XPLMDataAccess API

The data access API gives you a generic, flexible, high performance way to read and write data to and from X-Plane and other plug-ins. For example, this API allows you to read and set the nav radios, get the plane location, determine the current effective graphics frame rate, etc.

The data access APIs are the way that you read and write data from the sim as well as other plugins.

The API works using opaque data references. A data reference is a source of data; you do not know where it comes from, but once you have it you can read the data quickly and possibly write it.

Dataref Lookup

Data references are identified by verbose, permanent string names; by convention these names use path separates to form a hierarchy of datarefs, e.g. (sim/cockpit/radios/nav1_freq_hz). The actual opaque numeric value of the data reference, as returned by the XPLM API, is implementation defined and changes each time X-Plane is launched; therefore you need to look up the dataref by path every time your plugin runs.

The task of looking up a data reference is relatively expensive; look up your data references once based on the verbose path strings, and save the opaque data reference value for the duration of your plugin’s operation. Reading and writing data references is relatively fast (the cost is equivalent to two function calls through function pointers).

X-Plane publishes over 4000 datarefs; a complete list may be found in the reference section of the SDK online documentation (from the SDK home page, choose Documentation).

Dataref Types

A note on typing: you must know the correct data type to read and write. APIs are provided for reading and writing data in a number of ways. You can also double check the data type for a data ref. Automatic type conversion is not done for you.

Dataref types are a set, e.g. a dataref can be more than one type. When this happens, you can choose which API you want to use to read. For example, it is not uncommon for a dataref to be of type float and double. This means you can use either XPLMGetDatad or XPLMGetDataf to read it.

Creating New Datarefs

X-Plane provides datarefs that come with the sim, but plugins can also create their own datarefs. A plugin creates a dataref by registering function callbacks to read and write the dataref. The XPLM will call your plugin each time some other plugin (or X-Plane) tries to read or write the dataref. You must provide a read (and optional write) callback for each data type you support.

A note for plugins sharing data with other plugins: the load order of plugins is not guaranteed. To make sure that every plugin publishing data has published their data references before other plugins try to subscribe, publish your data references in your start routine but resolve them the first time your ‘enable’ routine is called, or the first time they are needed in code.

When a plugin that created a dataref is unloaded, it becomes “orphaned”. The dataref handle continues to be usable, but the dataref is not writable, and reading it will always return 0 (or 0 items for arrays). If the plugin is reloaded and re-registers the dataref, the handle becomes un-orphaned and works again.

READING AND WRITING DATA

These routines allow you to access data from within X-Plane and sometimes modify it.

XPLMDataRef

typedef void * XPLMDataRef;

A data ref is an opaque handle to data provided by the simulator or another plugin. It uniquely identifies one variable (or array of variables) over the lifetime of your plugin. You never hard code these values; you always get them from XPLMFindDataRef.

XPLMDataTypeID

This is an enumeration that defines the type of the data behind a data reference. This allows you to sanity check that the data type matches what you expect. But for the most part, you will know the type of data you are expecting from the online documentation.

Data types each take a bit field; it is legal to have a single dataref be more than one type of data. Whe this happens, you can pick any matching get/set API.

NameValueDescription
xplmType_Unknown"0" Data of a type the current XPLM doesn't do.
xplmType_Int"1" A single 4-byte integer, native endian.
xplmType_Float"2" A single 4-byte float, native endian.
xplmType_Double"4" A single 8-byte double, native endian.
xplmType_FloatArray"8" An array of 4-byte floats, native endian.
xplmType_IntArray"16" An array of 4-byte integers, native endian.
xplmType_Data"32" A variable block of data.

XPLMFindDataRef

XPLM_API XPLMDataRef XPLMFindDataRef(
                         const char *         inDataRefName);    

Given a c-style string that names the data ref, this routine looks up the actual opaque XPLMDataRef that you use to read and write the data. The string names for datarefs are published on the X-Plane SDK web site.

This function returns NULL if the data ref cannot be found.

