The NDArray (N-Dimensional array) is the class that is used for passing detector data from drivers to plugins. An NDArray is a general purpose class for handling array data. An NDArray object is self-describing, meaning it contains enough information to describe the data itself. It can optionally contain “attributes” (class NDAttribute) which contain meta-data describing how the data was collected, etc.

An NDArray can have up to ND_ARRAY_MAX_DIMS dimensions, currently 10. A fixed maximum number of dimensions is used to significantly simplify the code compared to unlimited number of dimensions. Each dimension of the array is described by an NDDimension structure. The NDArray class documentationdescribes this class in detail.


The NDArrayPool class manages a free list (pool) of NDArray objects. Drivers allocate NDArray objects from the pool, and pass these objects to plugins. Plugins increase the reference count on the object when they place the object on their queue, and decrease the reference count when they are done processing the array. When the reference count reaches 0 again the NDArray object is placed back on the free list. This mechanism minimizes the copying of array data in plugins. The NDArrayPool class documentationdescribes this class in detail.


The NDAttribute is a class for linking metadata to an NDArray. An NDattribute has a name, description, data type, value, source type and source information. Attributes are identified by their names, which are case-sensitive. There are methods to set and get the information for an attribute.

It is useful to define some conventions for attribute names, so that plugins or data analysis programs can look for a specific attribute. The following are the attribute conventions used in current plugins:

  Conventions for standard attribute names
Attribute name Description Data type
ColorMode Color mode of the NDArray. Used by many plugins to determine how to process the array. NDAttrInt32 (NDColorMode_t)
BayerPattern Bayer patter of an image collect by a color camera with a Bayer mask. Could be used to convert to a an RGB color image. This capability may be added to NDPluginColorConvert. NDAttrInt32 (NDBayerPattern_t)
DriverFilename The name of the file originally collected by the driver. This is used by NDPluginFile to delete the original driver file if the DeleteDriverFile flag is set and the NDArray has been successfully saved in another file. NDAttrString
FilePluginDestination This is used by NDPluginFile to determine whether to process this NDArray. If this attribute is present and is “all” or the name of this plugin then the NDArray is processed, otherwise it is ignored. NDAttrString
FilePluginFileName This is used by NDPluginFile to set the file name when saving this NDArray. NDAttrString
FilePluginFileNumber This is used by NDPluginFile to set the file number when saving this NDArray. NDAttrInt32
FilePluginFileClose This is used by NDPluginFile to close the file after processing this NDArray. NDAttrInt32
[HDF dataset name] This is used by NDFileHDF5 to determine which dataset in the file this NDArray should be written to. The attribute name is the name of the HDF5 dataset. NDAttrString
[posName] This is used by NDFileHDF5 to determine which position in the dataset this NDArray should be written to. The attribute name is contained in a PosName record defined in NDFileHDF5.template. This is designed to allow, for example, “snake scan” data to be placed in the correct order in an HDF5 file. NDAttrInt32

Attribute names are case-sensitive. For attributes not in this table a good convention would be to use the corresponding driver parameter without the leading ND or AD, and with the first character of every “word” of the name starting with upper case. For example, the standard attribute name for ADManufacturer should be “Manufacturer”, ADNumExposures should be “NumExposures”, etc.

The NDAttribute class documentation describes this class in detail.


The NDAttributeList implements a linked list of NDAttribute objects. NDArray objects contain an NDAttributeList which is how attributes are associated with an NDArray. There are methods to add, delete and search for NDAttribute objects in an NDAttributeList. Each attribute in the list must have a unique name, which is case-sensitive.

When NDArrays are copied with the NDArrayPool methods the attribute list is also copied.

IMPORTANT NOTE: When a new NDArray is allocated using NDArrayPool::alloc() the behavior of any existing attribute list on the NDArray taken from the pool is determined by the value of the global variable eraseNDAttributes. By default the value of this variable is 0. This means that when a new NDArray is allocated from the pool its attribute list is not cleared. This greatly improves efficiency in the normal case where attributes for a given driver are defined once at initialization and never deleted. (The attribute values may of course be changing.) It eliminates allocating and deallocating attribute memory each time an array is obtained from the pool. It is still possible to add new attributes to the array, but any existing attributes will continue to exist even if they are ostensibly cleared e.g. asynNDArrayDriver::readNDAttributesFile() is called again. If it is desired to eliminate all existing attributes from NDArrays each time a new one is allocated then the global variable eraseNDAttributes should be set to 1. This can be done at the iocsh prompt with the command:

var eraseNDAttributes 1

The NDAttributeList class documentation describes this class in detail.


