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.

Document Conventions

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

symbolBeingDefined:

otherSymbol

alternateSymbolFollowedBy literal

one of: list of posible literal values

Common Symbols

The symbols described in this section are used in the grammar, but may be implemented as lexical tokens.

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.

Integer Constants

integerConstant:

positiveInteger

- positiveInteger

positiveInteger:

octalConstant

hexConstant

decimalConstant

octalConstant:

0

octalConstant octalDigit

octalDigit:

one of: 0-7

hexConstant:

0x hexDigit

0X hexDigit

hexConstant hexDigit

hexDigit:

one of: 0-9 a-f A-F

decimalConstant:

one of: 1-9

decimalConstant decimalDigit

decimalDigit:

one of: 0-9

This was meant to be a description of the C99 standard integer representation, but I made it up myself so it may be flawed. Note that we will not accept the C99 numeric suffixes u/U and l/L since (unlike a C compiler) we know the type of the number we're expecting.

Floating Point Constants

realConstant:

positiveReal

- positiveReal

positiveReal:

digitSequence

digitSequence .

digitSequence . exponentPart

digitSequence . digitSequence

digitSequence . digitSequence exponentPart

. digitSequence

. digitSequence exponentPart

digitSequence exponentPart

digitSequence:

decimalDigit

digitSequence decimalDigit

exponentPart:

e signedExponent

E signedExponent

signedExponent:

- digitSequence

+ digitSequence

digitSequence

In ANSI C source code, a sequence of decimal digits with neither a decimal point nor an exponent is an integer constant, not a floating-point constant. We will permit this however, since we always know the field type in advance.

Boolean Constants

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

booleanConstant:

booleanTrue

" booleanTrue "

booleanFalse

" booleanFalse "

booleanTrue:

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

booleanFalse:

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

I'm proposing all these possibilities for true/false as they are all obvious in meaning, and will allow a CA Put of any of these strings to a boolean field. We might even want to allow registration of boolean strings in other languages...

String Constants

stringConstant:

" escapedCharacterList "

escapedCharacterList:

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

escapeSequence:

simpleEscapeSequence

octalEscapeSequence

hexEscapeSequence

universalCharacterName

simpleEscapeSequence:

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

octalEscapeSequence:

\ octalDigit

\ octalDigit octalDigit

\ octalDigit octalDigit octalDigit

hexEscapeSequence:

\x hexDigit

hexEscapeSequence hexDigit

Note: C99 does not limit the number of hexadecimal digits that can appear in a hexEscapeSequence, but it does state that the behaviour is undefined if the resulting character value exceeds that of the largest character.

universalCharacterName:

\u hexQuad

\U hexQuad hexQuad

hexQuad:

hexDigit hexDigit hexDigit hexDigit

Database File

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

• comments
• substitution macros
• templates and ports

Record Definitions

recordDefinition:

recordType recordName = { recordBody }

recordType:

identifier

recordName:

recordNameChar

recordName recordNameChar

recordNameChar:

one of: 0-9 A-Z a-z _ - : ; < > [ ]

Any Unicode/UTF-8 character outside of the Basic Latin set

This extends the character set available to a V3 record name, adding all possible multi-byte characters. However, EPICS sites are strongly advised to confirm that such record names can be processed using all of their database and CA client tools before actually making use of this particular extension.

recordBody:

recordBodyItem

recordBody recordBodyItem

Record instance definitions in EPICS V4 look very similar to a C99 structure definition with initialization. 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 be terminated with a semi-colon (which is different from inside a struct). The reason for this difference is to prevent database instance files from becoming dependent on the order of fields in a record; if we permit record instances to be created from a single comma-separated list of field values without the field names, it could lead to significant confusion if the field order ever changes.

recordBodyItem:

infoAssignment

fieldAssignment

extraFieldAssigment

Information Fields

infoAssignment:

info infoName = stringConstant ;

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";

Field Assignment

fieldAssignment:

fieldName = initializer ;

fieldName:

identifier

initializer:

constant

structInitializer

arrayInitializer

devlinkInitializer

initializerList:

initializer

initializerList , initializer

The initializer in a field assignment is also the exact same syntax that will be used when converting a string value from a CA client for example into a field value that is being put into a field.

Basic and Enumerated Initializers

constant:

booleanConstant

integerConstant

realConstant

stringConstant

The syntax for the field initializer depends on the data type represented by fieldName. Basic types (numeric or string) should need no comment other than to note that values for numeric fields must not be given inside quotes (unlike EPICS V3). Menu field values may be given as either a string or an integer. For enum fields, if the related field that contains the strings is defined first, the enum field may be specified using a string; otherwise it can only be set using an integer value.

Examples:

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

Q: What about map fields? For record initialization we could parse these like an array of structures, where the first structure element is the key and the second is the value. However being able to access and modify elements of a map over CA may be harder, and needs some thought.

Structure Initializers

structInitializer:

{ structAssignmentList }

structAssignmentList:

initializerList

fieldName = initializerList

structAssignmentList ; fieldName = initializerList

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

Link and Device Initializers

devlinkInitializer:

linkType ( structAssignmentList )

deviceType ( structAssignmentList )

linkType:

identifier

deviceType:

identifier

These select a particular link or device support for the field, and set its address according to the structure type defined for that link or device type.

  calcout foo:temperature:controller = {
      output = ca("fum:baz:heater"; pp=y);
      inputs = [2] {
          { "setpoint", pv("foo:temperature:setpoint") }
          { "current", db("foo:temperature:sensor"; ms=y) }
      };
      expression = "(setpoint - current) > 0";
      ...
  }

  mbbi foo:bar:door = {
      input = acro9440(0, 5);
      ...
  }

Array Initializers

arrayInitializer:

{ arrayAssignmentList }

arrayType { arrayAssignmentList }

[ arrayCapacity ] { arrayAssignmentList }

arrayType [ arrayCapacity ] { arrayAssignmentList }

arrayAssignmentList:

initializerList

[ integerConstant ] = initializerList

arrayAssignmentList ; [ integerConstant ] = initializerList

arrayType:

one of: bool int16 uint16 int32 uint32

one of: float32 float64 octet string

arrayCapacity:

integerConstant

arrayCapacity , integerConstant

If the definition of the array field being set did not do so, an array field initialization must include the size of the array and/or the type of the data stored in it. Inside the braces data values are given in a comma-separated list; the index can also be set to initialize individual values, and any mixture of the two can be used as desired:

  mbbi foo:bar:door = {
      stateNames = [4] {"Broken", "Closed", "Open", "Moving"};
      stateSeverity = [4] {"Major"; [3] = "Minor"};
      ...
  }

For multi-dimensional arrays, data values can only appear inside the inner-most sets of braces, although index settings are permitted outside of these. These two definitions give the same result:

  matrix identity1 = {
      value = float32 [3,3] { {1, 0, 0}, {0, 1, 0}, {1, 0, 0}};
  }
  matrix identity2 = {
      value = float32 [3,3] { {1}, {[1] = 1}, {[2] = 1}};
  }

Extra Fields

extraFieldAssigment:

new extraField = extraInitializer ;

This syntax not fully developed yet...

Information needed includes:

• fieldName
• extension interface name
• configuration data

Things to think about here:

• Make sure the syntax doesn't make a misspelled fieldName in a fieldAssignment look like an extraFieldAssignment and thereby cause a misleading error message to be printed.
• Is any part of the new field writable through CA?
• If so, how much if it can be changed? Presumably just the configuration data.
• Can I delete an extraField, or change it to a different type of extension?