NOTE: this function is relatively expensive; save the XPLMDataRef this function returns for future use. Do not look up your data ref by string every time you need to read or write it.

XPLMCanWriteDataRef

XPLM_API int        XPLMCanWriteDataRef(
                         XPLMDataRef          inDataRef);    

Given a data ref, this routine returns true if you can successfully set the data, false otherwise. Some datarefs are read-only.

NOTE: even if a dataref is marked writable, it may not act writable. This can happen for datarefs that X-Plane writes to on every frame of simulation. In some cases, the dataref is writable but you have to set a separate “override” dataref to 1 to stop X-Plane from writing it.

XPLMIsDataRefGood

XPLM_API int        XPLMIsDataRefGood(
                         XPLMDataRef          inDataRef);    

This function returns true if the passed in handle is a valid dataref that is not orphaned.

Note: there is normally no need to call this function; datarefs returned by XPLMFindDataRef remain valid (but possibly orphaned) unless there is a complete plugin reload (in which case your plugin is reloaded anyway). Orphaned datarefs can be safely read and return 0. Therefore you never need to call XPLMIsDataRefGood to ‘check’ the safety of a dataref. (XPLMIsDatarefGood performs some slow checking of the handle validity, so it has a perormance cost.)

XPLMGetDataRefTypes

XPLM_API XPLMDataTypeID XPLMGetDataRefTypes(
                         XPLMDataRef          inDataRef);    

This routine returns the types of the data ref for accessor use. If a data ref is available in multiple data types, the bit-wise OR of these types will be returned.

DATA ACCESSORS

These routines read and write the data references. For each supported data type there is a reader and a writer.

If the data ref is orphaned or the plugin that provides it is disabled or there is a type mismatch, the functions that read data will return 0 as a default value or not modify the passed in memory. The plugins that write data will not write under these circumstances or if the data ref is read-only.

NOTE: to keep the overhead of reading datarefs low, these routines do not do full validation of a dataref; passing a junk value for a dataref can result in crashing the sim. The get/set APIs do check for NULL.

For array-style datarefs, you specify the number of items to read/write and the offset into the array; the actual number of items read or written is returned. This may be less to prevent an array-out-of-bounds error.

XPLMGetDatai

XPLM_API int        XPLMGetDatai(
                         XPLMDataRef          inDataRef);    

Read an integer data ref and return its value. The return value is the dataref value or 0 if the dataref is invalid/NULL or the plugin is disabled.

XPLMSetDatai

XPLM_API void       XPLMSetDatai(
                         XPLMDataRef          inDataRef,    
                         int                  inValue);    

Write a new value to an integer data ref. This routine is a no-op if the plugin publishing the dataref is disabled, the dataref is invalid, or the dataref is not writable.

XPLMGetDataf

XPLM_API float      XPLMGetDataf(
                         XPLMDataRef          inDataRef);    

Read a single precision floating point dataref and return its value. The return value is the dataref value or 0.0 if the dataref is invalid/NULL or the plugin is disabled.

XPLMSetDataf

XPLM_API void       XPLMSetDataf(
                         XPLMDataRef          inDataRef,    
                         float                inValue);    

Write a new value to a single precision floating point data ref. This routine is a no-op if the plugin publishing the dataref is disabled, the dataref is invalid, or the dataref is not writable.

XPLMGetDatad

XPLM_API double     XPLMGetDatad(
                         XPLMDataRef          inDataRef);    

Read a double precision floating point dataref and return its value. The return value is the dataref value or 0.0 if the dataref is invalid/NULL or the plugin is disabled.

XPLMSetDatad

XPLM_API void       XPLMSetDatad(
                         XPLMDataRef          inDataRef,    
                         double               inValue);    

Write a new value to a double precision floating point data ref. This routine is a no-op if the plugin publishing the dataref is disabled, the dataref is invalid, or the dataref is not writable.

