V4 DB Record Instance Syntax

This page still being worked on...

The syntax used for record instances has to change in EPICS V4, since we now have to support structured data. While it would have been possible to modify the V3 syntax to allow for this, a complete redesign of the syntax has been done to help improve parsing, and to provide commonality between the syntax of a DB file and the string representation of structured data values passed through Channel Access.

This syntax is presented below in the form of a grammar. The conventions I'm using are as follows:

symbolBeingDefined:

otherSymbol

alternateSymbol literal

General Symbols

These symbols are used but not defined in the grammar:

integerConstant:

floatingConstant:

Standard format numbers

identifier:

A legal C99 identifier. Note that C99 permits implementations to allow extended characters to be used in identifiers, but does not require it, so the use of extended characters may reduce portability and is not recommended.

stringConstant:

" escapedCharacterList "

escapedCharacterList:

A series of characters, using the C99 escapeSequence syntax defined below:

escapeSequence:

simpleEscapeSequence

octalEscapeSequence

hexadecimalEscapeSequence

universalCharacterName

simpleEscapeSequence:

one of: \ \" \? \\ \a \b \f \n \r \t \v

octalEscapeSequence:

\ octalDigit

\ octalDigit octalDigit

\ octalDigit octalDigit octalDigit

octalDigit:

one of: 0 1 2 3 4 5 6 7

hexadecimalEscapeSequence:

\x hexDigit

hexadecimalEscapeSequence hexDigit

hexDigit:

one of: 0 1 2 3 4 5 6 7 8 9 a b c d e f A B C D E F

universalCharacterName:

\u hexQuad

\U hexQuad hexQuad

hexQuad:

hexDigit hexDigit hexDigit hexDigit

The following symbols are like identifier, but may have a different set of legal characters TBD (but which cannot include space).

recordName:

Currently record names may contain A-Z a-z _ - ; : < > [ ]. We should probably allow non-ASCII Unicode/UTF-8 characters, but no new ASCII characters.

linkType:

deviceType:

I'd like these two to have to be valid C99 identifiers, but we could probably add some other ASCII characters if desired. No spaces, brackets or braces, for syntax reasons.

We could afford to be generous in what we accept as a boolean value:

booleanConstant:

booleanTrue

" booleanTrue "

booleanFalse

" booleanFalse "

booleanTrue:

one of: 1 T TRUE True t true Y YES Yes y yes

booleanFalse:

one of: 0 F FALSE False f false N NO No n no

Database File

I need to define these, which are really preprocessor objects:

• comments
• substitution macros
• templates and ports

Record Instances

recordDefinition:

recordType recordName = { recordBody }

recordType:

identifier

recordBody:

recordBodyItem

recordBody recordBodyItem

Record definitions will look very similar to a C99 structure definition with initialization in EPICS V4. For example:

ai foo:bar:temperature = {
    ...
}

Inside the body of the record definition, there are three possible kinds of statements, similar to a C assignment statement. Note these statements must all be terminated with a semi-colon (which is different from inside a struct).

recordBodyItem:

info infoName = stringConstant ;

fieldName = initializer ;

extraField = extraInitializer ;

infoName:

identifier

stringConstant

Info items provide additional configuration data about this record that can be accessed by other software running on the IOC.

info savePeriod = "30.0";
info restorePhase = "1";
info "my favourite things" = "raindrops on roses";

fieldName:

identifier

initializer:

constant

{ structAssignmentList }

{ arrayAssignmentList }

arrayType { arrayAssignmentList }

[ arrayCapacity ] { arrayAssignmentList }

arrayType [ arrayCapacity ] { arrayAssignmentList }

linkType { structAssignmentList }

deviceType { structAssignmentList }

constant:

booleanConstant

integerConstant

floatingConstant

stringConstant

The syntax for setting field values depends on the data type represented by fieldName. Basic types (numeric or string) should need no comment other than to note that numeric values should not be given inside quotes:

ai foo:bar:temperature = {
    inputSmoothing = 0.98;
    invalidValue = 1000;
    units = "Celcius";
    ...
}

structAssignmentList:

initializerList

fieldName = initializerList

structAssignmentList ; fieldName = initializerList

initializerList:

initializer

initializerList , initializer

Initializers for a structure field look similar to a nested record body, but the rules are slightly different:

• You can give a series of values for adjacent items using a simple comma-separated list (for a record body, you must name each field)
• Semi-colons are required between a value and a following named item.

For example:

ai foo:bar:temperature = {
    linearConvert = {
        mode = "Linear";
        low = -12.5, 133.5
    };
    displayLimit = { 0, 100 };
    ...
}

arrayAssignmentList:

initializerList

[ integerConstant ] = initializerList

arrayAssignmentList ; [ integerConstant ] = initializerList

arrayType:

bool

int16

uint16

int32

uint32

float32

float64

octet

string

arrayCapacity:

integerConstant

integerConstant , arrayCapacity