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plumbmsg - plumbing message module


include "plumbmsg.m";
plumbmsg := load Plumbmsg Plumbmsg->PATH;
Msg: import plumbmsg;

Msg: adt
    src:    string;
    dst:    string; 
    dir:    string;
    kind:   string;
    attr:   string;
    data:   array of byte;
    # used by applications
    send:   fn(msg: self ref Msg): int;
    recv:   fn(): ref Msg;
    # used by plumb and send, recv
    pack:   fn(msg: self ref Msg): array of byte;
    unpack: fn(b: array of byte): ref Msg;

Attr: adt
    name: string;
    val:  string;

init:         fn(willsend: int, rcvport: string, maxdata: int): int;
shutdown:     fn();
string2attrs: fn(s: string): list of ref Attr;
attrs2string: fn(l: list of ref Attr): string;
lookup:       fn(attrs: list of ref Attr, name: string): (int, string);


Plumbmsg is an interface for message-passing between applications via the plumber(8). It allows applications to receive messages from the plumber on a logical input port, and send messages to other applications via the plumber.

Init must be called once when the application starts, to set up its plumbing connections. Applications can choose to send messages, receive them, or do both. Note that the plumber must be running before any of these functions are useful. Normally, that is done by the window system's initialisation procedure, but in specialised systems, plumbing can be used for attribute-oriented communication even without a window system.

If the application will be sending messages via the plumber, the value willsend must be non-zero, and init will open an appropriate channel to the plumber; if the application will not send messages, the value should be zero.

If the application is prepared to receive messages, the parameter rcvport names its logical input port, which must also be known to the plumber (ie, it must be named as a possible destination in plumbing(6)); init will open an appropriate channel to receive messages from the plumber. The parameter maxdata gives the size in bytes of the largest message the application is prepared to receive. Applications that only send messages set rcvport to nil.

Init returns returns -1 if for any reason either connection cannot be set up correctly, in particular if the plumber is not running or the input port is unknown. Otherwise it returns a non-negative value.

The following program fragment establishes input and output plumbing for an application `edit':

plumbed := 0;
plumbmsg = load Plumbmsg Plumbmsg->PATH;
if(plumbmsg->init(1, "edit", 1000) >= 0)
	plumbed = 1;

The variable plumbed is set to allow the application to disable its plumbing user interface (and not attempt to send messages) if initialisation fails.

The Msg adt encapsulates the message data routed between applications and provides member functions to send and receive them. Its components are used as follows:

The name of the program generating the message.
The output port to which the plumber should route the message. In practice, destination is often left empty, and the destination port will be determined by the plumber applying the automatic routing rules (cf. plumbing(6))
The directory in which to interpret the data (eg, if the data is a local file name).
The format of the data. Currently, `text' is the only type that applications understand, but the plumbing system can route any kind of data.
A string containing name=value pairs (eg, click=7), separated by tabs. Normally the value should be created using attrs2string and parsed using string2attrs, described below.
The message to be conveyed. If kind is text, and the message is a string s, data will be `array of byte s' (ie, its UTF encoding).

Plumbing messages are created directly using Limbo's ref operator, giving the desired value to each field. For example:

	msg := ref Msg(
		array of byte text);

The plumbing messages are exchanged with the plumber using two member functions:

Writes a plumbing message to the plumber. It returns the number of bytes written (the result of write in sys-read(2) which does the writing). It returns -1 if the plumber cannot route the message.
Reads a plumbing message from the file representing the application's input port, previously opened by init. It waits for a message, and returns a reference to a Msg that contains the message data.

Shutdown sends a message to the plumber that shuts down plumbing for the application's input port rcvport. An application must call it before it exits if it has an active input port.

String2attrs takes a string containing a tab-separated list of attribute pairs and returns a list of references to Attr adts.

Attr2string converts a list of references to Attr adts into a string of the form name=valuename=value . . . . The name=value pairs are separated by a single tab.

Lookup searches attrs for an attribute name, and if found, returns the tuple (1,value). If name is not found, lookup returns (0, nil).

Packed message format
The format of plumbing messages as transmitted, and member functions that encapsulate it, are documented here for completeness, and in case the details are useful in interpreting plumbing messages outside the Inferno environment.

Plumbing messages have a fixed structure: five lines of text giving UTF representations of the corresponding fields of Msg, then a line giving the length of data in decimal, followed by the bytes of data:







n bytes

The details are encapsulated in two functions:

Pack packs the contents Msg m into an array of byte for subsequent transmission using write (see sys-read(2)).
Unpack unpacks an array of byte b to form a copy of the original Msg, which it returns.


file to send messages to the plumber
file to receive messages routed to the logical name rcvport




Shutdown should not be needed: the plumber(8), as a file server, must know that a particular client has vanished.

PLUMBMSG(2 ) Rev:  Thu Feb 15 14:43:27 GMT 2007