_basic.txt

一个通讯程序源码

.*s 1 "Basic Organization"

ECU forks to run as two separate
processes, a transmitter (XMTR) and a receiver (RCVR).
The two processes
communicate via signals and a System V shared memory segment.
XMTR controls RCVR and terminates it under certain circumstances,
called here auxiliary operations.  After an auxiliary operation
completes, XMTR forks again to recreate RCVR.

.*s 2 "Transmitter Process (XMTR)"

XMTR
accepts user input from the computer keyboard; input is recognized
as belonging to one of two types: 1) transmit data and 2)
ECU commands.  Keyboard input
is passed to the serial line driver until an ECU command
is detected.  Commands are prefixed with a 
.B HOME
key which causes ECU to accept keyboard data up to the next ENTER
key as command text.  After a command has been processed,
keyed data is again routed to the serial line.

.*s 3 "Keyboard Interface"

The keyboard driver is set into the raw mode.
XMTR reads characters one at a time from the driver.
Normally, characters read from the keyboard are passed directly to
the serial line driver.  The
ASCII ESC ("escape") character is handled as a special case.
When a function key is pressed, the keyboard generates
to XMTR an ESC character,
followed by more characters describing which function key has
been pressed.
Pressing the ESC key also causes XMTR to see an ESC character,
but with no subsequent function key "suffix".

When an ESC character is read, XMTR delays transmission of the
character to the line for a short period to determine whether
the ESC key has been pressed or a function key has been pressed.
If no "suffix" is detected, the ESC is passed to the line, having
suffered an insignificant delay given human typing speeds.

If a function key "suffix" is detected, the function key type
is decoded.  The
.B HOME
key indicates an ECU command follows.  Any
other function key is passed to the function key mapping feature
(described later).

.*s 3 "ECU Command Assembly"

After 
.B HOME
has been pressed, XMTR presents a reverse video prompt on the
display, indicating its readiness to accept a command.  During
input, the command may be edited using the same control keys
specified with stty(C).  Additonal editing
using the left and right arrow keys and the
Ins(ert) key are usually available.
Command input may be aborted by pressing ESC.
When a command string has been assembled, it is passed to the
command processor, which breaks the command arguments into a token
array similar to the argc/argv array.  When the command handler
returns, XMTR returns to its normal mode of copying keyboard data to
the serial line.

Further information on XMTR command line processing may be found in 
later sections titled Line Editing and Interactive Command History.

.*s 3 "Function Key Mapping"

While in tty-to-line mode, function keys other than 
.B HOME
are available to be mapped
to transmit short keystroke sequences on a connection by
connection basis.  Under control of the dialing command ("Dial"
described below)  or the function key control command ("FK"),
predefined function key maps may be loaded.

Function keys which may be mapped are
.B F1
through
.B F12 ,
.B PgUp ,
.B PgDn ,
.B End ,
.B Ins ,
.B Del ,
and
the cursor control keys.

The unshifted keypad '5' key is permanently mapped to
produce a screen snapshot when it is pressed and XMTR is
reading from the keyboard.

The BackTab (shift Tab) key is permanently mapped to a screen
redisplay feature.

.*s 3 "Auxiliary Operation Control"

Certain commands cause ECU to perform what is called an
auxiliary operation, requiring temporary termination of the
RCVR process.  Such operations are not as useful
with
.B cu(C) ,
because cu does NOT kill its receiver process at any time.
As a result, it impossible to run a modern file transfer
protocol since the cu receiver process eats some of the
characters sent by the remote protocol program.

There are two types of auxiliary operations:
1) internal interactive command or procedure
execution and 2) external program execution.
Certain
.B internal
.B commands
require tight control over the serial line.
For instance, the
.B dial
command requires transmitting modem command
strings and receiving modem response codes.  Such functions
are best accomplished by single-process control of the line.
.B External
.B program
execution is of two kinds, file transfer invocation
and local shell/command execution.

The RCVR process is terminated in any of these cases either to
avoid the RCVR swallowing characters intended for other
targets (the modem handler in XMTR or the file transfer protocol)
or to avoid having remote data interspersed with the output of
local programs.

.*s 2 "Receiver Process (RCVR)"

The receiver process reads the incoming serial data stream and
passes it to the user terminal driver through a filter which
scans for events such as the occurrence of ASCII BEL (bell)
characters or terminal control sequences.  RCVR also handles
the session logging and ANSI filter functions.

.*s 3 "ANSI Filter"

Since the term "ANSI" is used to describe
.B many
variations on the ANSI X3.64 recommendations for terminal control (read
"IBM pseudo-ANSI"), the ECU receiver process has an "ANSI filter" which
attempts to translate incompatible (read "MSDOS") ANSI-like control
sequences to sequences acceptable to the console terminal in use.
This includes support for the MS-DOS "save cursor" and "restore cursor"
sequences.
More information may be found later in "Supported Terminals."

.*s 3 "Session Logging"

When directed by the user, the RCVR process logs incoming serial
data to a file named on the log command line.  The default operation
is to filter unprintable characters (other than TAB and NL) from the
log, but raw logging is available with a command option.  In a like
manner, the default is for appending to an existing file, but a
command option may specify scratching any previous contents.

Log files receive header lines each time the file is
opened, stating the logical system name, the telephone number
and the date/time.