V4 Design: epicsTypes

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EPICS: epicsTypes

June 1 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:

  • epicsUnknownT - Type is unknown
  • epicsBooleanT, ..., epicsFloat64T - Primitive types, i.e. C++ fundamental types
  • epicsStringT - EpicsString which contains a UTF-8 Encoded Character String
  • epicsArrayT - EpicsArray which describes type and storage for a one dim array. The element type can be any epicsType.
  • epicsStructT - EpicsStruct which describes and provides storage for a set of fields each of some epicsType
  • epicsMDArrayT - EpicsMDArray which describes type and provides storage for a multidimensional array. The element type can be a primitive type or epicsStringT.


The actual types associated with the epicsTypes are: 

    typedef bool               epicsBoolean;
    typedef char               epicsOctet;
    typedef short              epicsInt16;
    typedef unsigned short     epicsUInt16;
    typedef int                epicsInt32;
    typedef unsigned int       epicsUInt32;
    typedef long long          epicsInt64;
    typedef unsigned long long epicsUInt64;
    typedef float              epicsFloat32;
    typedef double             epicsFloat64;

    /*EpicsString holds UTF-8 characters*/
    class EpicsString  {
    public:
        EpicsUTF_8Buffer    *pbuffer;
    };
    class EpicsArray {
    public:
        epicsType        type;
        EpicsArrayBuffer *pbuffer;
    };
    class EpicsStruct{
    public:
        EpicsStructDef *pstructDef;
        void    *pstorage;
    };
    class EpicsMDArray {
    public:
       epicsType type;
       EpicsMDArrayDescription *pdescription;
       EpicsMDArrayBuffer *pbuffer;
    };

The types are :

  • epicsBoolean, ... epicsFloat64 Primitive types. Each has an associated C++ fundamental type.
  • epicsOctet Eight bit byte. It is not an integer type.
  • EpicsString UTF-8 encoded character string. Storage is managed via an EpicsUTF_8Buffer interface.
  • EpicsArray Array of any epicsType. Storage is managed via an EpicsArrayBuffer interface.
  • EpicsStruct Container of fields each of which is any epicsType.
  • EpicsMDArray Multi-dimensional array of a primitive or string type.

epicsTypes

epicsTypes.h contains the following: 

    /* The following may require OSD definitions*/
    typedef bool               epicsBoolean;
    typedef char               epicsOctet;
    typedef short              epicsInt16;
    typedef unsigned short     epicsUInt16;
    typedef int                epicsInt32;
    typedef unsigned int       epicsUInt32;
    typedef long long          epicsInt64;
    typedef unsigned long long epicsUInt64;
    typedef float              epicsFloat32;
    typedef double             epicsFloat64;
   
    enum epicsType {
        epicsUnknownT,
        epicsBooleanT,     // epicsBoolean
        epicsOctetT,       // epicsOctet
        epicsInt16T,       // epicsInt16
        epicsUInt16T,      // epicsUInt16
        epicsInt32T,       // epicsInt32
        epicsUInt32T,      // epicsUInt32
        epicsInt64T,       // epicsInt64
        epicsUInt64T,      // epicsUInt64
        epicsFloat32T,     // epicsFloat32
        epicsFloat64T,     // epicsFloat64
        epicsStringT,      // EpicsString
        epicsArrayT,       // EpicsArray
        epicsStructT,      // EpicsStruct
        epicsMDArrayT      // EpicsMDArray
    };


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 epicsBooleanT, ..., epicsFloat64T 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.

The unsigned integer types should only be used for data that represents masks. If they are used for representing large numbers, e.g. an epicsUInt16 that is greater than 32767, then Java clients/servers will not work properly.

Java Notes

  • epicsOctet is a Java byte
  • Java does not support unsigned integers. It should, however, accept each unsigned type as the corresponding signed type.

