,
      RETROK_KP_PLUS        = 270,
      RETROK_KP_ENTER       = 271,
      RETROK_KP_EQUALS      = 272,
  
      RETROK_UP             = 273,
      RETROK_DOWN           = 274,
      RETROK_RIGHT          = 275,
      RETROK_LEFT           = 276,
      RETROK_INSERT         = 277,
      RETROK_HOME           = 278,
      RETROK_END            = 279,
      RETROK_PAGEUP         = 280,
      RETROK_PAGEDOWN       = 281,
  
      RETROK_F1             = 282,
      RETROK_F2             = 283,
      RETROK_F3             = 284,
      RETROK_F4             = 285,
      RETROK_F5             = 286,
      RETROK_F6             = 287,
      RETROK_F7             = 288,
      RETROK_F8             = 289,
      RETROK_F9             = 290,
      RETROK_F10            = 291,
      RETROK_F11            = 292,
      RETROK_F12            = 293,
      RETROK_F13            = 294,
      RETROK_F14            = 295,
      RETROK_F15            = 296,
  
      RETROK_NUMLOCK        = 300,
      RETROK_CAPSLOCK       = 301,
      RETROK_SCROLLOCK      = 302,
      RETROK_RSHIFT         = 303,
      RETROK_LSHIFT         = 304,
      RETROK_RCTRL          = 305,
      RETROK_LCTRL          = 306,
      RETROK_RALT           = 307,
      RETROK_LALT           = 308,
      RETROK_RMETA          = 309,
      RETROK_LMETA          = 310,
      RETROK_LSUPER         = 311,
      RETROK_RSUPER         = 312,
      RETROK_MODE           = 313,
      RETROK_COMPOSE        = 314,
  
      RETROK_HELP           = 315,
      RETROK_PRINT          = 316,
      RETROK_SYSREQ         = 317,
      RETROK_BREAK          = 318,
      RETROK_MENU           = 319,
      RETROK_POWER          = 320,
      RETROK_EURO           = 321,
      RETROK_UNDO           = 322,
  
      RETROK_LAST,
  
      RETROK_DUMMY          = INT_MAX /* Ensure sizeof(enum) == sizeof(int) */
  };
  
  enum retro_mod
  {
      RETROKMOD_NONE       = 0x0000,
  
      RETROKMOD_SHIFT      = 0x01,
      RETROKMOD_CTRL       = 0x02,
      RETROKMOD_ALT        = 0x04,
      RETROKMOD_META       = 0x08,
  
      RETROKMOD_NUMLOCK    = 0x10,
      RETROKMOD_CAPSLOCK   = 0x20,
      RETROKMOD_SCROLLOCK  = 0x40,
  
      RETROKMOD_DUMMY = INT_MAX /* Ensure sizeof(enum) == sizeof(int) */
  };
  
  /* If set, this call is not part of the public libretro API yet. It can 
   * change or be removed at any time. */
  #define RETRO_ENVIRONMENT_EXPERIMENTAL 0x10000
  /* Environment callback to be used internally in frontend. */
  #define RETRO_ENVIRONMENT_PRIVATE 0x20000
  
  /* Environment commands. */
  #define RETRO_ENVIRONMENT_SET_ROTATION  1  /* const unsigned * --
                                              * Sets screen rotation of graphics.
                                              * Is only implemented if rotation can be accelerated by hardware.
                                              * Valid values are 0, 1, 2, 3, which rotates screen by 0, 90, 180, 
                                              * 270 degrees counter-clockwise respectively.
                                              */
  #define RETRO_ENVIRONMENT_GET_OVERSCAN  2  /* bool * --
                                              * Boolean value whether or not the implementation should use overscan, 
                                              * or crop away overscan.
                                              */
  #define RETRO_ENVIRONMENT_GET_CAN_DUPE  3  /* bool * --
                                              * Boolean value whether or not frontend supports frame duping,
                                              * passing NULL to video frame callback.
                                              */
  
  /* Environ 4, 5 are no longer supported (GET_VARIABLE / SET_VARIABLES),
   * and reserved to avoid possible ABI clash.
   */
  
  #define RETRO_ENVIRONMENT_SET_MESSAGE   6  /* const struct retro_message * --
                                              * Sets a message to be displayed in implementation-specific manner 
                                              * for a certain amount of 'frames'.
                                              * Should not be used for trivial messages, which should simply be 
                                              * logged via RETRO_ENVIRONMENT_GET_LOG_INTERFACE (or as a 
                                              * fallback, stderr).
                                              */
  #define RETRO_ENVIRONMENT_SHUTDOWN      7  /* N/A (NULL) --
                                              * Requests the frontend to shutdown.
                                              * Should only be used if game has a specific
                                              * way to shutdown the game from a menu item or similar.
                                              */
  #define RETRO_ENVIRONMENT_SET_PERFORMANCE_LEVEL 8
  /* const unsigned * --
   * Gives a hint to the frontend how demanding this implementation
   * is on a system. E.g. reporting a level of 2 means
   * this implementation should run decently on all frontends
   * of level 2 and up.
   *
   * It can be used by the frontend to potentially warn
   * about too demanding implementations.
   *
   * The levels are "floating".
   *
   * This function can be called on a per-game basis,
   * as certain games an implementation can play might be
   * particularly demanding.
   * If called, it should be called in retro_load_game().
   */
  #define RETRO_ENVIRONMENT_GET_SYSTEM_DIRECTORY 9
  /* const char ** --
   * Returns the "system" directory of the frontend.
   * This directory can be used to store system specific
   * content such as BIOSes, configuration data, etc.
   * The returned value can be NULL.
   * If so, no such directory is defined,
   * and it's up to the implementation to find a suitable directory.
   *
   * NOTE: Some cores used this folder also for "save" data such as
   * memory cards, etc, for lack of a better place to put it.
   * This is now discouraged, and if possible, cores should try to
   * use the new GET_SAVE_DIRECTORY.
   */
  #define RETRO_ENVIRONMENT_SET_PIXEL_FORMAT 10
  /* const enum retro_pixel_format * --
   * Sets the internal pixel format used by the implementation.
   * The default pixel format is RETRO_PIXEL_FORMAT_0RGB1555.
   * This pixel format however, is deprecated (see enum retro_pixel_format).
   * If the call returns false, the frontend does not support this pixel
   * format.
   *
   * This function should be called inside retro_load_game() or
   * retro_get_system_av_info().
   */
  #define RETRO_ENVIRONMENT_SET_INPUT_DESCRIPTORS 11
  /* const struct retro_input_descriptor * --
   * Sets an array of retro_input_descriptors.
   * It is up to the frontend to present this in a usable way.
   * The array is terminated by retro_input_descriptor::description
   * being set to NULL.
   * This function can be called at any time, but it is recommended
   * to call it as early as possible.
   */
  #define RETRO_ENVIRONMENT_SET_KEYBOARD_CALLBACK 12
  /* const struct retro_keyboard_callback * --
   * Sets a callback function used to notify core about keyboard events.
   */
  #define RETRO_ENVIRONMENT_SET_DISK_CONTROL_INTERFACE 13
  /* const struct retro_disk_control_callback * --
   * Sets an interface which frontend can use to eject and insert
   * disk images.
   * This is used for games which consist of multiple images and
   * must be manually swapped out by the user (e.g. PSX).
   */
  #define RETRO_ENVIRONMENT_SET_HW_RENDER 14
  /* struct retro_hw_render_callback * --
   * Sets an interface to let a libretro core render with
   * hardware acceleration.
   * Should be called in retro_load_game().
   * If successful, libretro cores will be able to render to a
   * frontend-provided framebuffer.
   * The size of this framebuffer will be at least as large as
   * max_width/max_height provided in get_av_info().
   * If HW rendering is used, pass only RETRO_HW_FRAME_BUFFER_VALID or
   * NULL to retro_video_refresh_t.
   */
  #define RETRO_ENVIRONMENT_GET_VARIABLE 15
  /* struct retro_variable * --
   * Interface to acquire user-defined information from environment
   * that cannot feasibly be supported in a multi-system way.
   * 'key' should be set to a key which has already been set by
   * SET_VARIABLES.
   * 'data' will be set to a value or NULL.
   */
  #define RETRO_ENVIRONMENT_SET_VARIABLES 16
  /* const struct retro_variable * --
   * Allows an implementation to signal the environment
   * which variables it might want to check for later using
   * GET_VARIABLE.
   * This allows the frontend to present these variables to
   * a user dynamically.
   * This should be called as early as possible (ideally in
   * retro_set_environment).
   *
   * 'data' points to an array of retro_variable structs
   * terminated by a { NULL, NULL } element.
   * retro_variable::key should be namespaced to not collide
   * with other implementations' keys. E.g. A core called
   * 'foo' should use keys named as 'foo_option'.
   * retro_variable::value should contain a human readable
   * description of the key as well as a '|' delimited list
   * of expected values.
   *
   * The number of possible options should be very limited,
   * i.e. it should be feasible to cycle through options
   * without a keyboard.
   *
   * First entry should be treated as a default.
   *
   * Example entry:
   * { "foo_option", "Speed hack coprocessor X; false|true" }
   *
   * Text before first ';' is description. This ';' must be
   * followed by a space, and followed by a list of possible
   * values split up with '|'.
   *
   * Only strings are operated on. The possible values will
   * generally be displayed and stored as-is by the frontend.
   */
  #define RETRO_ENVIRONMENT_GET_VARIABLE_UPDATE 17
  /* bool * --
   * Result is set to true if some variables are updated by
   * frontend since last call to RETRO_ENVIRONMENT_GET_VARIABLE.
   * Variables should be queried with GET_VARIABLE.
   */
  #define RETRO_ENVIRONMENT_SET_SUPPORT_NO_GAME 18
  /* const bool * --
   * If true, the libretro implementation supports calls to
   * retro_load_game() with NULL as argument.
   * Used by cores which can run without particular game data.
   * This should be called within retro_set_environment() only.
   */
  #define RETRO_ENVIRONMENT_GET_LIBRETRO_PATH 19
  /* const char ** --
   * Retrieves the absolute path from where this libretro
   * implementation was loaded.
   * NULL is returned if the libretro was loaded statically
   * (i.e. linked statically to frontend), or if the path cannot be
   * determined.
   * Mostly useful in cooperation with SET_SUPPORT_NO_GAME as assets can
   * be loaded without ugly hacks.
   */
  
