V4 DBD Statement Syntax

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DBD Statement Syntax November 02 2005

Overview

This document presents the syntax for V4 database definition files.

NOTE This version of the database definition and database instance syntax does not provide:

  1. a direction for links
  2. a way to select a subset of the support definitions

Support can exist for either a link field or a struct field. The support definition is just 

   support(choiceName,supportStructName)

When a record instance is created any choiceName can be specified for a link or struct field. If an invalid choice is made, an error will be issued during initialization.

General Statements

include

include "filename"

where

filename
must be a valid filename

The file system search path that will be used to look for the file is determined by the build system, and cannot be modified by the DBD file itself.

Include statements can be used at the top level, or inside the braces of a menu, struct or record definition.

# comment

# anything

Anything on a line after a # character is a comment, and will be ignored. Comments may appear on a line by themselves, or at the end of another statement. They may not appear inside the parentheses belonging to another statement, but they are permitted inside braces. Inside a single- or double- quoted string the # character has no special meaning.


menu

A menu is an enumerated type where the choice strings are defined once for each IOC. Menus are defined like this: 

menu(menuName) {
    choice(choiceName, "choiceValue")
    ...
}

where

menuName
Must be a valid, unique C identifier.
choiceName
Must be a valid, unique C identifier. By convention, every choiceName should start with the menuName it belongs to. These names are only available to C/C++ source code using the header file generated from the menu definition; they are not stored in the IOC itself.
choiceValue
Can be any UTF-8 compatible string, which should be unique within the context of this menu.

In general menus should only be definable once, in the DBD file (first definition seen wins in the event of duplicates, give a warning for duplicates and error if subsequent definition is different). However we may want to include a syntax that allows specific menu definitions to be extended at database load time.

Example: 

menu(menuScan) {
    choice(menuScanPassive, "Passive")
    choice(menuScanEvent, "Event")
    choice(menuScanInterrupt, "Interrupt")
    choice(menuScan10second, "10 second")
    choice(menuScan5second, "5 second")
    choice(menuScan2second, "2 second")
    choice(menuScan1second, "1 second")
    choice(menuScan_5second, ".5 second")
    choice(menuScan_2second, ".2 second")
    choice(menuScan_1second, ".1 second")
}

Structures, Record types, Fields and Views

Structures and record types have significant commonality in that they both define a data structure type containing fields. The main difference is that you can't create or populate an instance of a structure outside of a record; only records can appear at the very top level. Record types also define views of the record, which is not possible for a structure.

struct

A structure is defined as follows: 

struct(name) {
    field(fieldName, fieldType) {
        fieldAttribute(attributeValue)
        ...
    }
    ...
}

where

name
The structure name must be a valid, unique C identifier.
fieldName
Must be a valid C identifier, unique within the context of this particular structure.
fieldType
See fieldType below.
fieldAttribute and attributeValue
See fieldAttribute below.

record

A record type is defined as follows: 

include "dbCommon.dbd"
record(name) extends RecordCommon {
    field(fieldName, fieldType) {
        fieldAttribute(attributeValue)
        ...
    }
    ...
    view(viewName) {
        property(propName, fieldPath)
        ...
    }
    ...
}
name
The record type name. It must be a valid, unique C identifier.
extends RecordCommon
This states that the record type extends the set of fields defined in "V4 DB RecordCommon". It should be permissable to name other record types instead of RecordCommon here, as long as the inheritance tree starts at RecordCommon. Inheritance from multiple record types is not supported; you can only have one extends phrase.
fieldName
Must be a valid C identifier, unique within the context of this particular record type and its parents (extends ...).
fieldType
See fieldType below.
fieldAttribute and attributeValue
See fieldAttribute below.
viewName, propName and fieldPath
See Views of a record below.

fieldType

Both struct and record define a field as: 

    field(fieldName, fieldType) {
        fieldAttribute(attributeValue)
        ...
    }

The syntax for fieldType depends of the field type. For the more complex types the field definition needs additional information to be provided, which is given inside parentheses following the type name.

Basic types: octet, boolean, numerics, and string

The following field types have no arguments: octet, bool, int16, int32, int64, float32, float64, and string.

Examples: 

    field(description,string)
    field(value,float64)

enum

An enum field takes one argument after the type name: 

    enum(fieldName)

where fieldName is the name of another field in the same record that must be of type array(string[]). The values in this other field at runtime define the available choice strings for the enum field.

Examples: 

    field(choices, array(string[])
    field(value, enum(choices))

menu

A menu field is defined like this: 

    menu(menuName)

where menuName is the name of the menu.

Example: 

    field(scan, menu(menuScan))

struct

A struct field is declared using the type name 

    struct(structName)

where structName is the name of a struct which must have been previously defined.


Example: 

     struct(Point) {
         field(x,float64)
         field(y,float64)
         field(z,float64)
      }
      ...
      record(haspoint) extends RecordCommon {
          ...
          field(point, struct(point))
      }

array

An array field uses definitions like these:

1-dimensional array
array(arrayType[capacity])
multi-dimensional array
array(arrayType[capacityX, capacityY, ...])
arbitrarily dimensioned array
array(arrayType)
1-dimensional array of unknown size
array(arrayType[])
3-dimensional array of unknown size
array(arrayType[,,])
1-dimensional array of unknown type
array([capacity])
multi-dimensional array of unknown type
array([capacityX, capacityY, ...])
arbitrarily dimensioned array of unknown type
array()
1-dimensional array of unknown type or size
array([])
2-dimensional array of unknown type or size
array([,])
  • arrayType may be any fieldType except array, or may be omitted completely in which case the data type stored is determined by record instance. If the record instance defines the type, it can only be one of the types listed above under Basic types
  • capacity is the array's size in the relevent dimension, and must be specified for all dimensions or for none. If not specified by the record type, the record instance determines an array's dimensionality and size.