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

The types epicsStringT, epicsArrayT, epicsStructT, and epicsMDArrayT 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. epicsStringT, epicsArrayT, epicsStructT, and epicsMDArrayT, 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.

epicsString

epicsString.h contains the following: 

    class EpicsUTF_8Buffer;
    
    /*EpicsString holds UTF-8 characters*/
    class EpicsString  {
    public:
        EpicsUTF_8Buffer *pbuffer;
    };

    class EpicsUTF_8Buffer {
    public:
        virtual epicsInt32 allocate(epicsInt32 capacity) = 0;
        virtual void release(bool onlyStorage) = 0;
        virtual epicsInt32 capacity() = 0;
        virtual epicsInt32 noctets() = 0;
        virtual void noctets(epicsInt32 n) = 0;
        virtual bool mutable() = 0;
        virtual void mutable(bool trueFalse) = 0;
        virtual epicsInt32 get(epicsOctet *pto,
                               epicsInt32 offset, epicsInt32 noctets) = 0;
        virtual epicsInt32 put(const epicsOctet *pfrom,
                               epicsInt32 offset, epicsInt32 noctets) = 0;
        virtual bool isEqual(const EpicsUTF_8Buffer *pbuffer) = 0;
        virtual bool isEqual(const epicsOctet *pstring, epicsInt32 noctets) = 0;
        virtual void expose(epicsInt32 offset, epicsInt32 noctetsRequest,
                            epicsOctet *pdata, epicsInt32 *noctets);
        virtual epicsUint32 hash(epicsInt16 nBitsHashIndex) = 0;
    };



    typedef EpicsUTF_8Buffer *(EpicsUTF_8BufferAllocate)();
    class EpicsUTF_8BufferFactory {
    public:
        static epicsUint16 typeToTypeID(const char *type);
        static EpicsUTF_8Buffer *allocate(epicsUint16 typeId);
        static void register(const char *type,
                         EpicsUTF_8BufferAllocate allocater);
    };


An EpicsString is an instance of an epicsStringT. It contains a UTF-8 encoded string, i.e. multiple bytes 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 the string can be printed via the printf family of methods.

EpicsString has the following fields:

pbuffer
The address of a EpicsUTF_8Buffer, which is a class that manages the string storage.


EpicsUTF_8Buffer

A string buffer is a container for a UTF_8 encoded character string.

Multiple string buffer implementations are available. At least the following are available:

  • contiguous - The string is stored in a contiguous set of bytes.
  • segmented - The string is stored in chunks. This form should be used for strings that are constantly modified.

Additional implementations can be provided. For example special implementations may be provided for managing network buffers.

A string buffer can only be accessed via interface EpicsUTF_8Buffer. In addition to holding storage for a string, a string buffer keeps the following information.

capacity
The number of octets allocated. Note that this is NOT the number of characters since a UTF-8 character may require several bytes.
noctets
The current size, i.e. the number of octets currently stored in the buffer.
mutable
Can the string be modified? The default is true.

Implementation Note Code must always set noctets so that it is at the end of a complete character.

EpicsUTF_8Buffer has the following methods:

allocate
Allocate space for up to capacity octets. The number of octets allocated is returned. An implementation attempts to allocate the requested capacity but some implementations, e.g. network buffers, may impose a maximum size. If capacity is not zero when this is called and new storage is allocated then the old storage is freed or reused and the octets spanned by position, limit appear in the newly allocated storage.
release
Storage for the string is released. If onlyStorage is true then storage for the buffer is also released.
capacity
returns the capacity
noctets
Two methods are available, one to get the current noctets and one to set the noctets.
mutable
Can the buffer be modified, i.e. are puts allowed. Two methods are available, one to determine the current state and one to set the state.
get
copies characters to pto and returns the number of octets transfered. Complete

characters are always copied.