The PVAttribute class is derived from NDAttribute. It obtains its value by monitor callbacks from an EPICS PV, and is thus used to associate current the value of any EPICS PV with an NDArray. The PVAttribute class documentation describes this class in detail.


The paramAttribute class is derived from NDAttribute. It obtains its value from the current value of a driver or plugin parameter. The paramAttribute class is typically used when it is important to have the current value of the parameter and the value of a corresponding PVAttribute might not be current because the EPICS PV has not yet updated. The paramAttribute class documentation describes this class in detail.


The functAttribute class is derived from NDAttribute. It obtains its value from a user-written C++ function. The functAttribute class is thus very general, and can be used to add almost any information to an NDArray. ADCore contains example code, myAttributeFunctions.cpp that demonstates how to write such functions. The functAttribute class documentation describes this class in detail.


asynNDArrayDriver inherits from asynPortDriver. It implements the asynGenericPointer functions for NDArray objects. This is the class from which both plugins and area detector drivers are indirectly derived. The asynNDArrayDriver class documentationdescribes this class in detail.

The file asynNDArrayDriver.h defines a number of parameters that all NDArray drivers and plugins should implement if possible. These parameters are defined by strings (drvInfo strings in asyn) with an associated asyn interface, and access (read-only or read-write). There is also an integer index to the parameter which is assigned by asynPortDriver when the parameter is created in the parameter library. The EPICS database NDArrayBase.template provides access to these standard driver parameters. The following table lists the standard driver parameters. The columns are defined as follows:

  • Parameter index variable: The variable name for this parameter index in the driver. There are several EPICS records in ADBase.template that do not have corresponding parameter indices, and these are indicated as Not Applicable (N/A).
  • asyn interface: The asyn interface used to pass this parameter to the driver.
  • Access: Read-write (r/w) or read-only (r/o).
  • drvInfo string: The string used to look up the parameter in the driver through the drvUser interface. This string is used in the EPICS database file for generic asyn device support to associate a record with a particular parameter. It is also used to associate a paramAttribute with a driver parameter in the XML file that is read by asynNDArrayDriver::readNDAttributesFile
  • EPICS record name: The name of the record in ADBase.template. Each record name begins with the two macro parameters $(P) and $(R). In the case of read/write parameters there are normally two records, one for writing the value, and a second, ending in _RBV, that contains the actual value (Read Back Value) of the parameter.
  • EPICS record type: The record type of the record. Waveform records are used to hold long strings, with length (NELM) = 256 bytes and EPICS data type (FTVL) = UCHAR. This removes the 40 character restriction string lengths that arise if an EPICS “string” PV is used. MEDM allows one to edit and display such records correctly. EPICS clients will typically need to convert such long strings from a string to an integer or byte array before sending the path name to EPICS. In IDL this is done as follows:
; Convert a string to a null-terminated byte array and write with caput
IDL> t = caput('13PS1:TIFF1:FilePath', [byte('/home/epics/scratch'),0B])
; Read a null terminated byte array
IDL> t = caget('13PS1:TIFF1:FilePath', v)
; Convert to a string
IDL> s = string(v)

In SPEC this is done as follows:

array _temp[256]
# Setting the array to "" will zero-fill it
_temp = ""
# Copy the string to the array.  Note, this does not null terminate, so if array already contains
# a longer string it needs to first be zeroed by setting it to "".
_temp = "/home/epics/scratch"
epics_put("13PS1:TIFF1:FilePath", _temp)

Note that for parameters whose values are defined by enum values (e.g NDDataType, NDColorMode, etc.), drivers can use a different set of enum values for these parameters. They can override the enum menu in ADBase.template with driver-specific choices by loading a driver-specific template file that redefines that record field after loading ADBase.template.

