V4 Design: epicsTypes

EPICS: epicsTypes

June 7, 2005

Overview

This document describes the C++ definitions for storing data that can be accessed without pre-complied code, i.e. it has self describing features.

epicsTypes can be used by any code that stores data accessable via V4 Channel Access. Some examples are:

1. IOC records - Most data accessable from outside record support is stored as an epicsType.
2. Channel Access Gateway - Can be used to store and transfer data.
3. Channel Access Clients - Can be used to get/receive data from CA server.

Standard support can be provided to access, via dataAccess, epicsType data. For example standard support can be provided to move IOC record data between record instances and a Channel Access server.

epicsType is an enum that defines the following:

• epicsTypeUnknown - Type is unknown
• epicsTypeBoolean - true or false
• epicsTypeOctet - An eight bit byte. It is NOT an integer type
• epicsTypeInt16 - 16 bit signed integer
• epicsTypeInt32 - 32 bit signed integer
• epicsTypeInt64 - 64 bit signed integer
• epicsTypeFloat32 - 32 bit IEEE float
• epicsTypeFloat64 - 64 bit IEEE float
• epicsTypeString - A UTF-8 Encoded Character String
• epicsTypeArray - One dimensional array of any epicsType
• epicsTypeMDArray - Multidimensional array of type epicsTypeBoolean,..epicsTypeFloat64, or epicsTypeString.
• epicsTypeEnum - An index and associated set of choice strings.
• epicsTypeStruct - A structure with fields each of which is any epicsType

The actual types associated with the epicsTypes are:

    typedef bool               epicsBoolean;
    typedef char               epicsOctet;
    typedef short              epicsInt16;
    typedef int                epicsInt32;
    typedef long long          epicsInt64;
    typedef float              epicsFloat32;
    typedef double             epicsFloat64;

    //EpicsString holds UTF-8 characters
    class EpicsString; // see below for details;

    class EpicsArray; // see below for details;

    class EpicsMDArray; // see below for details;

    class EpicsEnum; // see below for details;

    class EpicsStruct; // see below for details;

NOTE: In this document primitive type means one of epicsBoolean, epicsOctet, epicsInt16, epicsInt32, epicsInt64, epicsFloat32, or epicsFloat64.

epicsTypes

epicsTypes.h contains the following:

    /* The following may require OSD definitions*/
    typedef bool               epicsBoolean;
    typedef char               epicsOctet;
    typedef short              epicsInt16;
    typedef int                epicsInt32;
    typedef long long          epicsInt64;
    typedef float              epicsFloat32;
    typedef double             epicsFloat64;
   
    enum epicsType {
        epicsTypeUnknown,
        epicsTypeBoolean,
        epicsTypeOctet,
        epicsTypeInt16,
        epicsTypeInt32,
        epicsTypeInt64,
        epicsTypeFloat32,
        epicsTypeFloat64,
        epicsTypeString,
        epicsTypeArray,
        epicsTypeMDArray,
        epicsTypeEnum,
        epicsTypeBitmask,
        epicsTypeStruct
    };

epicsTypes provides classes for describing data that can be introspected and can be passed between different platforms. All data that is sent to or received from EPICS records will be composed of epicsTypes.

The types epicsBoolean, ..., epicsFloat64 all map to a C++ standard type. It may be necessary to provide operating system dependent definitions for some of the types. For example on some architectures an epicsInt64 may have to be defined as a long rather than a long long.

epicsTypeUnknown is for anything that is not one of the other epicsTypes.

The non-primitive type, except, epicsTypeUnknown are designed so that the data they contain can be described, introspected, and passed over a network. Each is described in the following sections.

Locking Issues

epicsTypes does not provide any facilities for preventing simultaneous access by multiple threads. For example the class definitions for non-primitive types, i.e. epicsString, epicsArray, epicsStruct, and epicsMDArray, all provide a method expose which returns the address of data. In order to make expose safe, some rules must be established.

As an example of rules, a lock can be associated with each object that supports expose. Code can only call expose and access the exposed data while it holds the lock.

The rules are not specified by epicsTypes since they are application dependent.

