GitHub: Go back to a previous commit

 You can see commit history by clicking the commits link:

You copy the SHA of the commit you want to revert to, open a terminal, go to source folder and type

git reset --hard <SHA>

C++: Dynamically create array of objects

Dynamically creating array of object on the heap:

MyClass** objects;

arrSize = 100;

objects = new MyClass*[arrSize]; //Note the '*' before arrSize

for (size_t i = 0; i < arrSize; i++) {

objects[i] = new MyClass();

}

Freeing up heap memory allocated to object array:

for (size_t i = 0; i < arrSize; i++) {

delete objects[i];

}

delete[] objects;

Tip: You can use Visual Studio's Memory Usage diagnostic tool to check heap memory changes which gives clues about memory leaks. To get the function names where memory is allocated but then never released, use valgrind on WSL (Linux) as follows:

valgrind --tool=memcheck --leak-check=yes ./<app name>

Generating Linux programs in Windows Visual Studio C++

You can use Windows 10 and Visual Studio 2022 to generate, debug and analyze Linux binaries:

1. Install WSL, restart computer
2. Open cmd, type wsl to enable linux prompt, install tools: 
1. sudo apt-get update
2. sudo apt install g++ gdb make ninja-build rsync zip
3. Open Visual Studio, on menu Tools - Get Tools and Features, add "Linux development with C++" if you haven't done already:
4. 
5. In Visual Studio, 
1. Create new project and select CMake project:
2. 
3. After project is created, change Local Machine to WSL:Ubuntu. Change Startup item to your project name:
4. 
5. If Visual Studio notifies you that CMake needs to be updated/installed, let it do it.
6. Press debug button to build and run:
7. 
8. At the bottom of the screen, in Output window, change it to CMake:
9. 
10. Visual Studio tells you where it has generated the Linux binary. Copy this folder to clipboard:
11. 
12. Go to View - Terminal. In the opened PowerShell terminal at the bottom, type wsl and press enter, the prompt will change to linux :
13. 
14. Type cd and paste the folder you copied:
15. 
16. Now you can also run your Linux binary from terminal:
17. 
18. To use valgrind, first install it in PowerShell by typing sudo apt install valgrind
19. Now you can use valgrind on your binary without leaving Visual Studio by typing (replace ./CMakeProject4 with your own project name): valgrind --tool=memcheck --leak-check=yes ./CMakeProject4:
20. 
21. To copy the binary to a Windows folder, you can open File Explorer from Linux prompt by typing explorer.exe .:
22. 
23. Congratulations!

C++: Using arrays with threads

The following C++ code demonstrates how to safely use an array that is written to by one thread and read from in another thread:

	//How to safely use an array that is written to by one thread and read from in another thread.
	//Requires C++11 or later.
	//Şamil Korkmaz, 10.03.2022, public domain.
	#include <chrono>
	#include <iostream>
	#include <thread>
	#include <mutex>
	std::mutex mx;
	std::condition_variable cv;
	const size_t arraySize = 1000;
	double arr[arraySize];
	void writeArray() {
	size_t counter = 0;
	while (1) {
	auto currentTime = std::chrono::steady_clock::now();
	auto nextTime = currentTime + std::chrono::milliseconds(1);
	//printf("writeArray, counter = %d\n", counter);
	{ //critical section
	std::unique_lock<std::mutex> lock(mx);
	for (size_t i = 0; i < arraySize; i++) {
	arr[i] = counter;
	}
	} //lock is automatically released here
	cv.notify_all(); //let other threads to get lock
	counter++;
	std::this_thread::sleep_until(nextTime);
	}
	}
	void readArray() {
	while (1) {
	auto currentTime = std::chrono::steady_clock::now();
	auto nextTime = currentTime + std::chrono::milliseconds(1);
	double arrStart;
	double arrEnd;
	{ //critical section
	std::unique_lock<std::mutex> lock(mx);
	cv.wait(lock); //wait for writeArray to finish
	arrStart = arr[0];
	arrEnd = arr[arraySize - 1];
	} //lock is automatically released here
	auto arrDiff = arrEnd - arrStart;
	printf("* readArray, arrStart = %1.0f, arrEnd = %1.0f, arrDiff = %1.1f\n", arrStart, arrEnd, arrDiff);
	if (arrDiff < 0) //happens when the end of array is lagging behing beginning of array. You trigger it by commenting out the lock line above
	{
	printf("End of array lagging behind beginning of array! arrDiff = %1.1f < 0\n", arrDiff);
	std::cout << "Press enter..." << std::endl;
	std::cin.get();
	exit(1);
	}
	std::this_thread::sleep_until(nextTime);
	}
	}
	int main() {
	std::thread worker1(writeArray);
	std::thread worker2(readArray);
	worker1.join();
	worker2.join();
	std::cout << "Press enter..." << std::endl;
	std::cin.get();
	}

view raw threadSafeArrayWriteRead.cpp hosted with ❤ by GitHub

Java: Get raw text in JFormattedTextField

You could define a custom class that derives from JFormattedTextField and use KeyListener to get the raw text that the user is typing:

	public class MyFormattedTextField extends JFormattedTextField {
	private String rawText;
	private class MyKeyListener implements KeyListener {
	...
	@Override
	public void keyTyped(KeyEvent e) {
	rawText = getText();
	}
	}
	public MyFormattedTextField() {
	addKeyListener(new MyKeyListener());
	}
	public String getRawText() {
	return rawText;
	}
	}

view raw getRawText.java hosted with ❤ by GitHub

Linux cheat sheet

Linux commands I frequently use:

1. Change to root (on Ubuntu): sudo -i
2. uname -r 
3. sudo apt update && sudo apt upgrade
4. Show processes whose name contains a specific string: ps -ef | grep <string>
5. Forcefully kill process: pkill -f <process name>
• Example: pkill -f update-notifier
6. Run process in background: ./<process name> &
7. Show network card info: ifconfig
8. Show resource usage of running processes: top, htop
9. Reverse-i-search with ctrl+r
10. Change MAC address: sudo ifconfig enp2s0 hw ether 64:00:6a:28:fa:ac
11. Ubuntu: Allow GUI root login
12. Enable SSH to root user by adding the line PermitRootLogin yes to /etc/ssh/sshd_config
13. Enable receiving UDP packets 
1. Disable firewall completely: sudo ufw disable
2. Or you can allow for example on port 5000: sudo ufw allow from any to any port 5000 proto udp
14. Create image of USB with size of 8GB: dd if=/dev/sdb of=/home/myimage.img bs=1G count=8 status=progress

Tools:

1. sudo apt install net-tools
2. sudo apt install build-essential
• gcc --version
• make --version
3. Eclipse IDE for C++
1. In a project, when you remove a folder and copy another version of the same folder but with fewer files, during build, you might get the error "No rule to make target…". Right click on the project and select refresh, this rebuilds the index. Now you can build successfully.
4. To use psftp from another Windows computer for file transfer: sudo apt install openssh-server
5. sudo apt install rt-tests
• Sample usage: cyclictest l100000 -t 8 p95
6. sudo apt install htop
7. For remote connection from Windows: 
• sudo apt install xrdp
• sudo systemctl restart xrdp
• Note that you have to be logged out from Ubuntu for remote from Windows to work.