Parameter Definitions in asynNDArrayDriver.h and EPICS Record Definitions in NDArrayBase.template (file-related records are in NDFile.template)
Parameter index variable asyn interface Access Description drvInfo string EPICS record name EPICS record type
      Information about the version of ADCore and the plugin or driver
NDADCoreVersion asynOctet r/o ADCore version number. This can be used by Channel Access clients to alter their behavior depending on the version of ADCore that was used to build this driver or plugin. ADCORE_VERSION $(P)$(R)ADCoreVersion_RBV stringin
NDDriverVersion asynOctet r/o Driver or plugin version number. This can be used by Channel Access clients to alter their behavior depending on the version of the plugin or driver. DRIVER_VERSION $(P)$(R)DriverVersion_RBV stringin
      Information about the asyn port
NDPortNameSelf asynOctet r/o asyn port name PORT_NAME_SELF $(P)$(R)PortName_RBV stringin
      Data type
NDDataType asynInt32 r/w Data type (NDDataType_t). DATA_TYPE $(P)$(R)DataType, $(P)$(R)DataType_RBV mbbo, mbbi
      Color mode
NDColorMode asynInt32 r/w Color mode (NDColorMode_t). COLOR_MODE $(P)$(R)ColorMode, $(P)$(R)ColorMode_RBV mbbo, mbbi
NDBayerPattern asynInt32 r/o Bayer pattern (NDBayerPattern_t) of NDArray data. BAYER_PATTERN $(P)$(R)BayerPattern_RBV mbbi
      Actual dimensions of array data
NDNDimensions asynInt32 r/w Number of dimensions in the array ARRAY_NDIMENSIONS $(P)$(R)NDimensions, $(P)$(R)NDimensions_RBV longout, longin
NDDimensions asynInt32Array r/w Size of each dimension in the array ARRAY_DIMENSIONS $(P)$(R)Dimensions, $(P)$(R)Dimensions_RBV waveform (out), waveform (in)
N.A. N.A r/o Size of each array dimension, extracted from the $(P)$(R)Dimensions and $(P)$(R)Dimensions_RBV waveform records. Note that these are both longin record, i.e. readonly values using subarray records. In the future longout records may be added to write to the individual values in $(P)$(R)Dimensions. N.A. $(P)$(R)ArraySize[N] N=0-9 , (P)$(R)ArraySize[N]_RBV longin, longin
NDArraySizeX asynInt32 r/o Size of the array data in the X direction ARRAY_SIZE_X $(P)$(R)ArraySizeX_RBV longin
NDArraySizeY asynInt32 r/o Size of the array data in the Y direction ARRAY_SIZE_Y $(P)$(R)ArraySizeY_RBV longin
NDArraySizeZ asynInt32 r/o Size of the array data in the Z direction ARRAY_SIZE_Z $(P)$(R)ArraySizeZ_RBV longin
NDArraySize asynInt32 r/o Total size of the array data in bytes ARRAY_SIZE $(P)$(R)ArraySize_RBV longin
NDCodec asynOctet r/o The codec used to compress this array CODEC $(P)$(R)Codec_RBV stringin
NDCompressedSize asynInt32 r/o Compressed size of the array data in bytes. Only meaningful if NDCodec is not empty string. COMPRESSED_SIZE $(P)$(R)Compressed_RBV longin
      Array data
NDArrayCallbacks asynInt32 r/w Controls whether the driver or plugin does callbacks with the array data to registered plugins. 0=No, 1=Yes. Setting this to 0 in a driver can reduce overhead in the case that the driver is being used only to control the device, and not to make the data available to plugins or to EPICS clients. Setting this to 0 in a plugin can reduce overhead by eliminating the need to copy the NDArray if that plugin is not being used as a source of NDArrays to other plugins. ARRAY_CALLBACKS $(P)$(R)ArrayCallbacks, $(P)$(R)ArrayCallbacks_RBV bo, bi
NDArrayData asynGenericPointer r/w The array data as an NDArray object NDARRAY_DATA N/A. EPICS access to array data is through NDStdArrays plugin. N/A
NDArrayCounter asynInt32 r/w Counter that increments by 1 each time an array is acquired. Can be reset by writing a value to it. ARRAY_COUNTER $(P)$(R)ArrayCounter, $(P)$(R)ArrayCounter_RBV longout, longin
N/A N/A r/o Rate at which arrays are being acquired. Computed in the ADBase.template database. N/A $(P)$(R)ArrayRate_RBV calc
      Array attributes
NDAttributesFile asynOctet r/w The name of an XML file defining the NDAttributes to be added to each NDArray by this driver or plugin. The format of the XML file is described in the documentation for asynNDArrayDriver::readNDAttributesFile() ND_ATTRIBUTES_FILE $(P)$(R)NDAttributesFile waveform
