V4 DB Record Instance Syntax

This page still being written...

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

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

A sequence of characters within double-quotes, using the C99 escape sequence syntax defined below

escapeSequence

simpleEscapeSequence

octalEscapeSequence

hexadecimalEscapeSequence

universalCharacterName

simpleEscapeSequence

one of: \' \" \? \\ \a \b \f \n \r \t \v

octalEscapeSequence

\d

\dd

\ddd

where d is an octal digit.

hexadecimalEscapeSequence

\xhex-digits

universalCharacterName

\uhhhh

\Uhhhhhhhh

where h is a hex digit

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 extend that to include 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 expand that to add some other ASCII characters if desired. No spaces, brackets or braces though, for syntax reasons.

We can be generous in what we accept as a boolean value:

booleanConstant

booleanTrue

'"' booleanTrue '"'

booleanFalse

'"' booleanFalse '"'

booleanTrue

any of '1' 'T' 'TRUE' 'True' 't' 'true' 'Y' 'YES' 'Yes' 'y' 'yes'

booleanFalse

any 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