Author: David

Using heap memory with malloc

Using heap memory with malloc

Posted on by David

For some reason malloc seems to confuse new programmers but it’s very straightforward.  I didn’t use it or even talk about it in the ebook. Memory allocation is not something you want to be doing in a game loop, running 60 times a second. All the memory that was ever used in the asteroids game was for text strings, variables and arrays and you don’t have to explicitly allocate any memory for those.

When you have a program that needs to allocate and free memory, like say a compiler or text editor then you grab a block of RAM using malloc.  You have to specify how much RAM you need and you must type cast it, because malloc return a void *.

Here’s an example. The image shows the output in the Visual Studio debugger.


#include <stdio.h>
#include <stdlib.h>

char* ptr[10];
int main() {
	for (int i = 0; i < 10; i++) {
		ptr[i] = (char *)malloc(1000);			
		char* aptr = ptr[i];
		for (int j = 0; j < 1000; j++) {
			aptr[j] = i + 64;
		}
        }
	for (int i = 0; i < 10; i++) {
		free(ptr[i]);
	}
}

DEbugger showing array valuesThis is what the Visual Studio debugger shows just before the final loop that calls free.

It has allocated 1000 bytes of memory for each of the ten char * in ptr[] and then fills it with 1000 each of the ASCII values 64..73 @

Finally, it frees up the memory using free. For every malloc, there should be a corresponding free or else your program will develop a memory leak.

 

 

Do you use assert in your code?

Do you use assert in your code?

Posted on by David

assertIt’s a macro that checks an expression, and if that expression isn’t true (.e. non-zero) it outputs a message and halts the program.

Here’s an example.

#include 
#include 

int main() {
	int x = 0;
	assert(x != 0);
	printf("It is the end");
}

Because x is 0, it triggers the assert and the program never reaches the printf statement. It’s a bit of a crude tool. In other programming languages like C++ or Delphi it raises an exception which can be handles but C of course does not have exceptions.

My own preference is to check the value for example making sure a pointer is not null and nthen displaying an error but other programmers prefer to use assert and have it kill the program if things go wrong.

Thrusting in different directions

Thrusting in different directions

Posted on by David
Trigonometry
Image by dognamedseven from Pixabay

When you press the control key, the player’s ship will accelerate in whatever direction (0-23) it is facing. To make this possible, I pre-populated a couple of float arrays with values x and y for thrust in any of the 24 directions.

This function populate the two arrays. These are declared as

float thrustx[24];
float thrusty[24];

void InitThrustAngles() {
    const float pi = 3.14159265f;
    const float degreeToRad = pi / 180.0f;
    float degree = 0.0f;
    for (int i = 0; i<24; i++) {
        float radianValue = degree * degreeToRad;
        thrustx[i] = (float)sin(radianValue);
        thrusty[i] = (float)-cos(radianValue);
        degree += 15;
    }
}

It uses trigonometry to calculate the horizontal and vertical amounts for any of the angles 0,15,30..345 degrees. As sin and cos functions work in radians rather than degrees, the variable radianValue is calculated from the degree angle by multiplying by the value degreesToRad. If you remember your school trigonometry, one radian = 180/pi degrees. So the code divides by this value (or multiplies by its inverse in this case).

Then when you press the control key, it just adds the appropriate thrustx and thrusty value to the vx and vy variables. Simple and fast.

 

A command line editor in C

A command line editor in C

Posted on by David

Thompson Davis Editor (TDE)The screenshot is of an open source command line editor for Dos, Windows and Linux called TDE which is short for Thomson Davis Editor.

As the website says “TDE is a simple, public domain, multi-file/multi-window binary and text file editor written for IBM PCs and close compatibles running DOS, Win32 (console) or Linux. TDE is suitable for editing batch files, binary files, text files, and various computer language source code files (with configurable syntax highlighting). The only limit on the number and size of files that TDE can handle is the amount of memory. Likewise, the only limit on the number of windows is the amount of memory. There is no preset maximum number of files or windows that may be open at any one time.”

