MUX(3) 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 pro-
tocol (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 demul-
tiplexing 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:
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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(3)-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 rep-
resents 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)
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BUGS
Libmux does not know how to free protocol messages, so mes-
sage arriving with unexpected or invalid tags are leaked.
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