---

EpicsBuffer

epicsBuffer.h defines a generic interface EpicsBuffer to a buffer of octet data, and a buffer factory EpicsBufferFactory that provides a central registry of different implementations of EpicsBuffer and uses this to create instances of a particular buffer type on demand.

    class EpicsBuffer { // interface
    public:
        virtual void reserve(epicsInt32 capacity) = 0;
        virtual epicsInt32 capacity() const = 0;
        virtual void resize(epicsInt32 newsize) = 0;
        virtual epicsInt32 size() const = 0;
        virtual epicsInt32 maxSize() const = 0;
        virtual void destruct() = 0;
        virtual bool mutable() const = 0;
        virtual bool isEqual(const EpicsBuffer &cmp) const = 0;
        virtual bool isEqual(epicsInt32 offset, epicsInt32 len,
                             const epicsOctet *pdata) const = 0;
        virtual void expose(epicsInt32 offset, epicsInt32 &len,
                            epicsOctet *&pdata) = 0;
        virtual void expose(epicsInt32 offset, epicsInt32 &len,
                            const epicsOctet *&pdata) const = 0;
    };

An EpicsBuffer is a container for epicsOctet data values, and may be used to store things such as character strings containing UTF-8 characters or arrays of some other data type.

Multiple implementations of EpicsBuffer are needed with different characteristics, but all buffers will be accessed using the same EpicsBuffer interface. At least the following buffer types will be provided:

• readonly - A contiguous array that already contains the desired data is provided at buffer initialization time, and cannot be modified. This buffer is for use with literal character strings, and probably won't be registered with the EpicsBufferFactory.
• contiguous - The data is stored in a contiguous array of octets.
• segmented - The data is stored in fixed-size chunks. This form should be used for buffers that are frequently allocated and released, or which often need to change in size.

Additional implementations may be provided for other purposes such as managing network buffers. There might be a need for a contiguous zero-terminated buffer type for code that regularly talks to C string handling routines.

In addition to storing the data, the buffer keeps the following information:

capacity

The number of octets of storage allocated for the buffer.

size

The number of octets of data currently held in the buffer.

Implementations of EpicsBuffer must provide the following methods, the names of which have been designed (where possible) to match the names of the equivalent methods in the C++ standard template library's container classes:

void reserve(epicsInt32 capacity)

Allocate space to store capacity octets. Some implementations may impose a max_size() smaller than the limits of memory. If either limit is exceeded by the request, an exception will be thrown. This method is used to request an increase or decrease in the amount of storage alloted for the buffer and may cause the data to move in memory, but does not change the data stored or its size. In particular it cannot reduce the capacity below the amount of data currently stored in the buffer, so to release all the data storage an application would call resize(0) then reserve(0).

epicsInt32 capacity() const

Returns the allocated capacity of the buffer.

void resize(epicsInt32 newsize)

Sets the number of octets currently stored, up to the current capacity.

epicsInt32 size() const

Returns the number of octets currently stored in the buffer.

epicsInt32 maxSize() const

Returns the size of the largest buffer that may be allocated. An attempt to allocate more than this amount of space will fail for certain, but there is no guarantee that this amount of memory is available. This method provides a way for application code to discover any limits that may be imposed by the particular buffer type.

void destruct()

All storage for both the data and the buffer class itself is released.

bool mutable() const

Returns true if the buffer data can be modified, i.e. are puts allowed.

isEqual(const EpicsBuffer &cmp)

Compares the contents of this buffer with the contents of the buffer cmp, and returns true if the data is identical.

isEqual(epicsInt32 offset, epicsInt32 len, const epicsOctet *pdata)

Compares len octets of data starting offset octets into this buffer with the octet array pdata supplied by the caller. This method is mainly provided to simplify the implementation of the other isEqual method.

void expose(epicsInt32 offset, epicsInt32 &len, epicsOctet *&pdata)

A request for the address of len octets of buffer data starting offset octets into the buffer. If the buffer implementation uses segmented memory the maximum number of contiguous octets exposed is the segment size, which may be less than the length requested. In this case the value of len will be reduced before the method returns. The caller must process the data provided and call expose again with an increased offset to retrieve or modify subsequent segments.