XPLMGetDatavi

XPLM_API int        XPLMGetDatavi(
                         XPLMDataRef          inDataRef,    
                         int *                outValues,    /* Can be NULL */
                         int                  inOffset,    
                         int                  inMax);    

Read a part of an integer array dataref. If you pass NULL for outValues, the routine will return the size of the array, ignoring inOffset and inMax.

If outValues is not NULL, then up to inMax values are copied from the dataref into outValues, starting at inOffset in the dataref. If inMax + inOffset is larger than the size of the dataref, less than inMax values will be copied. The number of values copied is returned.

Note: the semantics of array datarefs are entirely implemented by the plugin (or X-Plane) that provides the dataref, not the SDK itself; the above description is how these datarefs are intended to work, but a rogue plugin may have different behavior.

XPLMSetDatavi

XPLM_API void       XPLMSetDatavi(
                         XPLMDataRef          inDataRef,    
                         int *                inValues,    
                         int                  inoffset,    
                         int                  inCount);    

Write part or all of an integer array dataref. The values passed by inValues are written into the dataref starting at inOffset. Up to inCount values are written; however if the values would write “off the end” of the dataref array, then fewer values are written.

Note: the semantics of array datarefs are entirely implemented by the plugin (or X-Plane) that provides the dataref, not the SDK itself; the above description is how these datarefs are intended to work, but a rogue plugin may have different behavior.

XPLMGetDatavf

XPLM_API int        XPLMGetDatavf(
                         XPLMDataRef          inDataRef,    
                         float *              outValues,    /* Can be NULL */
                         int                  inOffset,    
                         int                  inMax);    

Read a part of a single precision floating point array dataref. If you pass NULL for outVaules, the routine will return the size of the array, ignoring inOffset and inMax.

If outValues is not NULL, then up to inMax values are copied from the dataref into outValues, starting at inOffset in the dataref. If inMax + inOffset is larger than the size of the dataref, less than inMax values will be copied. The number of values copied is returned.

Note: the semantics of array datarefs are entirely implemented by the plugin (or X-Plane) that provides the dataref, not the SDK itself; the above description is how these datarefs are intended to work, but a rogue plugin may have different behavior.

XPLMSetDatavf

XPLM_API void       XPLMSetDatavf(
                         XPLMDataRef          inDataRef,    
                         float *              inValues,    
                         int                  inoffset,    
                         int                  inCount);    

Write part or all of a single precision floating point array dataref. The values passed by inValues are written into the dataref starting at inOffset. Up to inCount values are written; however if the values would write “off the end” of the dataref array, then fewer values are written.

Note: the semantics of array datarefs are entirely implemented by the plugin (or X-Plane) that provides the dataref, not the SDK itself; the above description is how these datarefs are intended to work, but a rogue plugin may have different behavior.

XPLMGetDatab

XPLM_API int        XPLMGetDatab(
                         XPLMDataRef          inDataRef,    
                         void *               outValue,    /* Can be NULL */
                         int                  inOffset,    
                         int                  inMaxBytes);    

Read a part of a byte array dataref. If you pass NULL for outVaules, the routine will return the size of the array, ignoring inOffset and inMax.

If outValues is not NULL, then up to inMax values are copied from the dataref into outValues, starting at inOffset in the dataref. If inMax + inOffset is larger than the size of the dataref, less than inMax values will be copied. The number of values copied is returned.

Note: the semantics of array datarefs are entirely implemented by the plugin (or X-Plane) that provides the dataref, not the SDK itself; the above description is how these datarefs are intended to work, but a rogue plugin may have different behavior.

XPLMSetDatab

XPLM_API void       XPLMSetDatab(
                         XPLMDataRef          inDataRef,    
                         void *               inValue,    
                         int                  inOffset,    
                         int                  inLength);    

Write part or all of a byte array dataref. The values passed by inValues are written into the dataref starting at inOffset. Up to inCount values are written; however if the values would write “off the end” of the dataref array, then fewer values are written.

Note: the semantics of array datarefs are entirely implemented by the plugin (or X-Plane) that provides the dataref, not the SDK itself; the above description is how these datarefs are intended to work, but a rogue plugin may have different behavior.