put
copies characters from pfrom, puts them into the buffer, and returns the number of octets transfered. The internal noctets value is modified.
isEqual(EpicsUTF_8Buffer *)
Compares the string stored in the buffer with a string stored in a different buffer. This is normally called by code that uses an EpicsUTF_8Buffer.
isEqual(epicsOctet *pstring, epicsInt32 len)
Compares the string stored in the buffer with a string supplied by the caller. This is normally called by EpicsUTF_8Buffer itself to compare it's string with the string stored in another buffer.
expose
A request to return the address of actual bytes of storage. Since a buffer implementation may used segmented memory the number of bytes exposed may be less than the amount requested.
hash
implement a hash on the octets stored in the buffer.

expose Notes

expose is provided for efficiency. The caller must follow some conventions:

  • Must not modify the data if mutable is false.
  • Must call pbuffer->noctets(n) if the noctets is modified.
  • Must never access storage outside the noctets returned by expose. The caller may have to make multiple expose calls to read or write a complete string.
  • Must never set noctets so that it specifies a partial character.


EpicsUTF_8BufferFactory

This is a class for allocating an EpicsUTF_8Buffer and also for registering EpicsUTF_8Buffer implementations. Note that allocating a buffer is different than allocating storage for the data that is stored in the buffer.

epicsArray

epicsArray.h contains the following: 

    class EpicsArrayBuffer;

    class EpicsArray {
    public:
        epicsType        type; 
        EpicsArrayBuffer *pbuffer;
    };

    class EpicsArrayBuffer {
    public:
        virtual epicsInt32 allocate(
                    epicsInt32 capacity,epicsUint16 elementSize) = 0;
        virtual void release(bool onlyStorage) = 0;
        virtual epicsInt32 capacity() = 0;
        virtual epicsInt32 elementSize() = 0;
        virtual epicsInt32 length() = 0;
        virtual void length(epicsInt32 newLength) = 0;
        virtual bool mutable() = 0;
        virtual void mutable(bool trueFalse) = 0;
        virtual epicsInt32 get(void *pto,
                               epicsInt32 offset, epicsInt32 nelements) = 0;
        virtual epicsInt32 put(const void *pfrom,
                               epicsInt32 offset, epicsInt32 nelements) = 0;
        virtual void expose(epicsInt32 offset, epicsInt32 nelementsRequest,
                                   void *pdata, epicsInt32 *nelements);
    }



    typedef EpicsArrayBuffer *(EpicsArrayBufferAllocate)();
    class EpicsArrayBufferFactory {
    public:
        static epicsUint16 typeToTypeID(const char *type);
        static EpicsArrayBuffer *allocate(epicsUint16 typeId);
        static void register(const char *type,
                         EpicsArrayBufferAllocate allocater);
    };


An EpicsArray can contain data of any epicsType. It has the following fields:

type
Any epicsType.
pbuffer
The address of an EpicsArrayBuffer that provides access to the array.

EpicsArrayBuffer

Multiple array buffer implementations are available. An implementation can provide the following semantics:

  • contiguous - The array is stored in a contiguous set of bytes.
  • segmented - The array is stored in chunks. This form should be used for strings that are constantly modified.
  • circular - The storage is contiguous but the data is stored as a circular buffer.

Since the fields can be of type epicsArrayT arrays of arrays are supported. This is analogous to multidimensional arrays in Java.

An array buffer can only be accessed via interface EpicsArrayBuffer. In addition to holding storage for the array an array buffer keeps the following information.

capacity
The number of elements for which storage is allocated.
elementSize
the number of bytes for each element
length
The current number of elements.
mutable
Can the data be modified? The default is true.