  /* Environment 20 was an obsolete version of SET_AUDIO_CALLBACK.
   * It was not used by any known core at the time,
   * and was removed from the API. */
  #define RETRO_ENVIRONMENT_SET_AUDIO_CALLBACK 22
  /* const struct retro_audio_callback * --
   * Sets an interface which is used to notify a libretro core about audio
   * being available for writing.
   * The callback can be called from any thread, so a core using this must
   * have a thread safe audio implementation.
   * It is intended for games where audio and video are completely
   * asynchronous and audio can be generated on the fly.
   * This interface is not recommended for use with emulators which have
   * highly synchronous audio.
   *
   * The callback only notifies about writability; the libretro core still
   * has to call the normal audio callbacks
   * to write audio. The audio callbacks must be called from within the
   * notification callback.
   * The amount of audio data to write is up to the implementation.
   * Generally, the audio callback will be called continously in a loop.
   *
   * Due to thread safety guarantees and lack of sync between audio and
   * video, a frontend  can selectively disallow this interface based on
   * internal configuration. A core using this interface must also
   * implement the "normal" audio interface.
   *
   * A libretro core using SET_AUDIO_CALLBACK should also make use of
   * SET_FRAME_TIME_CALLBACK.
   */
  #define RETRO_ENVIRONMENT_SET_FRAME_TIME_CALLBACK 21
  /* const struct retro_frame_time_callback * --
   * Lets the core know how much time has passed since last
   * invocation of retro_run().
   * The frontend can tamper with the timing to fake fast-forward,
   * slow-motion, frame stepping, etc.
   * In this case the delta time will use the reference value
   * in frame_time_callback..
   */
  #define RETRO_ENVIRONMENT_GET_RUMBLE_INTERFACE 23
  /* struct retro_rumble_interface * --
   * Gets an interface which is used by a libretro core to set
   * state of rumble motors in controllers.
   * A strong and weak motor is supported, and they can be
   * controlled indepedently.
   */
  #define RETRO_ENVIRONMENT_GET_INPUT_DEVICE_CAPABILITIES 24
  /* uint64_t * --
   * Gets a bitmask telling which device type are expected to be
   * handled properly in a call to retro_input_state_t.
   * Devices which are not handled or recognized always return
   * 0 in retro_input_state_t.
   * Example bitmask: caps = (1 << RETRO_DEVICE_JOYPAD) | (1 << RETRO_DEVICE_ANALOG).
   * Should only be called in retro_run().
   */
  #define RETRO_ENVIRONMENT_GET_SENSOR_INTERFACE (25 | RETRO_ENVIRONMENT_EXPERIMENTAL)
  /* struct retro_sensor_interface * --
   * Gets access to the sensor interface.
   * The purpose of this interface is to allow
   * setting state related to sensors such as polling rate,
   * enabling/disable it entirely, etc.
   * Reading sensor state is done via the normal
   * input_state_callback API.
   */
  #define RETRO_ENVIRONMENT_GET_CAMERA_INTERFACE (26 | RETRO_ENVIRONMENT_EXPERIMENTAL)
  /* struct retro_camera_callback * --
   * Gets an interface to a video camera driver.
   * A libretro core can use this interface to get access to a
   * video camera.
   * New video frames are delivered in a callback in same
   * thread as retro_run().
   *
   * GET_CAMERA_INTERFACE should be called in retro_load_game().
   *
   * Depending on the camera implementation used, camera frames
   * will be delivered as a raw framebuffer,
   * or as an OpenGL texture directly.
   *
   * The core has to tell the frontend here which types of
   * buffers can be handled properly.
   * An OpenGL texture can only be handled when using a
   * libretro GL core (SET_HW_RENDER).
   * It is recommended to use a libretro GL core when
   * using camera interface.
   *
   * The camera is not started automatically. The retrieved start/stop
   * functions must be used to explicitly
   * start and stop the camera driver.
   */
  #define RETRO_ENVIRONMENT_GET_LOG_INTERFACE 27
  /* struct retro_log_callback * --
   * Gets an interface for logging. This is useful for
   * logging in a cross-platform way
   * as certain platforms cannot use use stderr for logging.
   * It also allows the frontend to
   * show logging information in a more suitable way.
   * If this interface is not used, libretro cores should
   * log to stderr as desired.
   */
  #define RETRO_ENVIRONMENT_GET_PERF_INTERFACE 28
  /* struct retro_perf_callback * --
   * Gets an interface for performance counters. This is useful
   * for performance logging in a cross-platform way and for detecting
   * architecture-specific features, such as SIMD support.
   */
  #define RETRO_ENVIRONMENT_GET_LOCATION_INTERFACE 29
  /* struct retro_location_callback * --
   * Gets access to the location interface.
   * The purpose of this interface is to be able to retrieve
   * location-based information from the host device,
   * such as current latitude / longitude.
   */
  #define RETRO_ENVIRONMENT_GET_CONTENT_DIRECTORY 30 /* Old name, kept for compatibility. */
  #define RETRO_ENVIRONMENT_GET_CORE_ASSETS_DIRECTORY 30
  /* const char ** --
   * Returns the "core assets" directory of the frontend.
   * This directory can be used to store specific assets that the
   * core relies upon, such as art assets,
   * input data, etc etc.
   * The returned value can be NULL.
   * If so, no such directory is defined,
   * and it's up to the implementation to find a suitable directory.
   */
  #define RETRO_ENVIRONMENT_GET_SAVE_DIRECTORY 31
  /* const char ** --
   * Returns the "save" directory of the frontend.
   * This directory can be used to store SRAM, memory cards,
   * high scores, etc, if the libretro core
   * cannot use the regular memory interface (retro_get_memory_data()).
   *
   * NOTE: libretro cores used to check GET_SYSTEM_DIRECTORY for
   * similar things before.
   * They should still check GET_SYSTEM_DIRECTORY if they want to
   * be backwards compatible.
   * The path here can be NULL. It should only be non-NULL if the
   * frontend user has set a specific save path.
   */
  #define RETRO_ENVIRONMENT_SET_SYSTEM_AV_INFO 32
  /* const struct retro_system_av_info * --
   * Sets a new av_info structure. This can only be called from
   * within retro_run().
   * This should *only* be used if the core is completely altering the
   * internal resolutions, aspect ratios, timings, sampling rate, etc.
   * Calling this can require a full reinitialization of video/audio
   * drivers in the frontend,
   *
   * so it is important to call it very sparingly, and usually only with
   * the users explicit consent.
   * An eventual driver reinitialize will happen so that video and
   * audio callbacks
   * happening after this call within the same retro_run() call will
   * target the newly initialized driver.
   *
   * This callback makes it possible to support configurable resolutions
   * in games, which can be useful to
   * avoid setting the "worst case" in max_width/max_height.
   *
   * ***HIGHLY RECOMMENDED*** Do not call this callback every time
   * resolution changes in an emulator core if it's
   * expected to be a temporary change, for the reasons of possible
   * driver reinitialization.
   * This call is not a free pass for not trying to provide
   * correct values in retro_get_system_av_info(). If you need to change
   * things like aspect ratio or nominal width/height,
   * use RETRO_ENVIRONMENT_SET_GEOMETRY, which is a softer variant
   * of SET_SYSTEM_AV_INFO.
   *
   * If this returns false, the frontend does not acknowledge a
   * changed av_info struct.
   */
  #define RETRO_ENVIRONMENT_SET_PROC_ADDRESS_CALLBACK 33
  /* const struct retro_get_proc_address_interface * --
   * Allows a libretro core to announce support for the
   * get_proc_address() interface.
   * This interface allows for a standard way to extend libretro where
   * use of environment calls are too indirect,
   * e.g. for cases where the frontend wants to call directly into the core.
   *
   * If a core wants to expose this interface, SET_PROC_ADDRESS_CALLBACK
   * **MUST** be called from within retro_set_environment().
   */
  #define RETRO_ENVIRONMENT_SET_SUBSYSTEM_INFO 34
  /* const struct retro_subsystem_info * --
   * This environment call introduces the concept of libretro "subsystems".
   * A subsystem is a variant of a libretro core which supports
   * different kinds of games.
   * The purpose of this is to support e.g. emulators which might
   * have special needs, e.g. Super Nintendo's Super GameBoy, Sufami Turbo.
   * It can also be used to pick among subsystems in an explicit way
   * if the libretro implementation is a multi-system emulator itself.
   *
   * Loading a game via a subsystem is done with retro_load_game_special(),
   * and this environment call allows a libretro core to expose which
   * subsystems are supported for use with retro_load_game_special().
   * A core passes an array of retro_game_special_info which is terminated
   * with a zeroed out retro_game_special_info struct.
   *
   * If a core wants to use this functionality, SET_SUBSYSTEM_INFO
   * **MUST** be called from within retro_set_environment().
   */
  #define RETRO_ENVIRONMENT_SET_CONTROLLER_INFO 35
  /* const struct retro_controller_info * --
   * This environment call lets a libretro core tell the frontend
   * which controller types are recognized in calls to
   * retro_set_controller_port_device().
   *
   * Some emulators such as Super Nintendo
   * support multiple lightgun types which must be specifically
   * selected from.
   * It is therefore sometimes necessary for a frontend to be able
   * to tell the core about a special kind of input device which is
   * not covered by the libretro input API.
   *
   * In order for a frontend to understand the workings of an input device,
   * it must be a specialized type
   * of the generic device types already defined in the libretro API.
   *
   * Which devices are supported can vary per input port.
   * The core must pass an array of const struct retro_controller_info which
   * is terminated with a blanked out struct. Each element of the struct
   * corresponds to an ascending port index to
   * retro_set_controller_port_device().
   * Even if special device types are set in the libretro core,
   * libretro should only poll input based on the base input device types.
   */
  #define RETRO_ENVIRONMENT_SET_MEMORY_MAPS (36 | RETRO_ENVIRONMENT_EXPERIMENTAL)
  /* const struct retro_memory_map * --
   * This environment call lets a libretro core tell the frontend
   * about the memory maps this core emulates.
   * This can be used to implement, for example, cheats in a core-agnostic way.
   *
   * Should only be used by emulators; it doesn't make much sense for
   * anything else.
   * It is recommended to expose all relevant pointers through
   * retro_get_memory_* as well.
   *
   * Can be called from retro_init and retro_load_game.
   */
  #define RETRO_ENVIRONMENT_SET_GEOMETRY 37
  /* const struct retro_game_geometry * --
   * This environment call is similar to SET_SYSTEM_AV_INFO for changing
   * video parameters, but provides a guarantee that drivers will not be
   * reinitialized.
   * This can only be called from within retro_run().
   *
   * The purpose of this call is to allow a core to alter nominal
   * width/heights as well as aspect ratios on-the-fly, which can be
   * useful for some emulators to change in run-time.
   *
   * max_width/max_height arguments are ignored and cannot be changed
   * with this call as this could potentially require a reinitialization or a
   * non-constant time operation.
   * If max_width/max_height are to be changed, SET_SYSTEM_AV_INFO is required.
   *
   * A frontend must guarantee that this environment call completes in
   * constant time.
   */
  #define RETRO_ENVIRONMENT_GET_USERNAME 38
  /* const char **
   * Returns the specified username of the frontend, if specified by the user.
   * This username can be used as a nickname for a core that has online facilities
   * or any other mode where personalization of the user is desirable.
   * The returned value can be NULL.
   * If this environ callback is used by a core that requires a valid username,
   * a default username should be specified by the core.
   */
  #define RETRO_ENVIRONMENT_GET_LANGUAGE 39
  /* unsigned * --
   * Returns the specified language of the frontend, if specified by the user.
   * It can be used by the core for localization purposes.
   */
  
