UBFA(6)UBFA(6)NAMEubfa - universal binary format for data transport
DESCRIPTIONUBF(A) is the data transport encoding for Armstrong's Universal Binary
Format. It provides four primitive types: atoms (symbolic constants),
integers, strings, and binary data. There are two compound types:
fixed-length tuples and variable-length lists. Ubfa(2) provides basic
support in Limbo for reading and writing streams of UBF(A)-encoded
data.
The input syntax is defined by the following rules:
input ::= item* '$'
item ::= integer | atom | string | binary | tuple | list | store | push | comment | tag
integer ::= '-'?[0-9]+
atom ::= "'" ([^\'] | '\\' | "\'")* "'"
string ::= '"' ([^\"] | '\\' | '\"')* '"'
binary ::= '~' byte* '~' # preceded by integer byte count
tuple ::= '{' item* '}'
list ::= '#' (item '&')*
store ::= '>' reg
push ::= reg
reg ::= [^-%"~'`{}#& \n\r\t,0-9]
comment ::= '%' ([^\%] | '\\' | '\%')* '%'
tag ::= '`' ([^\`] | '\\' | '\`')* '`'
White space is any sequence of blank, tab, newline or carriage-return
characters, and can appear before or after any instance of item in the
grammar.
The input data is interpreted by a simple virtual machine. The machine
contains a stack of values of primitive and compound types, and a set
of registers also containing values of those types. White space and
comments are ignored. Primitive integer, atom and string values are
pushed onto the stack as they are recognised. Certain input bytes out‐
side any value act as operators:
{ Note the current stack depth.
} Pop stack values to restore the most recently noted stack depth.
Push a single value representing a tuple of those items; the
left-most value in the tuple is the last one popped (the first
in the original input stream).
~ Pop an integer value n from the stack. Read n bytes from the
input stream and push a value onto the stack that represents
them. The next byte must be the character ~, which is dis‐
carded.
# Push a value representing an empty list onto the stack.
& Pop a value v. Pop another value l, which must represent a
list. Push a value that represents the list v::l. (Note that
the items in a list therefore appear in reverse order in the
input stream.)
>reg Pop the top value from the stack and store it in a register
labelled by the byte reg.
reg Push the value of register reg (which must be non-null) onto the
stack.
tag Associate the tag string with the value on top of the stack.
The ubfa(2) implementation does so by replacing it by a special
Tag tuple.
$ End-of-input: there must be exactly one value on the stack,
which is the result.
Applications using UBF(A) typically take turns to exchange input values
on a communication channel.
SEE ALSOubfa(2), json(6), sexprs(6)
J L Armstrong, ``Getting Erlang to talk to the outside world'', ACM
SIGPLAN Erlang workshop 2002 , Pittsburg, PA USA
UBF web page, http://www.sics.se/~joe/ubf/
UBFA(6)