NDAttributesMacros asynOctet r/w A macro definition string that can be used to do macro substitution in the XML file. For example if this string is “CAMERA=13SIM1:cam1:,ID=ID13us:” then all $(CAMERA) and $(ID) strings in the XML file will be replaced with 13SIM1:cam1: and ID13us: respectively. ND_ATTRIBUTES_MACROS $(P)$(R)NDAttributesMacros waveform
NDAttributesStatus asynInt32 r/o The status of reading and parsing the XML attributes file. This is used to indicate if the file cannot be found, if there is an XML syntax error, or if there is a macro substitutions error. ND_ATTRIBUTES_STATUS $(P)$(R)NDAttributesStatus mbbi
      Array pool status
NDPoolMaxMemory asynFloat64 r/o The maximum number of NDArrayPool memory bytes that can be allocated. 0=unlimited. POOL_MAX_MEMORY $(P)$(R)PoolMaxMem ai
NDPoolUsedMemory asynFloat64 r/o The number of NDArrayPool memory bytes currently allocated. The SCAN rate of this record controls the scanning of all of the dynamic NDArrayPool status records. POOL_USED_MEMORY $(P)$(R)PoolUsedMem ai
NDPoolAllocBuffers asynInt32 r/o The number of NDArrayPool buffers currently allocated. POOL_ALLOC_BUFFERS $(P)$(R)PoolAllocBuffers longin
NDPoolFreeBuffers asynInt32 r/o The number of NDArrayPool buffers currently allocated but free. POOL_FREE_BUFFERS $(P)$(R)PoolFreeBuffers longin
N.A. N.A. r/o The number of NDArrayPool buffers currently in use. This is calculated as NDPoolAllocBuffers - NDPoolFreeBuffers. N.A. $(P)$(R)PoolUsedBuffers calc
NDPoolEmptyFreeList asynInt32 r/w Processing this record deletes all of the NDArrays on the freelist and sets the freelist size to 0. This provides a mechanism to free large amounts of memory and return it to the operating system, for example after a rapid acquisition with large plugin queues. On Windows the memory is returned to the operating system immediately. On Linux the freed memory may not actually be returned to the operating system even though it has been freed in the areaDetector process. On Centos7 (and presumably many other versions of Linux) setting the value of the environment variable MALLOC_TRIM_THRESHOLD_ to a small value will allow the memory to actually be returned to the operating system. POOL_EMPTY_FREELIST $(P)$(R)EmptyFreeList bo
NDNumQueuedArrays asynInt32 r/o The number of NDArrays from this driver’s NDArrayPool that are currently queued for processing by plugins. When this number goes to 0 the plugins have all completed processing. NUM_QUEUED_ARRAYS $(P)$(R)NumQueuedArrays longin
      Debugging control
N/A N/A N/A asyn record to control debugging (asynTrace) N/A $(P)$(R)AsynIO asyn
      File saving parameters (records are defined in NDFile.template)
NDFilePath asynOctet r/w File path FILE_PATH $(P)$(R)FilePath, $(P)$(R)FilePath_RBV waveform, waveform
NDFilePathExists asynInt32 r/o Flag indicating if file path exists FILE_PATH_EXISTS $(P)$(R)FilePathExists_RBV bi
NDFileName asynOctet r/w File name FILE_NAME $(P)$(R)FileName, $(P)$(R)FileName_RBV waveform, waveform
NDFileNumber asynInt32 r/w File number FILE_NUMBER $(P)$(R)FileNumber, $(P)$(R)FileNumber_RBV longout, longin
NDFileTemplate asynOctet r/w Format string for constructing NDFullFileName from NDFilePath, NDFileName, and NDFileNumber. The final file name (which is placed in NDFullFileName) is created with the following code: epicsSnprintf(FullFilename, sizeof(FullFilename), FileTemplate, FilePath, Filename, FileNumber); FilePath, Filename, FileNumber are converted in that order with FileTemplate. An example file format is “%s%s%4.4d.tif”. The first %s converts the FilePath, followed immediately by another %s for Filename. FileNumber is formatted with %4.4d, which results in a fixed field with of 4 digits, with leading zeros as required. Finally, the .tif extension is added to the file name. This mechanism for creating file names is very flexible. Other characters, such as _ can be put in Filename or FileTemplate as desired. If one does not want to have FileNumber in the file name at all, then just omit the %d format specifier from FileTemplate. If the client wishes to construct the complete file name itself, then it can just put that file name into NDFileTemplate with no format specifiers at all, in which case NDFilePath, NDFileName, and NDFileNumber will be ignored. FILE_TEMPLATE $(P)$(R)FileTemplate, $(P)$(R)FileTemplate_RBV waveform, waveform