  #define RETRO_MEMDESC_CONST     (1 << 0)   /* The frontend will never change this memory area once retro_load_game has returned. */
  #define RETRO_MEMDESC_BIGENDIAN (1 << 1)   /* The memory area contains big endian data. Default is little endian. */
  #define RETRO_MEMDESC_ALIGN_2   (1 << 16)  /* All memory access in this area is aligned to their own size, or 2, whichever is smaller. */
  #define RETRO_MEMDESC_ALIGN_4   (2 << 16)
  #define RETRO_MEMDESC_ALIGN_8   (3 << 16)
  #define RETRO_MEMDESC_MINSIZE_2 (1 << 24)  /* All memory in this region is accessed at least 2 bytes at the time. */
  #define RETRO_MEMDESC_MINSIZE_4 (2 << 24)
  #define RETRO_MEMDESC_MINSIZE_8 (3 << 24)
  struct retro_memory_descriptor
  {
      uint64_t flags;
  
      /* Pointer to the start of the relevant ROM or RAM chip.
       * It's strongly recommended to use 'offset' if possible, rather than
       * doing math on the pointer.
       *
       * If the same byte is mapped my multiple descriptors, their descriptors
       * must have the same pointer.
       * If 'start' does not point to the first byte in the pointer, put the
       * difference in 'offset' instead.
       *
       * May be NULL if there's nothing usable here (e.g. hardware registers and
       * open bus). No flags should be set if the pointer is NULL.
       * It's recommended to minimize the number of descriptors if possible,
       * but not mandatory. */
      void *ptr;
      size_t offset;
  
      /* This is the location in the emulated address space
       * where the mapping starts. */
      size_t start;
  