PUBLISHING YOUR PLUGIN'S DATA

These functions allow you to create data references that other plug-ins and X-Plane can access via the above data access APIs. Data references published by other plugins operate the same as ones published by X-Plane in all manners except that your data reference will not be available to other plugins if/when your plugin is disabled.

You share data by registering data provider callback functions. When a plug-in requests your data, these callbacks are then called. You provide one callback to return the value when a plugin ‘reads’ it and another to change the value when a plugin ‘writes’ it.

Important: you must pick a prefix for your datarefs other than “sim/” - this prefix is reserved for X-Plane. The X-Plane SDK website contains a registry where authors can select a unique first word for dataref names, to prevent dataref collisions between plugins.

XPLMGetDatai_f

typedef int (* XPLMGetDatai_f)(
                         void *               inRefcon);    

Data provider function pointers.

These define the function pointers you provide to get or set data. Note that you are passed a generic pointer for each one. This is the same pointer you pass in your register routine; you can use it to locate plugin variables, etc.

The semantics of your callbacks are the same as the dataref accessor above - basically routines like XPLMGetDatai are just pass-throughs from a caller to your plugin. Be particularly mindful in implementing array dataref read-write accessors; you are responsible for avoiding overruns, supporting offset read/writes, and handling a read with a NULL buffer.

XPLMSetDatai_f

typedef void (* XPLMSetDatai_f)(
                         void *               inRefcon,    
                         int                  inValue);    

XPLMGetDataf_f

typedef float (* XPLMGetDataf_f)(
                         void *               inRefcon);    

XPLMSetDataf_f

typedef void (* XPLMSetDataf_f)(
                         void *               inRefcon,    
                         float                inValue);    

XPLMGetDatad_f

typedef double (* XPLMGetDatad_f)(
                         void *               inRefcon);    

XPLMSetDatad_f

typedef void (* XPLMSetDatad_f)(
                         void *               inRefcon,    
                         double               inValue);    

XPLMGetDatavi_f

typedef int (* XPLMGetDatavi_f)(
                         void *               inRefcon,    
                         int *                outValues,    /* Can be NULL */
                         int                  inOffset,    
                         int                  inMax);    

XPLMSetDatavi_f

typedef void (* XPLMSetDatavi_f)(
                         void *               inRefcon,    
                         int *                inValues,    
                         int                  inOffset,    
                         int                  inCount);    

XPLMGetDatavf_f

typedef int (* XPLMGetDatavf_f)(
                         void *               inRefcon,    
                         float *              outValues,    /* Can be NULL */
                         int                  inOffset,    
                         int                  inMax);    

XPLMSetDatavf_f

typedef void (* XPLMSetDatavf_f)(
                         void *               inRefcon,    
                         float *              inValues,    
                         int                  inOffset,    
                         int                  inCount);    

XPLMGetDatab_f

typedef int (* XPLMGetDatab_f)(
                         void *               inRefcon,    
                         void *               outValue,    /* Can be NULL */
                         int                  inOffset,    
                         int                  inMaxLength);    

XPLMSetDatab_f

typedef void (* XPLMSetDatab_f)(
                         void *               inRefcon,    
                         void *               inValue,    
                         int                  inOffset,    
                         int                  inLength);    

XPLMRegisterDataAccessor

XPLM_API XPLMDataRef XPLMRegisterDataAccessor(
                         const char *         inDataName,    
                         XPLMDataTypeID       inDataType,    
                         int                  inIsWritable,    
                         XPLMGetDatai_f       inReadInt,    
                         XPLMSetDatai_f       inWriteInt,    
                         XPLMGetDataf_f       inReadFloat,    
                         XPLMSetDataf_f       inWriteFloat,    
                         XPLMGetDatad_f       inReadDouble,    
                         XPLMSetDatad_f       inWriteDouble,    
                         XPLMGetDatavi_f      inReadIntArray,    
                         XPLMSetDatavi_f      inWriteIntArray,    
                         XPLMGetDatavf_f      inReadFloatArray,    
                         XPLMSetDatavf_f      inWriteFloatArray,    
                         XPLMGetDatab_f       inReadData,    
                         XPLMSetDatab_f       inWriteData,    
                         void *               inReadRefcon,    
                         void *               inWriteRefcon);    