Two versions of the expose method are provided, one for use when modifying the buffer and one where the buffer (and the returned data pointer) is const.

• The const expose() method will only present data up to the current limit as reported by the size() method.
• The non-const expose() method will present data up to the current capacity of the buffer, so application code that is reading as well as modifying the buffer's contents must keep track of the current value of size() and stop reading when this is reached (or use the both methods, the const one for reading and the other for writing).
• The non-const version will throw an exception if called for a buffer that is immutable.

The design of the expose method is intended to increase the efficiency of data access. The caller must follow these conventions:

• Must call resize(n) if the data length is to be changed.
• Must not access storage outside the length returned by expose. The caller may have to make multiple expose calls to read or write a complete string.

EpicsBufferFactory

    typedef EpicsBuffer *(EpicsBufferAllocator)();
    
    class EpicsBufferFactory {
    public:
        static EpicsBuffer *allocate(const char *type);
        static epicsInt16 typeToTypeID(const char *type);
        static EpicsBuffer *allocate(epicsInt16 typeId);
        static void register(const char *type,
                         EpicsBufferAllocator allocator);
    };

This class provides a central registry of different EpicsBuffer implementations and is also responsible for creating an EpicsBuffer of any of these types on demand. Note that creating a buffer is not the same as allocating storage for the data that is to be stored in that buffer. The former is performed by EpicsBufferFactory, while the latter is the responsibilty of the EpicsBuffer implementation.

epicsString

epicsString.h defines a string class that uses an EpicsBuffer to hold its data.

    class EpicsString : public EpicsBuffer {
    public:
        EpicsString();
        EpicsString(const char *literal);
        EpicsString(const char *bufferType, epicsInt32 capacity);
        EpicsString(epicsInt16 bufferTypeId, epicsInt32 capacity);
        virtual ~EpicsString();
        EpicsString& operator=(const EpicsString &rhs);
        void useBuffer(const char *bufferType, epicsInt32 capacity = 0)
        void useBuffer(epicsInt16 bufferTypeId, epicsInt32 capacity = 0);
        epicsInt32 get(epicsInt32 offset, epicsInt32 len,
                       epicsOctet *pto) const;
        epicsInt32 put(epicsInt32 offset, epicsInt32 len,
                       const epicsOctet *pfrom);
        epicsInt32 hash(epicsInt16 nBitsHashIndex) const;
        
        // These routines are as described for EpicsBuffer
        void reserve(epicsInt32 capacity);
        epicsInt32 capacity() const;
        void resize(epicsInt32 newsize);
        epicsInt32 size() const;
        epicsInt32 maxSize() const;
        void destruct();
        bool mutable() const;
        bool isEqual(const EpicsBuffer &cmp) const;
        bool isEqual(epicsInt32 offset, epicsInt32 len,
                     const epicsOctet *pdata) const;
        bool expose(epicsInt32 offset, epicsInt32 &len,
                    epicsOctet *&pdata);
        bool expose(epicsInt32 offset, epicsInt32 &len,
                    const epicsOctet *&pdata) const;
    protected:
        EpicsBuffer *pbuffer;
    private:
        EpicsString(const EpicsString &str);   // No copy constructor
    };
    
    epicsBoolean operator==(const EpicsString &lhs, const EpicsString &rhs);
    epicsBoolean operator!=(const EpicsString &lhs, const EpicsString &rhs);

An EpicsString is a datatype represented by the epicsType enumeration value epicsTypeString. It holds a UTF-8 encoded string, i.e. multiple octets may be needed to store a single character. As long as it does not assume that each character is a single byte, most code can just ignore the fact that the string is UTF-8 encoded. If the local computer has been internationalized for the particular UTF-8 encoding then a UTF-8 string can be printed via the printf family of methods.

The EpicsString data is not guaranteed to be null-terminated and in most cases will not be, so applications using the string must take precautions when interfacing with routines that expect the string to end with a zero byte. We may provide a type of contiguous EpicsBuffer that does guarantee a null terminator.

