MUX(3)

NAME
Mux, muxinit, muxrpc, muxthreads - protocol multiplexor

SYNOPSIS
#include <mux.h>

struct Mux
{
	uint mintag;
	uint maxtag;
	int (*settag)(Mux *mux, void *msg, uint tag);
	int (*gettag)(Mux *mux, void *msg);
	int (*send)(Mux *mux, void *msg);
	void *(*recv)(Mux *mux);
	void *(*nbrecv)(Mux *mux);
	void *aux;
	...   /* private fields follow */
};

void	muxinit(Mux *mux);

void*	muxrpc(Mux *mux, void *request);

void	muxprocs(Mux *mux);

Muxrpc*	muxrpcstart(Mux *mux, void *request);

void*	muxrpccanfinish(Muxrpc *rpc);

DESCRIPTION
Libmux is a generic protocol multiplexor. A client program initializes a Mux structure with information about the protocol (mainly in the form of helper functions) and can then use muxrpc to execute individual RPCs without worrying about details of multiplexing requests and demultiplexing responses.

Libmux assumes that the protocol messages contain a tag (or message ID) field that exists for the sole purpose of demultiplexing messages. Libmux chooses the tags and then calls a helper function to put them in the outgoing messages. Libmux calls another helper function to retrieve tags from incoming messages. It also calls helper functions to send and receive packets.

A client should allocate a Mux structure and then call muxinit to initialize the library's private elements. The client must initialize the following elements:

mintag, maxtag
    The range of valid tags; maxtag is the maximum valid tag plus one, so that maxtag-mintag is equal to the number of valid tags. If libmux runs out of tags (all tags are being used for RPCs currently in progress), a new call to muxrpc will block until an executing call finishes.

settag, gettag
    Set or get the tag value in a message.

send, recv, nbrecv
    Send or receive protocol messages on the connection. Recv should block until a message is available and should return nil if the connection is closed. Nbrecv should not block; it returns nil if there is no message available to be read. Libmux will arrange that only one call to recv or nbrecv is active at a time.

aux An auxiliary pointer for use by the client. Once a client has initialized the Mux structure, it can call muxrpc to execute RPCs. The request is the message passed to settag and send. The return value is the response packet, as provided by recv, or nil if an error occurred. Muxprocs allocates new procs (see thread(3)) in which to run send and recv. After a call to muxprocs, muxrpc will run send and recv in these procs instead of in the calling proc. This is useful if the implementation of either (particularly recv) blocks an entire proc and there are other threads in the calling proc that need to remain active.

Libmux also provides a non-blocking interface, useful for programs forced to use a select(2)-based main loop. Muxrpcstart runs the first half of muxrpc: it assigns a tag and sends the request, but does not wait for the reply. Instead it returns a pointer to a Muxrpc structure that represents the in-progress call. Muxrpccanfinish checks whether the given call can finish. If no mux procs have been started, muxrpccanfinish may call nbrecv to poll for newly arrived responses.

EXAMPLE
See /usr/local/plan9/src/lib9pclient/fs.c for an example of using libmux with 9P (see intro(9p)).

SOURCE
/usr/local/plan9/src/libmux

SEE ALSO
thread(3), intro(9p)

BUGS
Libmux does not know how to free protocol messages, so message arriving with unexpected or invalid tags are leaked.