NDFullFileName asynOctet r/o Full file name constructed using the algorithm described in NDFileTemplate FULL_FILE_NAME $(P)$(R)FullFileName_RBV waveform, waveform
NDAutoIncrement asynInt32 r/w Auto-increment flag. Controls whether FileNumber is automatically incremented by 1 each time a file is saved (0=No, 1=Yes) AUTO_INCREMENT $(P)$(R)AutoIncrement, $(P)$(R)AutoIncrement_RBV bo, bi
NDAutoSave asynInt32 r/w Auto-save flag (0=No, 1=Yes) controlling whether a file is automatically saved each time acquisition completes. AUTO_SAVE $(P)$(R)AutoSave, $(P)$(R)AutoSave_RBV bo, bi
NDFileFormat asynInt32 r/w File format. The format to write/read data in (e.g. TIFF, netCDF, etc.) FILE_FORMAT $(P)$(R)FileFormat, $(P)$(R)FileFormat_RBV mbbo, mbbi
NDWriteFile asynInt32 r/w Manually save the most recent array to a file when value=1 WRITE_FILE $(P)$(R)WriteFile, $(P)$(R)WriteFile_RBV busy, bi
NDReadFile asynInt32 r/w Manually read a file when value=1 READ_FILE $(P)$(R)ReadFile, $(P)$(R)ReadFile_RBV busy, bi
NDFileWriteMode asynInt32 r/w File saving mode (Single, Capture, Stream)(NDFileMode_t) WRITE_MODE $(P)$(R)FileWriteMode, $(P)$(R)FileWriteMode_RBV mbbo, mbbi
NDFileWriteStatus asynInt32 r/o File write status. Gives status information on last file open or file write operation. Values are WriteOK (0) and WriteError (1). WRITE_STATUS $(P)$(R)FileWriteStatus mbbi
NDFileWriteMessage asynOctet r/o File write error message. An error message string if the previous file open or file write operation resulted in an error. WRITE_MESSAGE $(P)$(R)FileWriteMessage waveform
NDFileCapture asynInt32 r/w Start (1) or stop (0) file capture or streaming CAPTURE $(P)$(R)Capture, $(P)$(R)Capture_RBV busy, bi
NDFileNumCapture asynInt32 r/w Number of frames to acquire in capture or streaming mode NUM_CAPTURE $(P)$(R)NumCapture, $(P)$(R)NumCapture_RBV longout, longin
NDFileNumCaptured asynInt32 r/o Number of arrays currently acquired capture or streaming mode NUM_CAPTURED $(P)$(R)NumCaptured_RBV longin
NDFileDeleteDriverFile asynInt32 r/w Flag to enable deleting original driver file after a plugin has re-written the file in a different format. This can be useful for detectors that must write the data to disk in order for the areaDetector driver to read it back. Once a file-writing plugin has rewritten the data in another format it can be desireable to then delete the original file. DELETE_DRIVER_FILE $(P)$(R)DeleteDriverFile, $(P)$(R)DeleteDriverFile_RBV bo, bi
NDFileLazyOpen asynInt32 r/w Flag to defer the creation of a new file until the first NDArray to write has been received. This removes the need for passing an extra NDArray through the file writing plugin to initialise dimensions and possibly NDAttribute list before opening the file. The downside is that file creation can potentially be time-consuming so processing the first NDArray may be slower than subsequent ones. , Only makes sense to use with file plugins which support multiple frames per file and only in “Stream” mode. FILE_LAZY_OPEN $(P)$(R)LazyOpen, $(P)$(R)LazyOpen_RBV bo, bi
NDFileCreateDir asynInt32 r/w This parameter is used to automatically create directories if they don’t exist. If it is zero (default), no directories are created. If it is negative, then the absolute value is the maximum of directories that will be created (i.e. -1 will create a maximum of one directory to complete the path, -2 will create a maximum of 2 directories). If it is positive, then at least that many directories in the path must exist (i.e. a value of 1 will create all directories below the root directory and 2 will not create a directory in the root directory). CREATE_DIR $(P)$(R)CreateDirectory, $(P)$(R)CreateDirectory_RBV longout, longin
NDFileTempSuffix asynOctet r/w If this string is non-null, the file is opened with this suffix temporarily appended to the file name. When the file is closed it is then renamed to the correct file name without the suffix. This is useful for processing software watching for the file to appear since the file appears as an atomic operation when it is ready to be opened. FILE_TEMP_SUFFIX $(P)$(R)TempSuffix, $(P)$(R)TempSuffix_RBV stringout, stringin