      /* Which bits must be same as in 'start' for this mapping to apply.
       * The first memory descriptor to claim a certain byte is the one
       * that applies.
       * A bit which is set in 'start' must also be set in this.
       * Can be zero, in which case each byte is assumed mapped exactly once.
       * In this case, 'len' must be a power of two. */
      size_t select;
  
      /* If this is nonzero, the set bits are assumed not connected to the
       * memory chip's address pins. */
      size_t disconnect;
  
      /* This one tells the size of the current memory area.
       * If, after start+disconnect are applied, the address is higher than
       * this, the highest bit of the address is cleared.
       *
       * If the address is still too high, the next highest bit is cleared.
       * Can be zero, in which case it's assumed to be infinite (as limited
       * by 'select' and 'disconnect'). */
      size_t len;
  
      /* To go from emulated address to physical address, the following
       * order applies:
       * Subtract 'start', pick off 'disconnect', apply 'len', add 'offset'.
       *
       * The address space name must consist of only a-zA-Z0-9_-,
       * should be as short as feasible (maximum length is 8 plus the NUL),
       * and may not be any other address space plus one or more 0-9A-F
       * at the end.
       * However, multiple memory descriptors for the same address space is
       * allowed, and the address space name can be empty. NULL is treated
       * as empty.
       *
       * Address space names are case sensitive, but avoid lowercase if possible.
       * The same pointer may exist in multiple address spaces.
       *
       * Examples:
       * blank+blank - valid (multiple things may be mapped in the same namespace)
       * 'Sp'+'Sp' - valid (multiple things may be mapped in the same namespace)
       * 'A'+'B' - valid (neither is a prefix of each other)
       * 'S'+blank - valid ('S' is not in 0-9A-F)
       * 'a'+blank - valid ('a' is not in 0-9A-F)
       * 'a'+'A' - valid (neither is a prefix of each other)
       * 'AR'+blank - valid ('R' is not in 0-9A-F)
       * 'ARB'+blank - valid (the B can't be part of the address either, because
       * there is no namespace 'AR')
       * blank+'B' - not valid, because it's ambigous which address space B1234
       * would refer to.
       * The length can't be used for that purpose; the frontend may want
       * to append arbitrary data to an address, without a separator. */
      const char *addrspace;
  };
  
  /* The frontend may use the largest value of 'start'+'select' in a 
   * certain namespace to infer the size of the address space.
   *
   * If the address space is larger than that, a mapping with .ptr=NULL 
   * should be at the end of the array, with .select set to all ones for 
   * as long as the address space is big.
   *
   * Sample descriptors (minus .ptr, and RETRO_MEMFLAG_ on the flags):
   * SNES WRAM:
   * .start=0x7E0000, .len=0x20000
   * (Note that this must be mapped before the ROM in most cases; some of the 
   * ROM mappers 
   * try to claim $7E0000, or at least $7E8000.)
   * SNES SPC700 RAM:
   * .addrspace="S", .len=0x10000
   * SNES WRAM mirrors:
   * .flags=MIRROR, .start=0x000000, .select=0xC0E000, .len=0x2000
   * .flags=MIRROR, .start=0x800000, .select=0xC0E000, .len=0x2000
   * SNES WRAM mirrors, alternate equivalent descriptor:
   * .flags=MIRROR, .select=0x40E000, .disconnect=~0x1FFF
   * (Various similar constructions can be created by combining parts of 
   * the above two.)
   * SNES LoROM (512KB, mirrored a couple of times):
   * .flags=CONST, .start=0x008000, .select=0x408000, .disconnect=0x8000, .len=512*1024
   * .flags=CONST, .start=0x400000, .select=0x400000, .disconnect=0x8000, .len=512*1024
   * SNES HiROM (4MB):
   * .flags=CONST,                 .start=0x400000, .select=0x400000, .len=4*1024*1024
   * .flags=CONST, .offset=0x8000, .start=0x008000, .select=0x408000, .len=4*1024*1024
   * SNES ExHiROM (8MB):
   * .flags=CONST, .offset=0,                  .start=0xC00000, .select=0xC00000, .len=4*1024*1024
   * .flags=CONST, .offset=4*1024*1024,        .start=0x400000, .select=0xC00000, .len=4*1024*1024
   * .flags=CONST, .offset=0x8000,             .start=0x808000, .select=0xC08000, .len=4*1024*1024
   * .flags=CONST, .offset=4*1024*1024+0x8000, .start=0x008000, .select=0xC08000, .len=4*1024*1024
   * Clarify the size of the address space:
   * .ptr=NULL, .select=0xFFFFFF
   * .len can be implied by .select in many of them, but was included for clarity.
   */
  
  struct retro_memory_map
  {
      const struct retro_memory_descriptor *descriptors;
      unsigned num_descriptors;
  };
  
  struct retro_controller_description
  {
      /* Human-readable description of the controller. Even if using a generic
       * input device type, this can be set to the particular device type the
       * core uses. */
      const char *desc;
  
      /* Device type passed to retro_set_controller_port_device(). If the device
       * type is a sub-class of a generic input device type, use the
       * RETRO_DEVICE_SUBCLASS macro to create an ID.
       *
       * E.g. RETRO_DEVICE_SUBCLASS(RETRO_DEVICE_JOYPAD, 1). */
      unsigned id;
  };
  
  struct retro_controller_info
  {
      const struct retro_controller_description *types;
      unsigned num_types;
  };
  
  struct retro_subsystem_memory_info
  {
      /* The extension associated with a memory type, e.g. "psram". */
      const char *extension;
  
      /* The memory type for retro_get_memory(). This should be at
       * least 0x100 to avoid conflict with standardized
       * libretro memory types. */
      unsigned type;
  };
  
  struct retro_subsystem_rom_info
  {
      /* Describes what the content is (SGB BIOS, GB ROM, etc). */
      const char *desc;
  
      /* Same definition as retro_get_system_info(). */
      const char *valid_extensions;
  
      /* Same definition as retro_get_system_info(). */
      bool need_fullpath;
  
      /* Same definition as retro_get_system_info(). */
      bool block_extract;
  
      /* This is set if the content is required to load a game.
       * If this is set to false, a zeroed-out retro_game_info can be passed. */
      bool required;
  
      /* Content can have multiple associated persistent
       * memory types (retro_get_memory()). */
      const struct retro_subsystem_memory_info *memory;
      unsigned num_memory;
  };
  
  struct retro_subsystem_info
  {
      /* Human-readable string of the subsystem type, e.g. "Super GameBoy" */
      const char *desc;
  
      /* A computer friendly short string identifier for the subsystem type.
       * This name must be [a-z].
       * E.g. if desc is "Super GameBoy", this can be "sgb".
       * This identifier can be used for command-line interfaces, etc.
       */
      const char *ident;
  
      /* Infos for each content file. The first entry is assumed to be the
       * "most significant" content for frontend purposes.
       * E.g. with Super GameBoy, the first content should be the GameBoy ROM,
       * as it is the most "significant" content to a user.
       * If a frontend creates new file paths based on the content used
       * (e.g. savestates), it should use the path for the first ROM to do so. */
      const struct retro_subsystem_rom_info *roms;
  
      /* Number of content files associated with a subsystem. */
      unsigned num_roms;
  
      /* The type passed to retro_load_game_special(). */
      unsigned id;
  };
  
  typedef void (*retro_proc_address_t)(void);
  
  /* libretro API extension functions:
   * (None here so far).
   *
   * Get a symbol from a libretro core.
   * Cores should only return symbols which are actual 
   * extensions to the libretro API.
   *
   * Frontends should not use this to obtain symbols to standard 
   * libretro entry points (static linking or dlsym).
   *
   * The symbol name must be equal to the function name, 
   * e.g. if void retro_foo(void); exists, the symbol must be called "retro_foo".
   * The returned function pointer must be cast to the corresponding type.
   */
  typedef retro_proc_address_t (*retro_get_proc_address_t)(const char *sym);
  
  struct retro_get_proc_address_interface
  {
      retro_get_proc_address_t get_proc_address;
  };
  
  enum retro_log_level
  {
      RETRO_LOG_DEBUG = 0,
      RETRO_LOG_INFO,
      RETRO_LOG_WARN,
      RETRO_LOG_ERROR,
  
