V4 DB Record Instance Syntax

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Revision as of 13:05, 22 September 2005 by MartyKraimer (talk | contribs)
Record Instance Syntax Sept 22 2005

Overview

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:
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

This section will eventually define what can appear in a .db file. That currently means:

  • record instances
  • comments
  • macro instances, including where they will be allowed
  • template files and substitution macro definitions
  • port definitions for template instances
  • data for tools such as VDCT, that will not be discarded by .db processing tools.

The templates, macros and ports design should be very similar to the ideas produced for R3.14 VDCT templates.

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";
      ...
  }


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:
choiceName ( structAssignmentList )
choiceName:
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);
      input = [2] {
          {link = {
              MonitorLink{
                  pvname = "foo:temperature:setpoint";
                  process = false
              }
          }
          {link = {
              InputLink{
                  pvname = "foo:temperature:sensor";
                  process = true;
                  inheritSeverity = true
              }
          }
      };
      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}};
  }