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SLOWBAUD(1) Urchlay's Useless Stuff SLOWBAUD(1)
NAME
slowbaud - simulate a low bitrate serial connection
SYNOPSIS
slowbaud [<bits-per-sec>] [<file> ...]
slowbaud [<bits-per-sec>] -c [<command> [<arg> ...]]
slowbaud [<bits-per-sec>] -e <string> [<string> ...]
DESCRIPTION
slowbaud by default acts as a filter, or like the cat(1) command. It reads
files or its standard input, and writes the contents unmodified to standard
output... but slowly, at the given bits-per-second rate. Input and output are
unbuffered.
slowbaud can also act like echo(1) (the -e option), or run an interactive com‐
mand (the -c option).
The <bits-per-sec> argument is optional. If it's not given, the bit rate will
be set from SLOWBAUD_BPS in the environment, or a built-in default of 2400 if
not set.
OPTIONS
bits-per-sec
The bit ("baud") rate to simulate. This must be the first argument.
slowbaud assumes that if the first argument is a number, it's the bit
rate. If you're trying to pass a filename that consists only of dig‐
its, give -- as the first argument, or use e.g. ./filename.
-e Echo mode. Prints all further arguments as strings to stdout, separated
by a single space, at the given bit rate. Does not support back‐
slash-escapes, or any of the options of the regular echo command.
-c Command mode. Next argument (if present) is the command to run, any
remaining arguments become arguments to the command. With no arguments
after -c, a shell is spawned. This creates a pseudo-tty, so the com‐
mand can be interactive.
ENVIRONMENT
SLOWBAUD_BPS
Can be used to set the bit rate, when no <bits-per-sec> argument is
used.
SLOWBAUD_DEBUG
Set this (to any value) in the environment to see verbose debug output
on stderr, including timing accuracy stats.
SHELL Standard *nix environment variable, used to determine what shell to run
when -c is given with no <command>. If unset, /bin/sh is used.
EXIT STATUS
Without -c, 0 for success, non-zero on any error such as nonexistent/unread‐
able files. slowbaud exits immediately on such errors (this is unlike cat(1)).
With -c, exit status is that of the child process, or 127 if the child process
couldn't be spawned (e.g. command not found). Of course, the child process
could also exit with status 127...
NOTES
The bitrate has a range of 1 to 500000. Timing accuracy depends on your OS,
kernel config (HZ and/or NO_HZ on Linux), and system load. No "fancy" tech‐
niques like realtime scheduling or hardware event timers are used. At bitrates
up to 57600, on a typical unloaded Linux system, the timing should be at least
99.7% accurate.
We can't really insert a delay between the bits of a byte, since I/O is done
with byte granularity. For calculation purposes, <bits-per-sec> is divided by
10 to get bytes per second. This simulates "8-N-1": one start bit, 8 data
bits, no parity, and 1 stop bit (total of 10 bits per byte).
The timing code works by calculating how long to sleep after each character
(in microseconds), but actually sleeping slightly less than that, then
busy-waiting until the rest of the interval expires. At slower bitrates, this
works well, and the CPU overhead is barely noticeable (at least on reasonably
fast modern systems).
The timing error will almost always result in the bitrate being slightly too
slow at lower bitrates and slightly too fast at higher ones.
Timing is more accurate on Linux than OSX. It's done with getitimer() and sig‐
wait(). This works out to be slightly more accurate than using usleep() on
both Linux and OSX. It would be possible to use the realtime timer_create()
and clock_gettime() API on Linux, for possibly even better accuracy, but OSX
doesn't have these (and I want to be portable).
If this were a truly useful application, it would be worth trying to decrease
latency further, with realtime process scheduling. I didn't do this because
slowbaud is just a toy, and because the RT stuff tends to be unportable and
require elevated privileges (root, or something like setrtlimit or extended
filesystem attributes to manage capabilities).
About the name... I'm aware that "baud" is not synonymous with bps. I just
think "slowbaud" sounds better than "slowbps", as a name. Anyway the stty com‐
mand on Linux misuses the term ("speed 38400 baud"), so I'm in good company.
BUGS
With -c, signals aren't handled gracefully. Window size changes (SIGWINCH)
don't get propagated to the child process, and pressing ^C doesn't interrupt
the process. Yet.
COPYRIGHT
slowbaud is copyright 2021, B. Watson <yalhcru@gmail.com>. Released under the
WTFPL. See http://www.wtfpl.net/txt/copying/ for details.
0.0.1 2021-07-21 SLOWBAUD(1)
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