This routine creates a new item of data that can be read and written. Pass in the data’s full name for searching, the type(s) of the data for accessing, and whether the data can be written to. For each data type you support, pass in a read accessor function and a write accessor function if necessary. Pass NULL for data types you do not support or write accessors if you are read-only.

You are returned a data ref for the new item of data created. You can use this data ref to unregister your data later or read or write from it.

XPLMUnregisterDataAccessor

XPLM_API void       XPLMUnregisterDataAccessor(
                         XPLMDataRef          inDataRef);    

Use this routine to unregister any data accessors you may have registered. You unregister a data ref by the XPLMDataRef you get back from registration. Once you unregister a data ref, your function pointer will not be called anymore.

SHARING DATA BETWEEN MULTIPLE PLUGINS

The data reference registration APIs from the previous section allow a plugin to publish data in a one-owner manner; the plugin that publishes the data reference owns the real memory that the data ref uses. This is satisfactory for most cases, but there are also cases where plugnis need to share actual data.

With a shared data reference, no one plugin owns the actual memory for the data reference; the plugin SDK allocates that for you. When the first plugin asks to ‘share’ the data, the memory is allocated. When the data is changed, every plugin that is sharing the data is notified.

Shared data references differ from the ‘owned’ data references from the previous section in a few ways:

  • With shared data references, any plugin can create the data reference; with owned plugins one plugin must create the data reference and others subscribe. (This can be a problem if you don’t know which set of plugins will be present).

  • With shared data references, every plugin that is sharing the data is notified when the data is changed. With owned data references, only the one owner is notified when the data is changed.

  • With shared data references, you cannot access the physical memory of the data reference; you must use the XPLMGet… and XPLMSet… APIs. With an owned data reference, the one owning data reference can manipulate the data reference’s memory in any way it sees fit.

Shared data references solve two problems: if you need to have a data reference used by several plugins but do not know which plugins will be installed, or if all plugins sharing data need to be notified when that data is changed, use shared data references.

XPLMDataChanged_f

typedef void (* XPLMDataChanged_f)(
                         void *               inRefcon);    

An XPLMDataChanged_f is a callback that the XPLM calls whenever any other plug-in modifies shared data. A refcon you provide is passed back to help identify which data is being changed. In response, you may want to call one of the XPLMGetDataxxx routines to find the new value of the data.

XPLMShareData

XPLM_API int        XPLMShareData(
                         const char *         inDataName,    
                         XPLMDataTypeID       inDataType,    
                         XPLMDataChanged_f    inNotificationFunc,    
                         void *               inNotificationRefcon);    

This routine connects a plug-in to shared data, creating the shared data if necessary. inDataName is a standard path for the data ref, and inDataType specifies the type. This function will create the data if it does not exist. If the data already exists but the type does not match, an error is returned, so it is important that plug-in authors collaborate to establish public standards for shared data.

If a notificationFunc is passed in and is not NULL, that notification function will be called whenever the data is modified. The notification refcon will be passed to it. This allows your plug-in to know which shared data was changed if multiple shared data are handled by one callback, or if the plug-in does not use global variables.

A one is returned for successfully creating or finding the shared data; a zero if the data already exists but is of the wrong type.

XPLMUnshareData

XPLM_API int        XPLMUnshareData(
                         const char *         inDataName,    
                         XPLMDataTypeID       inDataType,    
                         XPLMDataChanged_f    inNotificationFunc,    
                         void *               inNotificationRefcon);    

This routine removes your notification function for shared data. Call it when done with the data to stop receiving change notifications. Arguments must match XPLMShareData. The actual memory will not necessarily be freed, since other plug-ins could be using it.