Posts by Chang Park

Virtual Consoles

linux terminal

Virtual Consoles enable multi-user logins on a single machine. For example, you can login with your account and also as a root.


By default, most Linux versions provide virtual consoles with these keys.

Ctrl + Alt + F1 – F6: Virtual Consoles

Ctl + Alt + F7: A console where X server is running.



  • is an application that manages one or more graphical displays and input devices(mouse, keyboard, etc.)
  • works as a server.

You have probably heard about X11, X server, or X window.

All of these mean the same, and X11 just means eleventh version of X.


You can use a command ‘w’ which shows a list of currently logged-in users. 

w result

tty represents regular terminal.

pts represents pseudo terminal slave(e.g. ssh connection).



Methodology: Qualitative vs. Quantitative


These two terms can be used as research methods.


  • used to gain an understanding of underlying reasons, opinions, and motivations
  • provides insights into the problem or helps to develop ideas or hypotheses


  • used to quantify the problem by generating numerical data
  • uses measurable data to formulate facts and uncover patterns in research.
  • much more structured than qualitative.


How above methodologies can be applied in research project:

  1. Formulating hypotheses (Qualitative) – Can initiate a research by discovering the problems or opportunities people are thinking.
  2. Validating your hypotheses (Quantitative) – Can get numbers to apply statical analysis for validating hypotheses.
  3. Finding general answers (Quantitative) – Helps to get definite answers for broad questions.
  4. Incorporating the human element (Qualitative) – People’s own words can help to uncover blind spots.



Down Call vs. Up Call


Down Call

  • User code(unprivileged) enters to kernel with trap(interrupt).

Up Call

  • Kernel(privileged) enters to user code with IPC(Interprocess Communication).


White Box, Black Box, and Gray Box


In software testing, there are three different types of testing depending on how much knowledge the testers have on the software being tested.

White Box Testing

  • With full knowledge of the software. (internal structures, source codes, etc.)
  • Usually tests how the software performs.
  • Suitable for software developers.

Black Box Testing

  • Without having any knowledge of the software. (internal structures, source codes, etc.)
  • Usually tests for functionality of the software.
  • Suitable for third-party testers.

Gray Box Testing

  • Test with partial knowledge of the software.

Testing Methods


These are currently popular methods to test software.

Fuzz Testing

  • Input massive invalid, unexpected, or random data(Fuzz) to find errors or vulnerabilities in the software.
  • Effective on blackbox testing


Stochastic Testing

  • Input random sequence of datas 


Model-based Testing

  • Generates meaningful test case models which can detect errors on the desired behavior.


Symbolic Testing

  • Replace inputs of program with symbolic variables.
  • Find the feasibility of all paths in the program.
  • Quite expensive for finding feasibility of path.
x = read();
if (x > 3) {
  y = 1;
  if (x < 0)
    y = 2;
} else y = 3;

After changing with symbolic variables, it checks the feasibility of paths. On above example, the expression (y = 2; on line #5) will not be reached.


Concolic Testing (Concrete + Symbolic)

  • Input actual concrete value to the program, then generate new input by changing the path condition.
  • Less expensive then Symbolic testing.

What does the symbol ‘&’ mean in C and C++?


‘&’ symbol has two meanings in C and C++.

  • Address of Operator: returns address of a variable.
    • int a = 3;
    • printf(“%d”, a);
      • will return value of a: 3
    • printf(“%d”, &a);
      • will return address of variable a.


  • Bit-wise Operator &: operates on bitlevel
    • Example:table_bitwise_operator
    • int a = 13&4;
      • 13 = 1101
      • 4  =0100
      • 13&4 = 0100
      • a = 4

compileSdkVersion, minSdkVersion, and targetSdkVersion

Screen Shot 2017-11-15 at 12.37.43 AM


  • can choose which version of Android SDK to compile the app with
  • strongly recommended to compile with latest SDK (the app can use latest API)
  • this compile SDK will not be included in the app.
  • If using support library, the version should be matched with this compileSdkVersion.



  • lower bound of your app (minimum version of user devices the app can be installed).
  • Google Play Store uses to determine which devices the app can be installed on.
  • If using support libraries, it should be higher than the version of the support libraries.
    • On July 2017, minSdkVersion of all support libraries were increased to 14.
  • Check DashBoard to see current distribution of all Android versions.



  • Indicates which Android version the app designed for.
  • Informs that developer tested the app against target SDK version.
  • If target SDK version is lower than running API version, system enables compatible behaviors to maintain the app’s forward compatibility.



compileSdkVersion = targetSdkVersion = latest Android Version

  • so that the app can work on latest Android version devices.

Recommend to check latest SDK and update the app

  • Set targetSdkversion and compileSdkVersion to latest version.
  • In that way, you can check if there will be any error when running the app on newer version.
  • You don’t need to use any new features of newer versions.

Euclid’s Algorithm

Screen Shot 2017-10-02 at 10.32.14 PM

Euclid’s(Euclidean) Algorithm

  • Algorithm to find GCD of two large numbers

GCD(N1, N2), suppose that N1 is larger than N2

N1 = N2 * q1 + r1   <– Larger number divide by Small number

N2 = r1 * q2 + r2   <– Small number divide by remainder of previous

r1 = r2 * q3 + r3


N = r * q + 0    <– When remainder becomes zero, then r is the GCD



  • GCD(1701,3768)
    • 3768 = 1701*2 + 366
    • 1701 = 366*4 + 237
    • 366 = 237*1 + 129
    • 237 = 129*1 + 108
    • 129 = 108*1 + 21
    • 108 = 21*5  + 3
    • 21 = 3*7 + 0       <– 3 is the GCD of 1701, 3768


Deprecated APIs



  • As Android Versions keep being updated, many APIs and their methods are being revised.
  • Even though those API’s methods get deprecated, developers can still use the methods. (but they will see compiler’s warning)
  • However, developers are discouraged to keep using these methods because it is dangerous or there is better alternatives.





Multi Core vs. Hyper-threading


Multi Core

  • Provides multiple cores in CPU.
  • Dual-core can run two different processes at a time. (quad-core = 4 cores = 4 processes at a time)



  • Make one physical processing unit to two logical processing units.
  • Little bit of cheat to make it looks like two different processing unit.
  • Keep switching between two tasks, but actually one at a time.
  • Even though divided logically, CPU with hyper-threading performs better than CPU without hyper-threading.



  • i5: 4 cores without hyper-threading => 4 processing units
  • i7: 4 cores with hyper-threading => 8 processing units(logically)




Diagrams are from UB CSE421 lecture slides (Professor: Karthik Dantu)