EpicsString provides the following methods:

EpicsString()

If a string is default constructed, one of the useBuffer() methods must be called to set the underlying buffer type before any data can be stored.

EpicsString(const char *literal)

Use of this constructor causes the string to have a readonly buffer that holds just the literal string given, without copying. This is intended to be efficient as it is likely to be commonly used.

EpicsString(const char *bufferType, epicsInt32 capacity)

EpicsString(picsInt16 bufferTypeId, epicsInt32 capacity)

Here a buffer type is specified, either by name or by identifier, so the selected buffer type will be created using the EpicsBufferFactory, and if capacity is non-zero the space will be reserved for at least that number of octets of data.

~EpicsString()

The destructor will destruct() the EpicsBuffer if one has been allocated.

void useBuffer(const char *bufferType, epicsInt32 capacity)

void useBuffer(epicsInt16 bufferTypeId, epicsInt32 capacity)

These methods create an EpicsBuffer to hold the string data, and if capacity is non-zero the space will be reserved for at least that number of octets of data.

What happens if a buffer type has already been selected for the string? Throw an exception, or copy the old data to the new buffer and destroy the old buffer? Start with the exception, switch if we need to.

epicsInt32 get(epicsInt32 offset, epicsInt32 len, epicsOctet *pto)

Copies up to len octets starting at offset from the string buffer to pto, and returns the number of octets transfered. The return value will be less than len if offset+len > size().

epicsInt32 put(epicsInt32 offset, epicsInt32 len, const epicsOctet *pfrom)

Copies up to len octets from pfrom into the buffer starting offset octets from the beginning, and returns the number of octets copied. The return value will be less than len if offset+len > capacity(). The string size will be updated if this call extends the string beyond its original size. However on entry offset must not be greater than size() or an exception will be thrown.

epicsInt32 hash(epicsInt16 nBitsHashIndex) const

Calculates an n-bit hash of the octets stored in the string buffer.

EpicsString also implements all of the routines in the EpicsBuffer interface. If an EpicsBuffer has been allocated these calls are forwarded to the underlying EpicsBuffer method. If an EpicsBuffer has not been allocated however, all methods except for useBuffer() will throw an exception. The destruct() causes the underlying buffer storage to be completely released, after which one of the useBuffer() methods must be used before data can be stored in the string again.

---

epicsArray

epicsArray.h defines an array class that uses an EpicsBuffer to hold its data.

    class EpicsArray {
        EpicsArray(epicsInt32 elementSize = 0);
        EpicsArray(const char *bufferType, epicsInt32 capacity,
                   epicsInt32 elementSize = 0);
        EpicsArray(epicsInt16 bufferTypeId, epicsInt32 capacity,
                   epicsInt32 elementSize = 0);
        virtual ~EpicsArray();
        void setElementSize(epicsInt32 elementSize);
        epicsInt32 getElementSize() const;
        void useBuffer(const char *bufferType, epicsInt32 capacity = 0);
        void useBuffer(epicsInt16 bufferTypeId, epicsInt32 capacity = 0);
        epicsOctet * element(epicsInt32 index);
        const epicsOctet * element(epicsInt32 index) const;
        epicsInt32 get(epicsInt32 offset, epicsInt32 len,
                       epicsOctet *pto) const;
        epicsInt32 put(epicsInt32 offset, epicsInt32 len,
                       const epicsOctet *pfrom);
        
        // These are similar to EpicsBuffer, measured in elements not octets
        void reserve(epicsInt32 capacity);
        epicsInt32 capacity() const;
        void resize(epicsInt32 newsize);
        epicsInt32 size() const;
        epicsInt32 maxSize() const;
        void destruct();
        bool mutable() const;
        void expose(epicsInt32 offset, epicsInt32 &len,
                    epicsOctet *&pdata);
        void expose(epicsInt32 offset, epicsInt32 &len,
                    const epicsOctet *&pdata) const;
    protected:
        epicsInt32 elementSize;
        EpicsBuffer *pbuffer;
    private:
        EpicsArray(const EpicsArray &);            // No copy constructor
        EpicsArray& operator=(const EpicsArray &); // No assignment operator
    };

An EpicsArray is a datatype represented by the epicsType enumeration value epicsTypeArray. It holds an array of elements of constant size.

