V4 DB Record Instance Syntax
- The information on this page should now be reasonably stable, although it's not yet complete (the definition of extraFieldAssignment given towards the end still needs fleshing out, but I don't see that as a high priority right now and it doesn't affect anything else defined here).
- Andrew Johnson, 2005-05-25
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?
