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HELP MUX                                           R.Evans December 1986

Lib MUX provides a simple interface to the 'SELECT' Unix system call
(see Unix MAN (2) select), allowing POPLOG to monitor (MUltipleX)
several devices simultaneously for input. See LIB *VEDMUX for an example
of use.

See also *VEDMUX  *VSH

NB: LIB MUX is supported only on Unix systems which provide the BSD 4.2
    version of select(2). Loading the library on other systems will
    cause a mishap.

[This and related bsd facilities referred to herein are unsupported, and
therefore included in the $usepop/pop/lib/lib/ directory.
    See DOC * SYSSPEC.

It is therefore necessary to compile MUX explicitly e.g. with one of
the following commands
    uses mux
    lib mux
The latter will print a warning message.

It is hoped that although the facility is unsupported it will serve as
a useful programming example.]

         CONTENTS - (Use <ENTER> g to access sections)


-- MUX_SELECT ---------------------------------------------------------

    mux_select(mask,table) -> entries;

The procedure MUX_SELECT provides the basic interface to the 'select'
Unix system call. It takes two arguments, a mask which is an integer in
which bits are set corresponding to the descriptors of input devices
that you want to monitor, and a table of arbitrary pop objects, each
corresponding to a given device (indexed on descriptor). MUX_SELECT
monitors all the devices specified simultaneously until characters are
available for reading from one or more of them, or until a signal (eg
alarm) is received. It returns a list of table entries corresponding
to the devices with characters available for reading - usually only one
device. If interrupted by a signal, it returns [].

In order to use MUX_SELECT, you need to know the Unix file descriptor (a
number) corresponding to a pop11 device. To obtain this, the system procedure
DEVICE_OS_CHANNEL may be used:

    device_os_channel(dev) -> n;

This takes an open device and returns its descriptor. It is this
descriptor which is used to identify the device in the mask and table
given to MUX_SELECT.

NB: MUX_SELECT does not actually read characters from any device, it
    merely tells you which device chacarters can be read from, thus
    avoiding the need for polling, or hanging up on a particular device
    waiting for input.

-- MUX_INPUT and MUX_ENTRY --------------------------------------------

MUX_SELECT requires a certain amount of specialist knowledge. The
procedure MUX_INPUT and MUX_ENTRY provide a simpler interface, relieving
the user from the need to know about file descriptors, masks etc.

    entry -> mux_entry(dev);
    mux_entry(dev) -> entry;

MUX_ENTRY and its updater allow you to access and define table entries
for pop11 devices in a standard table (stored in the variable
MUX_TABLE). An entry of FALSE means the device is not to be monitored
for input. As well as allowing access to the table, MUX_ENTRY maintains
a mask suitable for use with MUX_SELECT (although normally used via

    mux_input() -> entries;

MUX_INPUT executes a call of MUX_SELECT on the standard table and mask
maintained by MUX_ENTRY, and returns a list of table entries for devices
ready for reading.

--- C.unix/help/mux
--- Copyright University of Sussex 1992. All rights reserved. ----------