SE-3910 Real-time Systems Week 5, Class 2 – Lab turn-in page is up! – Lost-and-not-found power supply – Quick-Quiz (Ungraded) – Use interrupts in a Linux/C

SE-3910Real-time Systems

• Week 5, Class 2– Lab turn-in page is up!– Lost-and-not-found power supply– Quick-Quiz (Ungraded) – Use interrupts in a Linux/C environment

• Watchdog follow-up– Watchdog demo– (Tentative) HOW to set time-out for watchdog

– Bash Scripting– Real-Time OS’s

SE-3910 - Dr. Josiah YoderSlide style: Dr. Hornick

Much Material: Dr. Schilling, Some from Dr. Hornick, etc.1

Quick Quiz! (1)

How do you get a reference (pointer) to the first element in an array?int x[5] = {1,2,3,4,5}boolean foo(int *ip);a)foo(&x);b)foo(&x[0]);c)foo(x);d)foo(x[0]);


Much Material: Dr. Schilling2

Quick Quiz! (2)How do you get a reference (pointer) to the second element in an array?int x[5] = {1,2,3,4,5}boolean foo(int *ip);a)foo(&x+1);b)foo(&x[0]);c)foo(x+1);d)foo(x[0]);e)foo(&(x+1));



Real-Time Operating Systems• What does an operating system REALLY need?

– Essential• Scheduler• Memory allocation (e.g. malloc)• File system• Interrupts

– Important• Protection (security) (if on network)

– Less important • Networking• Protection (security) (if off network)

– Nice to have• GUI• Virtual memory• Multithreading



UNIX Operating System• Common driver strategies

– Write to a file– Read from a file– ioctl (system call)– Short programs that wrap ioctl calls– [Addendum] “system calls” from C programs (which

don’t really call the kernel, but rather execute the short command-line programs mentioned above or execute a program that reads/writes from a file…



Scripting• Why learn scripting?

– Faster to write than compiled code

• Why learn command-line (bash) scripting?– Command line works very naturally with files– UNIX/Linux runs on files



Starting a Script• (More details)• Put the following into a file (e.g. script)

#!/bin/bashecho "Hello World"# other fun

• Now, run this command to make it executablechmod u+x script

• Now run the script like this:./script



http://linuxcommand.org/wss0010.php#write

Variables

Set a variableexport myVar="This is a variable"

Read a variable, printing to standard out:echo $myVar

Increment a Variableexport myVar=$((myVar+1))

Other integer mathexport myVar=$((myVar+1))



Arguments• Suppose we run our script like this:

./script arg1 arg2 arg3

• And inside the script, we run this command:echo "argument 3 is: $3"

• This will print:argument 3 is: arg3



Quoting - Escapes• Suppose we run

./script "this is a test" "of" "arguments"

• The quotes group the words …echo "arg1: $1"echo "arg2: $2"echo "arg3: $3"

• will printarg1: this is a testarg2: ofarg3: arguments



Quoting - EscapesIn the example from the previous section,

echo "arg1: $1"

prints arg1: this is a test

but echo 'arg1: $1'

printsarg1: $1

Escapes (e.g. "\\, \n, \r") are also useful.SE-3910 - Dr. Josiah Yoder

Slide style: Dr. HornickMuch Material: Dr. Schilling

11

Quoting Challenge:• How do I run an ssh command like this

– ssh host “command to run”

• But give “to run” as a single argument?• Hint: Use escapes!• Hint2: Command to run will be interpretted

twice.



Conditionals(More details) (UPDATED link at end of lab)Comparison

if test "$var1" = "$var2”; thenecho "true"

elseecho "false"

fiIf file “$var1” exists…

if test –e "$var1"; thenecho "true"

fi



“Counting” LoopsCounting loop (my preferred approach)

for i in `seq 1 10`; doecho $i;

done;

Processing files in current directoryfor i in *.pdf; do

echo cp $i ${i/.pdf}.bak.pdf;done;# (Untested – test before use!)



“Sentinal” loopsLoop while command succeeds (returns 0)

while commandecho "looping"

done;

Keep looping as long as the command fails.while !command

echo "looping"done;

Keep looping as long as test is “true”while test …

echo "looping"done;



Useful commands• Useful commands

– sleep 0.5 will sleep for half a second– time command args1 arg2 arg3– will measure how long it takes– command arg1 arg2 arg3 to run



References for scripting

One of the main resources linked from above:http://linuxcommand.org/writing_shell_scripts.php

For if test …, I like this one betterhttp://www.tldp.org/LDP/Bash-Beginners-

Guide/html/sect_07_01.htmlIntro to variables (possibly useful)http://www.tldp.org/LDP/abs/html/parameter-substitution.html



Watchdog On the Beaglebone• http://beaglebone.cameon.net/home/watchdog-

timer• Open the file /dev/watchdog• Do not close the file• Write something (e.g. "\n") to the file at least

every 59 seconds to keep the system runningDEMO• Yes, it is possible to change the time.DEMO? DETAILS?



Command-Line Cheat-sheet• I’m working on one• I’ve got links to online ones I like• Just ask me



Demo – Connecting to Beaglebone• [TODO]

– I plan to put instructions for setting up a DHCP server on your laptop using “connection sharing”

– You know my number.



Demo – Watchdog• [See computer]



Documents

SE-3910 Real-time Systems Week 5, Class 2 – Lab turn-in page is up! – Lost-and-not-found power supply – Quick-Quiz (Ungraded) – Use interrupts in a Linux/C