      RETRO_LOG_DUMMY = INT_MAX
  };
  
  /* Logging function. Takes log level argument as well. */
  typedef void (*retro_log_printf_t)(enum retro_log_level level,
                                     const char *fmt, ...);
  
  struct retro_log_callback
  {
      retro_log_printf_t log;
  };
  
  /* Performance related functions */
  
  /* ID values for SIMD CPU features */
  #define RETRO_SIMD_SSE      (1 << 0)
  #define RETRO_SIMD_SSE2     (1 << 1)
  #define RETRO_SIMD_VMX      (1 << 2)
  #define RETRO_SIMD_VMX128   (1 << 3)
  #define RETRO_SIMD_AVX      (1 << 4)
  #define RETRO_SIMD_NEON     (1 << 5)
  #define RETRO_SIMD_SSE3     (1 << 6)
  #define RETRO_SIMD_SSSE3    (1 << 7)
  #define RETRO_SIMD_MMX      (1 << 8)
  #define RETRO_SIMD_MMXEXT   (1 << 9)
  #define RETRO_SIMD_SSE4     (1 << 10)
  #define RETRO_SIMD_SSE42    (1 << 11)
  #define RETRO_SIMD_AVX2     (1 << 12)
  #define RETRO_SIMD_VFPU     (1 << 13)
  #define RETRO_SIMD_PS       (1 << 14)
  #define RETRO_SIMD_AES      (1 << 15)
  #define RETRO_SIMD_VFPV3    (1 << 16)
  #define RETRO_SIMD_VFPV4    (1 << 17)
  
  typedef uint64_t retro_perf_tick_t;
  typedef int64_t retro_time_t;
  
  struct retro_perf_counter
  {
      const char *ident;
      retro_perf_tick_t start;
      retro_perf_tick_t total;
      retro_perf_tick_t call_cnt;
  
      bool registered;
  };
  
  /* Returns current time in microseconds.
   * Tries to use the most accurate timer available.
   */
  typedef retro_time_t (*retro_perf_get_time_usec_t)(void);
  
  /* A simple counter. Usually nanoseconds, but can also be CPU cycles.
   * Can be used directly if desired (when creating a more sophisticated 
   * performance counter system).
   * */
  typedef retro_perf_tick_t (*retro_perf_get_counter_t)(void);
  
  /* Returns a bit-mask of detected CPU features (RETRO_SIMD_*). */
  typedef uint64_t (*retro_get_cpu_features_t)(void);
  
  /* Asks frontend to log and/or display the state of performance counters.
   * Performance counters can always be poked into manually as well.
   */
  typedef void (*retro_perf_log_t)(void);
  
  /* Register a performance counter.
   * ident field must be set with a discrete value and other values in 
   * retro_perf_counter must be 0.
   * Registering can be called multiple times. To avoid calling to 
   * frontend redundantly, you can check registered field first. */
  typedef void (*retro_perf_register_t)(struct retro_perf_counter *counter);
  
  /* Starts a registered counter. */
  typedef void (*retro_perf_start_t)(struct retro_perf_counter *counter);
  
  /* Stops a registered counter. */
  typedef void (*retro_perf_stop_t)(struct retro_perf_counter *counter);
  
  /* For convenience it can be useful to wrap register, start and stop in macros.
   * E.g.:
   * #ifdef LOG_PERFORMANCE
   * #define RETRO_PERFORMANCE_INIT(perf_cb, name) static struct retro_perf_counter name = {#name}; if (!name.registered) perf_cb.perf_register(&(name))
   * #define RETRO_PERFORMANCE_START(perf_cb, name) perf_cb.perf_start(&(name))
   * #define RETRO_PERFORMANCE_STOP(perf_cb, name) perf_cb.perf_stop(&(name))
   * #else
   * ... Blank macros ...
   * #endif
   *
   * These can then be used mid-functions around code snippets.
   *
   * extern struct retro_perf_callback perf_cb;  * Somewhere in the core.
   *
   * void do_some_heavy_work(void)
   * {
   *    RETRO_PERFORMANCE_INIT(cb, work_1;
   *    RETRO_PERFORMANCE_START(cb, work_1);
   *    heavy_work_1();
   *    RETRO_PERFORMANCE_STOP(cb, work_1);
   *
   *    RETRO_PERFORMANCE_INIT(cb, work_2);
   *    RETRO_PERFORMANCE_START(cb, work_2);
   *    heavy_work_2();
   *    RETRO_PERFORMANCE_STOP(cb, work_2);
   * }
   *
   * void retro_deinit(void)
   * {
   *    perf_cb.perf_log();  * Log all perf counters here for example.
   * }
   */
  
  struct retro_perf_callback
  {
      retro_perf_get_time_usec_t    get_time_usec;
      retro_get_cpu_features_t      get_cpu_features;
  
      retro_perf_get_counter_t      get_perf_counter;
      retro_perf_register_t         perf_register;
      retro_perf_start_t            perf_start;
      retro_perf_stop_t             perf_stop;
      retro_perf_log_t              perf_log;
  };
  
  /* FIXME: Document the sensor API and work out behavior.
   * It will be marked as experimental until then.
   */
  enum retro_sensor_action
  {
      RETRO_SENSOR_ACCELEROMETER_ENABLE = 0,
      RETRO_SENSOR_ACCELEROMETER_DISABLE,
  
      RETRO_SENSOR_DUMMY = INT_MAX
  };
  
  /* Id values for SENSOR types. */
  #define RETRO_SENSOR_ACCELEROMETER_X 0
  #define RETRO_SENSOR_ACCELEROMETER_Y 1
  #define RETRO_SENSOR_ACCELEROMETER_Z 2
  
  typedef bool (*retro_set_sensor_state_t)(unsigned port,
                                           enum retro_sensor_action action, unsigned rate);
  
  typedef float (*retro_sensor_get_input_t)(unsigned port, unsigned id);
  
  struct retro_sensor_interface
  {
      retro_set_sensor_state_t set_sensor_state;
      retro_sensor_get_input_t get_sensor_input;
  };
  
  enum retro_camera_buffer
  {
      RETRO_CAMERA_BUFFER_OPENGL_TEXTURE = 0,
      RETRO_CAMERA_BUFFER_RAW_FRAMEBUFFER,
  
      RETRO_CAMERA_BUFFER_DUMMY = INT_MAX
  };
  
  /* Starts the camera driver. Can only be called in retro_run(). */
  typedef bool (*retro_camera_start_t)(void);
  
  /* Stops the camera driver. Can only be called in retro_run(). */
  typedef void (*retro_camera_stop_t)(void);
  
  /* Callback which signals when the camera driver is initialized 
   * and/or deinitialized.
   * retro_camera_start_t can be called in initialized callback.
   */
  typedef void (*retro_camera_lifetime_status_t)(void);
  
  /* A callback for raw framebuffer data. buffer points to an XRGB8888 buffer.
   * Width, height and pitch are similar to retro_video_refresh_t.
   * First pixel is top-left origin.
   */
  typedef void (*retro_camera_frame_raw_framebuffer_t)(const uint32_t *buffer,
                                                       unsigned width, unsigned height, size_t pitch);
  
