UBFA(6) UBFA(6)
NAME
ubfa - universal binary format for data transport
DESCRIPTION
UBF(A) is the data transport encoding for Armstrong's Uni-
versal 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 com-
pound 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).
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UBFA(6) UBFA(6)
~ 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 char-
acter ~, which is discarded.
# Push a value representing an empty list onto the stack.
& Pop a value v. Pop another value l, which must repre-
sent 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 reg-
ister 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 ALSO
ubfa(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/
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