6CCS3RSC/7CCSMRTS
Assignment:Real-time Programming with pthreads
17th February 2021
For this assignment, you will need the template code critical.c and multithread.c. Download this
from the module’s KEATS page and save it to your computer. Use the following command to compile the
source code:
gcc filename.c -o filename -lrt -lpthread
where filename is the name of the source code file. The resultant executable should be run using sudo,
i.e., use the command:
sudo ./filename
Check that the above works for the source code provided, and complete the following exercises.
POSIX support for mutexes is illustrated in program critical.c. Examine this code and make sure that
you understand what it does.
1. Compile and run the program critical.c. To run this program you need to use sudo (i.e., use the
command: sudo ./critical). Record the output. Briefly explain what this program does and how
the scheduling gives rise to the observed behaviour. Include in your report a timing diagram; be sure
to show the state of each task (including the main routine) at all times.
[5 marks]
2. Modify the code so that the mutexes are no longer “commented-out”. Run the program again (using
sudo) and record the output. Briefly explain the execution of the modified program. Draw a timing
diagram of the modified program.
[5 marks]
Next, you will need the code for program multithread.c. Download this from the module’s KEATS page
before continuing with the below.
Examine the code for program multithread.c and make sure that you understand what it does.
3. Run the code and record the output. To run this program you need to use sudo (i.e., use the com-
mand: sudo ./multithread). Briefly explain what this program does and how the scheduling gives
rise to the observed behaviour. Include in your report a timing diagram; be sure to show the state
of each task (including the main routine) at all times.
[5 marks]
Dr M. Howard Department of Engineering
King’s College London
Modify the threadA function so that after printing half the letters it increases the priority of threadB
using the instructions:
param.sched priority = priority min+2;
pthread setschedparam(threadB id,policy,¶m);
Make sure that you understand what these changes do.
4. Run the modified program (using sudo) and record the output. Draw a timing diagram showing the
state of each task (including the main routine) at all times. Explain the scheduling of the threads.
Modify the code to decrease the priority of threadB after threadA has printed half its letters.
Report what happens and comment on this result. Modify the priority of only the executing thread
and report what happens when the priority is increased and when it is decreased. Comment on and
explain these results.
[5 marks]
5. Modify the original program so that threadA sleeps for 1 millisecond after printing half its letters
(use the nanosleep command). Run the code (using sudo) and record the output. Draw a timing
diagram and explain the scheduling of the threads. Include a print-out of your program code.
[5 marks]
6. Change the scheduling policy used by multithread.c to be SCHED RR rather than SCHED FIF0.
Perform experiments to determine how these scheduling policies differ. Describe the experiments
that you performed and summarise the results. Describe in your report how both scheduling policies
determine the scheduling of tasks both when threads have equal priority and when when threads have
unequal priorities. Include a print-out of your program code.
[5 marks]
Completed assignments should be submitted to KEATS on 5pm, 9th March 2021.
Your report must be no longer than four A4-pages. It should be submitted as a single PDF with a
minimum font size of 11pt, and margins of no less than 2cm.
This assignment is worth 15% of the module mark.
Dr M. Howard 2 Department of Engineering
King’s College London




























































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