  /* A callback for when OpenGL textures are used.
   *
   * texture_id is a texture owned by camera driver.
   * Its state or content should be considered immutable, except for things like 
   * texture filtering and clamping.
   *
   * texture_target is the texture target for the GL texture.
   * These can include e.g. GL_TEXTURE_2D, GL_TEXTURE_RECTANGLE, and possibly 
   * more depending on extensions.
   *
   * affine points to a packed 3x3 column-major matrix used to apply an affine 
   * transform to texture coordinates. (affine_matrix * vec3(coord_x, coord_y, 1.0))
   * After transform, normalized texture coord (0, 0) should be bottom-left 
   * and (1, 1) should be top-right (or (width, height) for RECTANGLE).
   *
   * GL-specific typedefs are avoided here to avoid relying on gl.h in 
   * the API definition.
   */
  typedef void (*retro_camera_frame_opengl_texture_t)(unsigned texture_id,
                                                      unsigned texture_target, const float *affine);
  
  struct retro_camera_callback
  {
      /* Set by libretro core.
       * Example bitmask: caps = (1 << RETRO_CAMERA_BUFFER_OPENGL_TEXTURE) | (1 << RETRO_CAMERA_BUFFER_RAW_FRAMEBUFFER).
       */
      uint64_t caps;
  
      unsigned width; /* Desired resolution for camera. Is only used as a hint. */
      unsigned height;
      retro_camera_start_t start; /* Set by frontend. */
      retro_camera_stop_t stop; /* Set by frontend. */
  
      /* Set by libretro core if raw framebuffer callbacks will be used. */
      retro_camera_frame_raw_framebuffer_t frame_raw_framebuffer;
      /* Set by libretro core if OpenGL texture callbacks will be used. */
      retro_camera_frame_opengl_texture_t frame_opengl_texture;
  
      /* Set by libretro core. Called after camera driver is initialized and
       * ready to be started.
       * Can be NULL, in which this callback is not called.
       */
      retro_camera_lifetime_status_t initialized;
  
      /* Set by libretro core. Called right before camera driver is
       * deinitialized.
       * Can be NULL, in which this callback is not called.
       */
      retro_camera_lifetime_status_t deinitialized;
  };
  
  /* Sets the interval of time and/or distance at which to update/poll 
   * location-based data.
   *
   * To ensure compatibility with all location-based implementations,
   * values for both interval_ms and interval_distance should be provided.
   *
   * interval_ms is the interval expressed in milliseconds.
   * interval_distance is the distance interval expressed in meters.
   */
  typedef void (*retro_location_set_interval_t)(unsigned interval_ms,
                                                unsigned interval_distance);
  
  /* Start location services. The device will start listening for changes to the
   * current location at regular intervals (which are defined with 
   * retro_location_set_interval_t). */
  typedef bool (*retro_location_start_t)(void);
  
  /* Stop location services. The device will stop listening for changes 
   * to the current location. */
  typedef void (*retro_location_stop_t)(void);
  
  /* Get the position of the current location. Will set parameters to 
   * 0 if no new  location update has happened since the last time. */
  typedef bool (*retro_location_get_position_t)(double *lat, double *lon,
                                                double *horiz_accuracy, double *vert_accuracy);
  
  /* Callback which signals when the location driver is initialized 
   * and/or deinitialized.
   * retro_location_start_t can be called in initialized callback.
   */
  typedef void (*retro_location_lifetime_status_t)(void);
  
  struct retro_location_callback
  {
      retro_location_start_t         start;
      retro_location_stop_t          stop;
      retro_location_get_position_t  get_position;
      retro_location_set_interval_t  set_interval;
  
      retro_location_lifetime_status_t initialized;
      retro_location_lifetime_status_t deinitialized;
  };
  
  enum retro_rumble_effect
  {
      RETRO_RUMBLE_STRONG = 0,
      RETRO_RUMBLE_WEAK = 1,
  
      RETRO_RUMBLE_DUMMY = INT_MAX
  };
  
  /* Sets rumble state for joypad plugged in port 'port'. 
   * Rumble effects are controlled independently,
   * and setting e.g. strong rumble does not override weak rumble.
   * Strength has a range of [0, 0xffff].
   *
   * Returns true if rumble state request was honored. 
   * Calling this before first retro_run() is likely to return false. */
  typedef bool (*retro_set_rumble_state_t)(unsigned port,
                                           enum retro_rumble_effect effect, uint16_t strength);
  
  struct retro_rumble_interface
  {
      retro_set_rumble_state_t set_rumble_state;
  };
  
  /* Notifies libretro that audio data should be written. */
  typedef void (*retro_audio_callback_t)(void);
  
  /* True: Audio driver in frontend is active, and callback is 
   * expected to be called regularily.
   * False: Audio driver in frontend is paused or inactive. 
   * Audio callback will not be called until set_state has been 
   * called with true.
   * Initial state is false (inactive).
   */
  typedef void (*retro_audio_set_state_callback_t)(bool enabled);
  
  struct retro_audio_callback
  {
      retro_audio_callback_t callback;
      retro_audio_set_state_callback_t set_state;
  };
  
  /* Notifies a libretro core of time spent since last invocation 
   * of retro_run() in microseconds.
   *
   * It will be called right before retro_run() every frame.
   * The frontend can tamper with timing to support cases like 
   * fast-forward, slow-motion and framestepping.
   *
   * In those scenarios the reference frame time value will be used. */
  typedef int64_t retro_usec_t;
  typedef void (*retro_frame_time_callback_t)(retro_usec_t usec);
  struct retro_frame_time_callback
  {
      retro_frame_time_callback_t callback;
      /* Represents the time of one frame. It is computed as
       * 1000000 / fps, but the implementation will resolve the
       * rounding to ensure that framestepping, etc is exact. */
      retro_usec_t reference;
  };
  
  /* Pass this to retro_video_refresh_t if rendering to hardware.
   * Passing NULL to retro_video_refresh_t is still a frame dupe as normal.
   * */
  #define RETRO_HW_FRAME_BUFFER_VALID ((void*)-1)
  
  /* Invalidates the current HW context.
   * Any GL state is lost, and must not be deinitialized explicitly.
   * If explicit deinitialization is desired by the libretro core,
   * it should implement context_destroy callback.
   * If called, all GPU resources must be reinitialized.
   * Usually called when frontend reinits video driver.
   * Also called first time video driver is initialized, 
   * allowing libretro core to initialize resources.
   */
  typedef void (*retro_hw_context_reset_t)(void);
  
  /* Gets current framebuffer which is to be rendered to.
   * Could change every frame potentially.
   */
  typedef uintptr_t (*retro_hw_get_current_framebuffer_t)(void);
  
  /* Get a symbol from HW context. */
  typedef retro_proc_address_t (*retro_hw_get_proc_address_t)(const char *sym);
  
  enum retro_hw_context_type
  {
      RETRO_HW_CONTEXT_NONE             = 0,
      /* OpenGL 2.x. Driver can choose to use latest compatibility context. */
              RETRO_HW_CONTEXT_OPENGL           = 1,
      /* OpenGL ES 2.0. */
              RETRO_HW_CONTEXT_OPENGLES2        = 2,
      /* Modern desktop core GL context. Use version_major/
       * version_minor fields to set GL version. */
              RETRO_HW_CONTEXT_OPENGL_CORE      = 3,
      /* OpenGL ES 3.0 */
              RETRO_HW_CONTEXT_OPENGLES3        = 4,
      /* OpenGL ES 3.1+. Set version_major/version_minor. For GLES2 and GLES3,
       * use the corresponding enums directly. */
              RETRO_HW_CONTEXT_OPENGLES_VERSION = 5,
  
      RETRO_HW_CONTEXT_DUMMY = INT_MAX
  };
  
  struct retro_hw_render_callback
  {
      /* Which API to use. Set by libretro core. */
      enum retro_hw_context_type context_type;
  
      /* Called when a context has been created or when it has been reset.
       * An OpenGL context is only valid after context_reset() has been called.
       *
       * When context_reset is called, OpenGL resources in the libretro
       * implementation are guaranteed to be invalid.
       *
       * It is possible that context_reset is called multiple times during an
       * application lifecycle.
       * If context_reset is called without any notification (context_destroy),
       * the OpenGL context was lost and resources should just be recreated
       * without any attempt to "free" old resources.
       */
      retro_hw_context_reset_t context_reset;
  
      /* Set by frontend. */
      retro_hw_get_current_framebuffer_t get_current_framebuffer;
  