What interested me with this was that the editor should work for Linux. It includes a viewer mode where files are loaded read-only,  file search (text or regular), can load files as binary, run a file as a Macro and display several source files with formatting. It certainly sounds a loty better than nano which is a terminal editor.

 

Using small delays in C with SDL ticks

Using small delays in C with SDL ticks

Posted on by David

Asteroids game - player ship rotationRunning a game at 60 frames per second means that handling things like key presses can be interesting. In the game, I call various SDL functions and via a giant switch statement set flags. Then later in the game loop, those flags are used to determine actions

So if you press Q to rotate the player ship anti-clockwise (counter-clockwise to yanks!) without some limiting thing, it would whizz round through 900 degrees each second. (There are 24 rotation angles for the ship, each 15 degrees. 60 x 15 = 900) .

I use a very simple technique to limit it. SDL has a function called SDL_GetTicks that returns the number of ticks since SDL was initialized in the game, i.e. when it started running. A tick is 1/1000th of a second, i.e. a millisecond. By calling this function twice, you can measure a time period. It’s not as precise as the nanosecond CPU clock that I use but for the kind of delays I’m talking about it is more than sufficient.

This is the code that is called each frame in the game loop.

void RotatePlayerShip() {
	if (rotateFlag && (SDL_GetTicks() - rotTimer > 40)) {
		rotTimer = SDL_GetTicks();
		if (rotateFlag == 1) // CounterClockwise 
		{
			Player.dir += 23;
			Player.dir %= 24;
		}
		else
			if (rotateFlag == 2) // Clockwise
			{
				Player.dir++;
				Player.dir %= 24;
			}
	}
}

Because the game loop syncs to the vertical fly-back, the time between two successive calls of this would be about 16.666 milliseconds. (=1000/60), but the check to see if 40 ticks have passed slows it down to 25 x 15 = 375 degrees rotation per second, i.e. just over one complete revolution which is more manageable than 900/360 = 2.5 full rotations. Plus if you wish rotation speed to be faster just change the 40 to a lower value.

This measured time delay is used quite a few times in the game. You could use it as a means to make the game get harder by having shorter delays on say aliens moving or firing.

How to Draw a circle in C

How to Draw a circle in C

Posted on by David

Asteroids-with shield-round-player-shipIn the asteroids game, when you press the s button to put up a shield, it draws a circle.  I must confess, I didn’t know how to draw a cuircle so looked it up and found an example on StackOverflow. You can use code from StackOverflow, licensed under a MIT license.  I include the link to StackOverflow in the game code (in the chapter 48 zip file) in a comment.

Here’s the code in the game.

void DrawCircle(SDL_Renderer *Renderer, int _x, int _y, int radius)
{
	int x = radius - 1;
	int y = 0;
	int tx = 1;
	int ty = 1;
	int err = tx - (radius << 1); // shifting bits left by 1 effectively
								  // doubles the value. == tx - diameter
	while (x >= y)
	{
		//  Each of the following renders an octant (1/8th) of the circle
		SDL_RenderDrawPoint(Renderer, _x + x, _y - y);
		SDL_RenderDrawPoint(Renderer, _x + x, _y + y);
		SDL_RenderDrawPoint(Renderer, _x - x, _y - y);
		SDL_RenderDrawPoint(Renderer, _x - x, _y + y);
		SDL_RenderDrawPoint(Renderer, _x + y, _y - x);
		SDL_RenderDrawPoint(Renderer, _x + y, _y + x);
		SDL_RenderDrawPoint(Renderer, _x - y, _y - x);
		SDL_RenderDrawPoint(Renderer, _x - y, _y + x);

		if (err <= 0)
		{
			y++;
			err += ty;
			ty += 2;
		}
		else 
		{
			x--;
			tx += 2;
			err += tx - (radius << 1);
		}
	}
}

it’s as simple as that! To make it more interesting, it is called each frame with the shield throbbing  by increasing  the radius from 38 to 46 pixels by 2 then restarting at 38 again. Here’s the code for that. Note that when the shield energy is below 10, it no longer works.