EpicsArrayBuffer has the following methods:

allocate
This allocates space for up to capacity elements. The number of elements allocated is returned. An implementation attempts to allocate the requested capacity but some implemenations, e.g. network buffers, may impose a maximum size. If capacity is not zero when this is called and new storage is allocated then the old storage is freed or reused and the elements spanned by position, limit appear in the newly allocated storage.
release
Storage for the array is released. If onlyStorage is true then the storage for the buffer itself is also released.
capacity
return the capacity
elementSize
return the elementSize.
length
Two methods are available, one to get the current length and one to set the length.
mutable
Can the array be modified? The default is true.
get
copies elements to pto and returns the number of elements transfered.
put
copies characters from pfrom, puts them into the buffer, and returns the number of elements transfered. The internal length is modified.
expose
A request to return the address of actual storage. Since a buffer implementation may used segmented memory the amount of storage exposed may be less than the amount requested.

expose Notes

expose is provided for efficiency. The caller must follow some conventions:

  • Must not modify the data if mutable is false.
  • Must call pbuffer->length(newLength) if the length is modified.
  • Must never access storage outside the limit returned by expose. The caller may have to make multiple expose calls to read or write a complete string.
  • If the buffer is a circular buffer, an exception may be thrown if expose is called.


EpicsArrayBufferFactory

This is a class for allocating an EpicsArrayBuffer and also for registering EpicsArrayBuffer implementations. Note that allocating a buffer is different than allocating storage for the data that is stored in the buffer.

epicsStruct

epicsStruct.h contains the following: 

    class EpicsStructDef;
    class EpicsStructLifetime;

    class EpicsStruct{
    public:
        EpicsStructDef *pstructDef;
        void           *pstorage;
    };

    class EpicsStructField {
    public:
        EpicsString name;
        epicsType   type;
    };
   
    class EpicsStructDef{
    public:
        EpicsString         name;
        EpicsStructLifetime *plifetime;
        epicsInt16          nfields;
        EpicsStructField    *pfield[];//ptr to array of ptr to EpicsStructField
    };

    class EpicsStructLifetime {
    public:
        virtual void allocate(EpicsStruct *pstruct) = 0;
        virtual void destroy(EpicsStruct *pstruct) = 0;
        virtual void *exposeField(EpicsStruct *pstruct, 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
  • EpicsStructDef - Describes an epicsStruct
  • EpicsStructField - Describes a field of an epicsStruct
  • EpicsStructLifetime - 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 epicsUnknownT then it is likely that most will be extended.

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

An EpicsStruct contains two fields:

pstructDef
Address of a EpicsStructDef that describes the structure.
pstorage
Address of storage for the data contained in the structure.

EpicsStructDef

EpicsStructDef has the fields:

name
The structure name.
plifetime
Address of a EpicsStructLifetime interface. See below.
nfields
The number of fields in the structure.
pfield
addresss of an array of pointers to EpicsStructField, one for each field.

EpicsStructField

EpicsStructField has the fields:

name
The field name.
type
The field type, which can be any epicsType.

EpicsStructLifetime

EpicsStructLifetime 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.

epicsMDArray

An EpicsMDArray is an instance of an epicsMDArrayT 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.

epicsMDArrayT, 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 epicsBooleanT, ..., epicsStringT but it can not be type epicsUnknownT, epicsArrayT, epicsStructT, or epicsMDArrayT.


epicsMDArray.h contains the following: 

    class EpicsMDArrayDescription;

    class EpicsMDArray {
    public:
       EpicsMDArrayDescription *pdescription;
       EpicsArrayBuffer        *pbuffer;
    };

    class EpicsMDArrayBounds {
    public
        epicsInt32 low;
        epicsInt32 high;
    };
    class EpicsMDArrayDescription {
    public:
        epicsType   type; 
        epicsInt16  ndim;      // number of dimensions
        EpicsMDArrayBounds bounds[]; // bounds[ndim]
    };

EpicsMDArray

EpicsMDArray has the following fields:

pdescription
The address of an EpicsMDArrayDescription that describes the array
pbuffer
The address of an EpicsArrayBuffer that provides storage for the array data.

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.

EpicsMDArrayDescription

EpicsMDArrayDescription has the following fields:

type
The element type which must be one of bool,...,string
ndim
The number of dimensions
bounds
An array(ndim) of EpicsMDArrayBounds

EpicsMDArrayBounds has the fields:

low
high

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

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