How do we cope with padding?

EpicsArray has the following methods:

EpicsArray(epicsInt32 elementSize = 0)

If an array is default constructed, one of the useBuffer() methods must be called to set the underlying buffer type before any storage can be reserved for the array elements. The array must also be told its element size before buffer space can be reserved.

EpicsArray(const char *bufferType, epicsInt32 capacity, epicsInt32 elementSize = 0)

EpicsArray(epicsInt16 bufferTypeId, epicsInt32 capacity, epicsInt32 elementSize = 0)

Here a buffer type is specified, either by name or by identifier, so the selected butter type will be created using the EpicsBufferFactory, and if both capacity and elementSize are non-zero the space will be reserved for at least that number of array elements.

~EpicsArray

The destructor will destruct() the EpicsBuffer if one has been allocated.

void setElementSize(epicsInt32 elementSize)

This sets the size (in octets) of the element type to be stored in the array. If a buffer has already been allocated for previous array data, this will be released and the buffer capacity set to zero.

epicsInt32 getElementSize()

This returns the element size currently stored.

void useBuffer

These methods create an EpicsBuffer to hold the array data, and if capacity and elementSize are non-zero the space will be reserved for at least that number of array elements.

What happens if a buffer type has already been selected for the array? Throw an exception, or destroy the old buffer and create the new one? Start with the exception, switch if we need to.

epicsOctet * element(epicsInt32 index)

Returns a pointer giving direct access to the index'th element of the array, allowing this element to be modified. The pointer returned will have to be cast to an appropriate element type before use. Access to either adjacent array element must not be made by adjusting the pointer since the underlying buffer storage may be segmented - instead call element() again with the appropriately modified index.

const epicsOctet * element(epicsInt32 index) const

Returns a pointer giving const (read-only) access to the index'th element of the array, allowing this element to be read but not modified. If the underlying buffer type is not mutable, this method will throw an exception. The pointer returned will have to be cast to an appropriate element type before use. Access to either adjacent array element must not be made by adjusting the pointer since the underlying buffer storage may be segmented - instead call element() again with the appropriately modified index.

epicsInt32 get(epicsInt32 offset, epicsInt32 len, epicsOctet *pto)

Copies up to len elements starting at offset from the array buffer to pto, and returns the number of elements transfered. The return value will be less than len if offset+len > size().

epicsInt32 put(epicsInt32 offset, epicsInt32 len, const epicsOctet *pfrom)

Copies up to len elements from pfrom into the buffer starting offset elements from the beginning, and returns the number of elements copied. The return value will be less than len if offset+len > capacity(). The array size will be updated if this call extends the array beyond its original size. However on entry offset must not be greater than size() or an exception will be thrown.

EpicsArray also implements all of the routines in the EpicsBuffer interface, translating all capacity, size and offset values into element counts before passing them to the underlying EpicsBuffer. If an EpicsBuffer has not been allocated however, all these methods will throw an exception. The destruct() causes the underlying buffer storage to be completely released, after which one of the useBuffer() methods must be used before data can be stored in the array again.

---

epicsMDArray

An EpicsMDArray is an instance of an epicsTypeMDArray It specifies the element type, has the address of the array description, type and has an associated buffer that holds the storage for the array.

epicsTypeMDArray, i.e. multidimensional array data, is a supported type, because collection and display of two and three dimensional images is a common requirement.