      /* Set by frontend. */
      retro_hw_get_proc_address_t get_proc_address;
  
      /* Set if render buffers should have depth component attached. */
      bool depth;
  
      /* Set if stencil buffers should be attached. */
      bool stencil;
  
      /* If depth and stencil are true, a packed 24/8 buffer will be added.
       * Only attaching stencil is invalid and will be ignored. */
  
      /* Use conventional bottom-left origin convention. If false,
       * standard libretro top-left origin semantics are used. */
      bool bottom_left_origin;
  
      /* Major version number for core GL context or GLES 3.1+. */
      unsigned version_major;
  
      /* Minor version number for core GL context or GLES 3.1+. */
      unsigned version_minor;
  
      /* If this is true, the frontend will go very far to avoid
       * resetting context in scenarios like toggling fullscreen, etc.
       */
      bool cache_context;
  
      /* The reset callback might still be called in extreme situations
       * such as if the context is lost beyond recovery.
       *
       * For optimal stability, set this to false, and allow context to be
       * reset at any time.
       */
  
      /* A callback to be called before the context is destroyed in a
       * controlled way by the frontend. */
      retro_hw_context_reset_t context_destroy;
  
      /* OpenGL resources can be deinitialized cleanly at this step.
       * context_destroy can be set to NULL, in which resources will
       * just be destroyed without any notification.
       *
       * Even when context_destroy is non-NULL, it is possible that
       * context_reset is called without any destroy notification.
       * This happens if context is lost by external factors (such as
       * notified by GL_ARB_robustness).
       *
       * In this case, the context is assumed to be already dead,
       * and the libretro implementation must not try to free any OpenGL
       * resources in the subsequent context_reset.
       */
  
      /* Creates a debug context. */
      bool debug_context;
  };
  
  /* Callback type passed in RETRO_ENVIRONMENT_SET_KEYBOARD_CALLBACK. 
   * Called by the frontend in response to keyboard events.
   * down is set if the key is being pressed, or false if it is being released.
   * keycode is the RETROK value of the char.
   * character is the text character of the pressed key. (UTF-32).
   * key_modifiers is a set of RETROKMOD values or'ed together.
   *
   * The pressed/keycode state can be indepedent of the character.
   * It is also possible that multiple characters are generated from a 
   * single keypress.
   * Keycode events should be treated separately from character events.
   * However, when possible, the frontend should try to synchronize these.
   * If only a character is posted, keycode should be RETROK_UNKNOWN.
   *
   * Similarily if only a keycode event is generated with no corresponding 
   * character, character should be 0.
   */
  typedef void (*retro_keyboard_event_t)(bool down, unsigned keycode,
                                         uint32_t character, uint16_t key_modifiers);
  
  struct retro_keyboard_callback
  {
      retro_keyboard_event_t callback;
  };
  
  /* Callbacks for RETRO_ENVIRONMENT_SET_DISK_CONTROL_INTERFACE.
   * Should be set for implementations which can swap out multiple disk 
   * images in runtime.
   *
   * If the implementation can do this automatically, it should strive to do so.
   * However, there are cases where the user must manually do so.
   *
   * Overview: To swap a disk image, eject the disk image with 
   * set_eject_state(true).
   * Set the disk index with set_image_index(index). Insert the disk again 
   * with set_eject_state(false).
   */
  
  /* If ejected is true, "ejects" the virtual disk tray.
   * When ejected, the disk image index can be set.
   */
  typedef bool (*retro_set_eject_state_t)(bool ejected);
  
  /* Gets current eject state. The initial state is 'not ejected'. */
  typedef bool (*retro_get_eject_state_t)(void);
  
  /* Gets current disk index. First disk is index 0.
   * If return value is >= get_num_images(), no disk is currently inserted.
   */
  typedef unsigned (*retro_get_image_index_t)(void);
  
  /* Sets image index. Can only be called when disk is ejected.
   * The implementation supports setting "no disk" by using an 
   * index >= get_num_images().
   */
  typedef bool (*retro_set_image_index_t)(unsigned index);
  
  /* Gets total number of images which are available to use. */
  typedef unsigned (*retro_get_num_images_t)(void);
  
  struct retro_game_info;
  
  /* Replaces the disk image associated with index.
   * Arguments to pass in info have same requirements as retro_load_game().
   * Virtual disk tray must be ejected when calling this.
   *
   * Replacing a disk image with info = NULL will remove the disk image 
   * from the internal list.
   * As a result, calls to get_image_index() can change.
   *
   * E.g. replace_image_index(1, NULL), and previous get_image_index() 
   * returned 4 before.
   * Index 1 will be removed, and the new index is 3.
   */
  typedef bool (*retro_replace_image_index_t)(unsigned index,
                                              const struct retro_game_info *info);
  
  /* Adds a new valid index (get_num_images()) to the internal disk list.
   * This will increment subsequent return values from get_num_images() by 1.
   * This image index cannot be used until a disk image has been set 
   * with replace_image_index. */
  typedef bool (*retro_add_image_index_t)(void);
  
  struct retro_disk_control_callback
  {
      retro_set_eject_state_t set_eject_state;
      retro_get_eject_state_t get_eject_state;
  
      retro_get_image_index_t get_image_index;
      retro_set_image_index_t set_image_index;
      retro_get_num_images_t  get_num_images;
  
      retro_replace_image_index_t replace_image_index;
      retro_add_image_index_t add_image_index;
  };
  
  enum retro_pixel_format
  {
      /* 0RGB1555, native endian.
       * 0 bit must be set to 0.
       * This pixel format is default for compatibility concerns only.
       * If a 15/16-bit pixel format is desired, consider using RGB565. */
              RETRO_PIXEL_FORMAT_0RGB1555 = 0,
  
      /* XRGB8888, native endian.
       * X bits are ignored. */
              RETRO_PIXEL_FORMAT_XRGB8888 = 1,
  
      /* RGB565, native endian.
       * This pixel format is the recommended format to use if a 15/16-bit
       * format is desired as it is the pixel format that is typically
       * available on a wide range of low-power devices.
       *
       * It is also natively supported in APIs like OpenGL ES. */
              RETRO_PIXEL_FORMAT_RGB565   = 2,
  
      /* Ensure sizeof() == sizeof(int). */
              RETRO_PIXEL_FORMAT_UNKNOWN  = INT_MAX
  };
  
  struct retro_message
  {
      const char *msg;        /* Message to be displayed. */
      unsigned    frames;     /* Duration in frames of message. */
  };
  
  /* Describes how the libretro implementation maps a libretro input bind
   * to its internal input system through a human readable string.
   * This string can be used to better let a user configure input. */
  struct retro_input_descriptor
  {
      /* Associates given parameters with a description. */
      unsigned port;
      unsigned device;
      unsigned index;
      unsigned id;
  
      /* Human readable description for parameters.
       * The pointer must remain valid until
       * retro_unload_game() is called. */
      const char *description;
  };
  
  struct retro_system_info
  {
      /* All pointers are owned by libretro implementation, and pointers must
       * remain valid until retro_deinit() is called. */
  
      const char *library_name;      /* Descriptive name of library. Should not
                                     * contain any version numbers, etc. */
      const char *library_version;   /* Descriptive version of core. */
  
      const char *valid_extensions;  /* A string listing probably content
                                     * extensions the core will be able to 
                                     * load, separated with pipe.
                                     * I.e. "bin|rom|iso".
                                     * Typically used for a GUI to filter 
                                     * out extensions. */
  
      /* If true, retro_load_game() is guaranteed to provide a valid pathname
       * in retro_game_info::path.
       * ::data and ::size are both invalid.
       *
       * If false, ::data and ::size are guaranteed to be valid, but ::path
       * might not be valid.
       *
       * This is typically set to true for libretro implementations that must
       * load from file.
       * Implementations should strive for setting this to false, as it allows
       * the frontend to perform patching, etc. */
      bool        need_fullpath;
  
