pd's message-system

Non-audio-data are distributed via a message-system. Each message consists of a ``selector'' and a list of atoms.

atoms

There are three kinds of atoms:

• A_FLOAT: a numerical value (floatingpoint)
• A_SYMBOL: a symbolic value (string)
• A_POINTER: a pointer

Numerical values are always floatingpoint-values (t_float), even if they could be displayed as integer values.

Each symbol is stored in a lookup-table for reasons of performance. The command gensym looks up a string in the lookup-table and returns the address of the symbol. If the string is not yet to be found in the table, a new symbol is added.

Atoms of type A_POINTER are not very important (for simple externals).

The type of an atom a is stored in the structure-element a.a_type.

selectors

The selector is a symbol that defines the type of a message. There are five predefined selectors:

• ``bang'' labels a triggerevent. A ``bang''-message consists only of the selector and contains no lists of atoms.
• ``float'' labels a numerical value. The list of a ``float''-Message contains one single atom of type A_FLOAT
• ``symbol'' labels a symbolic value. The list of a ``symbol''-Message contains one single atom of type A_SYMBOL
• ``pointer'' labels a pointer value. The list of a ``pointer''-Message contains one single atom of type A_POINTER
• ``list'' labels a list of one or more atoms of arbitrary type.

Since the symbols for these selectors are used quite often, their address in the lookup-table can be queried directly, without having to use gensym:

selector	lookup-routine	lookup-address
bang	gensym("bang")	&s_bang
float	gensym("float")	&s_float
symbol	gensym("symbol")	&s_symbol
pointer	gensym("pointer")	&s_pointer
list	gensym("list")	&s_list
-- (signal)	gensym("signal")	&s_symbol

Other selectors can be used as well. The receiving class has to provide a method for a specifique selector or for ``anything'', which is any arbitrary selector.

Messages that have no explicit selector and start with a numerical value, are recognized automatically either as ``float''-message (only one atom) or as ``list''-message (several atoms).

For example, messages ``12.429'' and ``float 12.429'' are identical. Likewise, the messages ``list 1 for you'' is identical to ``1 for you''.