An EpicsMDArray can only hold primitive or string data. Thus it can be any of the types epicsTypeBoolean, ..., epicsTypeString but it can not be type epicsTypeUnknown, epicsTypeArray, epicsTypeStruct, or epicsTypeMDArray.

epicsMDArray.h contains the following:

    class EpicsMDArraySlice;

    class EpicsMDArray : public EpicsArrayBuffer {
        EpicsMDArray(epicsType type,epicsInt16 ndim);
        virtual ~EpicsMDArray();
        EpicsMDArray(epicsType type,epicsInt16 ndim,const char *bufferType);
        EpicsMDArray(epicsType type,epicsInt16 ndim,epicsInt16 bufferTypeId);
        EpicsMDArray(epicsType type,epicsInt16 ndim,
                     const char *bufferType,,epicsInt32 capacity);
        EpicsMDArray(epicsType type,epicsInt16 ndim,
                     epicsInt16 bufferTypeId,,epicsInt32 capacity);
        void allocateBuffer(const char *bufferType);
        void allocateBuffer(epicsInt16 bufferTypeId);
        void allocateBuffer(const char *bufferType,
                    epicsInt16 elementSize, epicsInt32 capacity)
        void allocateBuffer(epicsInt16 bufferTypeId,
                    epicsInt16 elementSize, epicsInt32 capacity)
        epicsType getType();
        epicsInt16 getElementSize();
    protected:
       epicsType type;
       epicsInt16 elementSize;
       EpicsMDArraySlice *pslice;
       EpicsMDArrayBuffer *pbuffer;
    private:
       EpicsMDArray(); // no default constructor
        EpicsMDArray(const EpicsMDArray &); // copy not allowed
        EpicsMDArray(const EpicsMDArray *); // copy not allowed
    };

    class EpicsMDArrayIndex {
    public
        epicsInt32 low;
        epicsInt32 high;
    };
    class EpicsMDArraySlice {
    public:
        epicsType   type; 
        epicsInt16  ndim;      // number of dimensions
        EpicsMDArrayIndex *paindex; // addr of array of EpicsMDArrayIndex
    };

EpicsMDArray

An EpicsMDArray is an instance of an epicsTypeMDArray. It can contain an array of any epicsType.

EpicsMDArray has the following methods:

EpicsMDArray

Several constructors are available. All require an epicsType and the number ofdimensions. If no buffer type is specified then, one of the allocate methods must be called. If a buffer type is specified then EpicsArrayBufferFactory:allocate is called to initialize pbuffer. If capacity is also specified EpicsArrayBuffer:allocate is called to allocate storage for data.

~EpicsMDArray

The descructor will release the EpicsArrayBuffer if it has been allocated.

void allocateBuffer

These methods allocate an EpicsArrayBuffer and if capacity is specified call EpicsArrayBuffer:allocate.

EpicsMDArray implements interface EpicsArrayBuffer. If an EpicsArrayBuffer has not been allocated all methods raise an exceptioon. If an EpicsArrayBuffer has been allocated it calls the associated EpicsArrayBuffer method.

EpicsArrayBuffer Notes

EpicsMDArray uses the same buffer interface as EpicsArray. Code that accesses a multidimensional array should be prepared to access both contiguous and segmented buffer implementations. Circular buffer implementations may not make sense.

EpicsMDArraySlice

EpicsMDArraySlice has the following fields:

type

The element type which must be one of bool,...,string

ndim

The number of dimensions

paindex

addr of array(ndim) of EpicsMDArrayIndex

EpicsMDArrayIndex has the fields:

low

high

The definitions describe a "slice" of a multi-dimensional array.

---

EpicsEnum

An EpicsEnum is a 16-bit index value, with an interface to convert between choice strings and their index values.

class EpicsEnumChoices {
public:
    virtual EpicsEnumChoices *duplicate() const = 0;
    virtual void release() = 0;
    virtual epicsInt16 nChoices() const = 0;
    virtual epicsInt16 index(const EpicsString &choice) const = 0;
    virtual void choice(epicsInt16 index, EpicsString &choice) const = 0;
};

class EpicsEnum {
public:
    enum {invalid = -1};
    