      /* If true, the frontend is not allowed to extract any archives before
       * loading the real content.
       * Necessary for certain libretro implementations that load games
       * from zipped archives. */
      bool        block_extract;
  };
  
  struct retro_game_geometry
  {
      unsigned base_width;    /* Nominal video width of game. */
      unsigned base_height;   /* Nominal video height of game. */
      unsigned max_width;     /* Maximum possible width of game. */
      unsigned max_height;    /* Maximum possible height of game. */
  
      float    aspect_ratio;  /* Nominal aspect ratio of game. If
                              * aspect_ratio is <= 0.0, an aspect ratio
                              * of base_width / base_height is assumed.
                              * A frontend could override this setting,
                              * if desired. */
  };
  
  struct retro_system_timing
  {
      double fps;             /* FPS of video content. */
      double sample_rate;     /* Sampling rate of audio. */
  };
  
  struct retro_system_av_info
  {
      struct retro_game_geometry geometry;
      struct retro_system_timing timing;
  };
  
  struct retro_variable
  {
      /* Variable to query in RETRO_ENVIRONMENT_GET_VARIABLE.
       * If NULL, obtains the complete environment string if more
       * complex parsing is necessary.
       * The environment string is formatted as key-value pairs
       * delimited by semicolons as so:
       * "key1=value1;key2=value2;..."
       */
      const char *key;
  
      /* Value to be obtained. If key does not exist, it is set to NULL. */
      const char *value;
  };
  
  struct retro_game_info
  {
      const char *path;       /* Path to game, UTF-8 encoded.
                              * Usually used as a reference.
                              * May be NULL if rom was loaded from stdin
                              * or similar. 
                              * retro_system_info::need_fullpath guaranteed 
                              * that this path is valid. */
      const void *data;       /* Memory buffer of loaded game. Will be NULL
                              * if need_fullpath was set. */
      size_t      size;       /* Size of memory buffer. */
      const char *meta;       /* String of implementation specific meta-data. */
  };
  
  /* Callbacks */
  
  /* Environment callback. Gives implementations a way of performing 
   * uncommon tasks. Extensible. */
  typedef bool (*retro_environment_t)(unsigned cmd, void *data);
  
  /* Render a frame. Pixel format is 15-bit 0RGB1555 native endian 
   * unless changed (see RETRO_ENVIRONMENT_SET_PIXEL_FORMAT).
   *
   * Width and height specify dimensions of buffer.
   * Pitch specifices length in bytes between two lines in buffer.
   *
   * For performance reasons, it is highly recommended to have a frame 
   * that is packed in memory, i.e. pitch == width * byte_per_pixel.
   * Certain graphic APIs, such as OpenGL ES, do not like textures 
   * that are not packed in memory.
   */
  typedef void (*retro_video_refresh_t)(const void *data, unsigned width,
                                        unsigned height, size_t pitch);
  
  /* Renders a single audio frame. Should only be used if implementation 
   * generates a single sample at a time.
   * Format is signed 16-bit native endian.
   */
  typedef void (*retro_audio_sample_t)(int16_t left, int16_t right);
  
  /* Renders multiple audio frames in one go.
   *
   * One frame is defined as a sample of left and right channels, interleaved.
   * I.e. int16_t buf[4] = { l, r, l, r }; would be 2 frames.
   * Only one of the audio callbacks must ever be used.
   */
  typedef size_t (*retro_audio_sample_batch_t)(const int16_t *data,
                                               size_t frames);
  
  /* Polls input. */
  typedef void (*retro_input_poll_t)(void);
  
  /* Queries for input for player 'port'. device will be masked with 
   * RETRO_DEVICE_MASK.
   *
   * Specialization of devices such as RETRO_DEVICE_JOYPAD_MULTITAP that 
   * have been set with retro_set_controller_port_device()
   * will still use the higher level RETRO_DEVICE_JOYPAD to request input.
   */
  typedef int16_t (*retro_input_state_t)(unsigned port, unsigned device,
                                         unsigned index, unsigned id);
  
  /* Sets callbacks. retro_set_environment() is guaranteed to be called 
   * before retro_init().
   *
   * The rest of the set_* functions are guaranteed to have been called 
   * before the first call to retro_run() is made. */
  RETRO_API void retro_set_environment(retro_environment_t);
  RETRO_API void retro_set_video_refresh(retro_video_refresh_t);
  RETRO_API void retro_set_audio_sample(retro_audio_sample_t);
  RETRO_API void retro_set_audio_sample_batch(retro_audio_sample_batch_t);
  RETRO_API void retro_set_input_poll(retro_input_poll_t);
  RETRO_API void retro_set_input_state(retro_input_state_t);
  
  /* Library global initialization/deinitialization. */
  RETRO_API void retro_init(void);
  RETRO_API void retro_deinit(void);
  
  /* Must return RETRO_API_VERSION. Used to validate ABI compatibility
   * when the API is revised. */
  RETRO_API unsigned retro_api_version(void);
  
  /* Gets statically known system info. Pointers provided in *info 
   * must be statically allocated.
   * Can be called at any time, even before retro_init(). */
  RETRO_API void retro_get_system_info(struct retro_system_info *info);
  
  /* Gets information about system audio/video timings and geometry.
   * Can be called only after retro_load_game() has successfully completed.
   * NOTE: The implementation of this function might not initialize every 
   * variable if needed.
   * E.g. geom.aspect_ratio might not be initialized if core doesn't 
   * desire a particular aspect ratio. */
  RETRO_API void retro_get_system_av_info(struct retro_system_av_info *info);
  
  /* Sets device to be used for player 'port'.
   * By default, RETRO_DEVICE_JOYPAD is assumed to be plugged into all 
   * available ports.
   * Setting a particular device type is not a guarantee that libretro cores 
   * will only poll input based on that particular device type. It is only a 
   * hint to the libretro core when a core cannot automatically detect the 
   * appropriate input device type on its own. It is also relevant when a 
   * core can change its behavior depending on device type. */
  RETRO_API void retro_set_controller_port_device(unsigned port, unsigned device);
  
  /* Resets the current game. */
  RETRO_API void retro_reset(void);
  
  /* Runs the game for one video frame.
   * During retro_run(), input_poll callback must be called at least once.
   * 
   * If a frame is not rendered for reasons where a game "dropped" a frame,
   * this still counts as a frame, and retro_run() should explicitly dupe 
   * a frame if GET_CAN_DUPE returns true.
   * In this case, the video callback can take a NULL argument for data.
   */
  RETRO_API void retro_run(void);
  
  /* Returns the amount of data the implementation requires to serialize 
   * internal state (save states).
   * Between calls to retro_load_game() and retro_unload_game(), the 
   * returned size is never allowed to be larger than a previous returned 
   * value, to ensure that the frontend can allocate a save state buffer once.
   */
  RETRO_API size_t retro_serialize_size(void);
  
  /* Serializes internal state. If failed, or size is lower than
   * retro_serialize_size(), it should return false, true otherwise. */
  RETRO_API bool retro_serialize(void *data, size_t size);
  RETRO_API bool retro_unserialize(const void *data, size_t size);
  
  RETRO_API void retro_cheat_reset(void);
  RETRO_API void retro_cheat_set(unsigned index, bool enabled, const char *code);
  
  /* Loads a game. */
  RETRO_API bool retro_load_game(const struct retro_game_info *game);
  
  /* Loads a "special" kind of game. Should not be used,
   * except in extreme cases. */
  RETRO_API bool retro_load_game_special(
          unsigned game_type,
          const struct retro_game_info *info, size_t num_info
  );
  
  /* Unloads a currently loaded game. */
  RETRO_API void retro_unload_game(void);
  
  /* Gets region of game. */
  RETRO_API unsigned retro_get_region(void);
  
  /* Gets region of memory. */
  RETRO_API void *retro_get_memory_data(unsigned id);
  RETRO_API size_t retro_get_memory_size(unsigned id);
  
  #ifdef __cplusplus
  }
  #endif
  
  #endif
  