Examples: 

    field(VAL1D,array(float64[]))  #1d array with arbitrary capacity
    field(VAL2D,array(float64[,])) #2d array with arbitrary capacities
    field(anyTypeAnyD,array())     #arbitrary type,number of dimensions, and capacities

link

This field type can get or put data from/to some source outside of the record. Link fields replace the DBF_INLINK, DBF_OUTLINK, and DTYP fields from EPICS V3.

A link field's choices come from link definitions - see the #link section below for more details. A link can be a link to another database record, to hardware device support, or something else.

The syntax is: 

    link

Examples: 

    field(disableLink,link)
    field(process, link)
    field(inp,link)

fieldAttribute

Each field definition has several associated attributes, the values of which are set like this: 

    default("fieldValue")
    readonly(yesNo)
    design(yesNo)
    special(yesNo)
    asl(securityLevel)
    link(yesNo)

Marty thinks we should get rid of these two: 

    prompt("promptString")
    group("promptGroup")

I am thinking about combining readonly and design into a single attribute called access, which takes one of four choices: design (the default), runtime, readonly, or none.

The attribute parameter values have the following meanings:

default("fieldValue")
Default value for an instance of this field, using the record instance value syntax. If a default is not specified, the field will initialize to all zero bits.
If the field is itself a structure, the default value for the instance of the whole structure can override default values declared for individual fields inside that structure. This can occur at multiple levels.
readonly(yesNo)
Can this field be modified via channel access or database links? Takes the value No if not specified.
design(yesNo)
Should a Database Configuration Tool allow the field to be configured at design time? If No, values for the field cannot be set when loading record instance data at startup. Takes the value Yes if not specified.
special(yesNo)
Does the record have to take special action if the field is modified? If this is Yes, the record types special processing will be invoked to actually change the field value, which will allow it to perform value checks or additional processing. Takes the value No if not specified.
asl(securityLevel)
Channel Access security level for this field, 0 or 1. Takes the value 1 if not specified.
link
This is only valid for string fields. It signifies the the field is the name of an external record. This is for use by Database Configuration Tools.

These attributes may disappear, see comment above:

prompt("promptString")
A description of this field for the database designer, this string will be displayed by a Database Configuration Tool. Empty if not specified. Not used within the IOC.
group("promptGroup")
A name that can be used by a Database Configuration Tool to group similar or related fields together. Empty if not specified. Not used within the IOC.

view

There needs to be more than one way to look at a record remotely (via Channel Access or some other similar network protocol). Often we just want to get the contents of the value field and some metadata associated with that value, but there are often several fields which can share metadata - engineering units for example. We can't do metadata using structures because that would mean replicating this metadata, so we add a level of indirection to allow us to group fields together.

A view of a record provides a hierarchical mapping of some of the record's fields from a named Data Access property catalog that can be reached using Channel Access. Records automatically get a view named "field" that provides direct access to the individual public fields of the record, with no metadata. Beyond that, record types can declare additional hierarchical views and define the fields that appear in them inside the DBD file. The first view defined for a record type is used as its default view (if no views are defined, the field view will become the default view; view parameters may not be permitted).

A simple view looks like this: 

view(viewName) {
    property(propName) {
        property(propName, fieldPath)
        ...
    }
    property(propName, fieldPath)
    ...
}
viewName
The view name must be a valid C identifier, which must be unique in the context of the particular record type.
propName
A Data Access property name, which must be a valid C identifier.
fieldPath
The path to a field in this record type. To use a field inside a structure field, give the full path to that field: controlLimit.upper for example.
If fieldPath resolves to a structure, a property catalog containing the whole structure will be sent, with property names matching the structure's field names.
The fieldPath may be omitted as long as there is a subordinate property catalog below this property.

Example: 

record(ao) extends RecordCommon {
    field(value, float64) { ... }
    field(outValue, float64) { ... }
    field(rawValue, int32) { ... }
    field(units, string) { ... }
    field(displayLimit, struct(displayLimit)) { ... }
    ...
    view(value)
        property(value, value) {
            property(units, units)
            property(timeStamp, time)
            property(alarmSeverity, alarmSeverity)
            property(alarmStatus, alarmStatus)
            property(displayLimit, displayLimit)
        }
    }
    view(outValue)
        property(value, outValue) {
            property(units, units)
            property(timeStamp, time)
            property(alarmSeverity, alarmSeverity)
            property(alarmStatus, alarmStatus)
        }
    }
    view(rawValue)
        property(value, rawValue) {
            property(timeStamp, time)
        }
    }
    ...
}

support

A support statement describes an implementation of support for a link or struct field. Link support can be any of the following:

  • A link to another record either local or remote
  • A link to hardware support
  • Something else.

The syntax for these is: 

support(choiceName)

or 

support(choiceName,supportStructName)

where

choiceName
UTF-8 string that describes the choice
supportStructName
The name of a struct containing configuration information for the support. Record support normally does not access this structure. Database configuration tools do prompt the user to assign values to the structure.

When a record instance is created the choiceName selects the support to attach to a record link.

Examples: 

    link(processLink,ProcessLink)
    link(monitorLink,MonitorLink)
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