    EpicsEnum();
    EpicsEnum(const EpicsEnum &e);
    EpicsEnum(EpicsEnumChoices *choices);
    EpicsEnum(EpicsEnumChoices *choices, epicsInt16 index);
    virtual ~EpicsEnum();
    EpicsEnum& operator=(const EpicsEnum &rhs);
    
    virtual void choices(EpicsEnumChoices *pchoices);
    EpicsEnumChoices *choices() const { return pchoices; }
    epicsInt16 nChoices() const { return pchoices ? pchoices->nChoices() : 0; }
    epicsInt16 get() const { return index; }
    virtual void get(EpicsString &choice) const;
    virtual void put(epicsInt16 index);
    virtual void put(const EpicsString &choice);
protected:
    EpicsEnumChoices *pchoices;
    epicsInt16 index;
};

---

epicsStruct

epicsStruct.h contains the following:

    class EpicsStructDescription;
    class EpicsStructManager;

    class EpicsStruct{
        EpicsStruct(EpicsStructDescription &structDef);
        ~EpicsStruct();
    protected:
        EpicsStructDescription *pstructDef;
        void    *pstorage;
    private:
        EpicsStruct(); // no default cobstructor
    };

    struct EpicsStringDefaults { // defaults for string
        const char * bufferType;
        epicsInt32 capacity;
    };

    struct EpicsArrayDefaults { // defaults for array
        epicsType  type;
        const char * bufferType;
        epicsInt32 capacity;
        epicsInt32 nelements;
    };

    struct EpicsEnumDefaults { // defaults for enum
         ???
    };

    struct EpicsBitMaskDefaults { // defaults for enum
         ???
    };

    struct EpicsMDArrayDefaults { // defaults for mdarray
        epicsType  type;
        epicsInt16 ndim
        const char *bufferType;
        epicsInt32 capacity;
    };

    struct EpicsStructDefaults { // defaults for struct
        EpicsStructDescription *pdescription;
    };

    class EpicsStructField {
    public:
        EpicsString name;
        epicsType   type;
        union {
            EpicsStringDefaults  *pstring;
            EpicsArrayDefaults   *parray;
            EpicsMDArrayDefaults *pmdarray;
            EpicsEnumDefaults    *penum;
            EpicsBitmaskDefaults *pbitmask;
            EpicsStructDefaults  *pstruct;
        } defaults;
    };
   
    class EpicsStructDescription{
    public:
        EpicsString         name;
        EpicsStructManager *plifetime;
        epicsInt16          nfields;
        EpicsStructField    *pafield;//ptr to array of EpicsStructField
    };

    class EpicsStructManager {
    public:
        virtual void allocate(EpicsStruct &struct) = 0;
        virtual void destroy(EpicsStruct &struct) = 0;
        virtual void *exposeField(EpicsStruct &struct, epicsInt16 index) = 0;
    };

An epicsStruct is a container with fields each of which is any epicsType including unknown.

The following classes are involved with an epicsStruct:

• EpicsStruct - An instance of an epicsStruct
• EpicsStructDescription - Describes an epicsStruct
• EpicsStructField - Describes a field of an epicsStruct
• EpicsStructDefaults - Provides default values for non-primitive fields. This is intended for code that initializes fields.
• EpicsStructManager - An interface that manages storage for an epicsStruct.

It is expected that each of these classes are extended. In fact if any field of an epicsStruct has type epicsTypeUnknown then it is likely that most will be extended.

epicsTypes does not specify how to locate an EpicsStructManager, i.e. the mechanism is application dependent.

An EpicsStruct contains two fields:

pstructDef

Address of a EpicsStructDescription that describes the structure.

pstorage

Address of storage for the data contained in the structure.

EpicsStructDescription

EpicsStructDescription has the fields:

name

The structure name.

plifetime

Address of a EpicsStructManager interface. See below.

nfields

The number of fields in the structure.

pafield

addresss of an array of EpicsStructField, one for each field.

EpicsStructField

EpicsStructField has the fields:

name

The field name.

type

The field type, which can be any epicsType.

defaults

For each of the non-primitive epicsTypes, default values can be specified. This is for use by code that must initialize field instances.

EpicsStructManager

EpicsStructManager is an interface that has three methods:

allocate

This sets pstructDef to the definition for the associated structure and sets pstorage to the address of storage for the fields in the structure.

destroy

releases the storage for the structure and sets pstructDef null.

exposeField

This returns the address of the storage for the data associated with the field.