void DisplayShield(SDL_Rect * target) {
	if (shieldFlag && shieldStrength >10) {
		SDL_SetRenderDrawColor(renderer, 0xff, 0xff, 0xff, 0xff);
		DrawCircle(renderer, target->x + (SHIPWIDTH/2), target->y + (SHIPHEIGHT/2), shieldRadius);
		shieldRadius += 2;
		if (shieldRadius == 46) {
			shieldRadius = 38;
		}
	}
	if (shieldStrength < 100) {
		TextAt(target->x + 10, target->y + 58, sltoa(shieldStrength), 0.67f);
	}
}

The number under the player ship is the shield energy which drains while the shield is being displayed and recharges back up to 100 when you take your finger off the shield button. The number is only shown when the value is less than 100.

Open source repositories are worth a trawl

Open source repositories are worth a trawl

Posted on by David
Repository
Image by Jagrit Parajuli from Pixabay

How many C language projects do you think there are on GitHub? If you put language:C in the search box, you get the astonishing figure of 980,752! That would take some trawling through.

There are other search terms you can use as well. Click the cheat sheet link on the bottom left of the page and it will popup a form explaining how to filter for various things.  You are probably as well reading the GitHub search help. There’s a lot more to search there.

Confusingly the searches sometimes seem to come up with different values. I’ve seen it vary between 578,000 and over a million!   Add game to the search and there’s only 31,426 projects! Only…

Should you use #pragma once?

Should you use #pragma once?

Posted on by David

Pragma as found by google image searchThe traditional way of using an include guard is to put all of the header inside a #ifdef like this.

#ifndef hr_time
  #define hr_time
  #include <linux/time.h>
  #include 

// rest of code here
#endif

However the modern way is to put this at the top of the header file.

#pragma once

And this seems supported by most compilers I’ve tried. Certainly Visual Studio C/C++, gcc and clang all work.

In fact when you add a new file and choose header in Visual Studio, it put the #pragma once in automatically for you!

Given that those three C compilers are the main ones I use, I much prefer this pragma and use it.  But I would be interested in hearing of any C compilers that don’t  use it.

 

Willing to take a Risc?

Willing to take a Risc?

Posted on by David

One of the understated and wrong assumptions about the Raspberry Pi is that you can try any flavour of Linux on it but Linux is really the only OS.  It is true that most of the 20+ OSRISC Os that you can try are based on Linux but there are a few that aren’t.

  • Windows IOT Core – a limited version of Windows
  • Risc OS – Created by the inventor of ARM
  • RaspBSD – More Unix than Linux.
  • Chromium OS. Turn your Pi into a Chromebook.

Of these, I think Risc OS looks the most interesting. It certainly isn’t Linux and it reminds me of the Archimedes, the first ARM computer.

Risc OS is 33 years old, it started in Cambridge in 1987 and is a descendant of the BBC Micro OS. And yes they do have a BBC BASIC available.  There’s  lot more background to it on this page  but bear in mind that it and the comments date back to 2012. It has an  ARM C compiler called Norcroft.

I won’t be oing any more with Risc OS, but thought it worth a mention. All of the stuff I’m doing will be on Linux OS.

Not. ARM is in the news because Apple have just announced that they will be transitioning from Intel CPUs to ARM over the next year or two.

 

How to find files in Ubuntu

How to find files in Ubuntu

Posted on by David

In this I’m looking for the SDL2 header files. These are installed when you install libsdl2-dev as we saw yesterday.

While you can do it from a terminal with the find command, I find it easier to do it from the GUI. We need the Files utility which you get by clicking on this icon on the left toolbar. files-icon Now click Other Locations on the left and you should see something like this.

On This computer
Click Computer. This will let you search through the entire Linux file tree.

Click on the magnifying glass icon and in the text box that appears type in sdl2 and press enter. It will spend a few seconds or minutes searching and then find a number of files/folders.

The first result was SDL2 and the usr/include is a big clue. Double-click on this and it will open on a folder full of header files!That’s what we want.SDL2 Folder

 

When you are configuring gcc/clang and want to add paths for include files like SDL2, it’s important to know where those files are located.