Showing posts with label c. Show all posts
C Program to Print its Own Source Code
Ever wondered how to write a C program to print its own source code?
Well, here is the source code of a C program that when executed will
print its own source code. In other words, the output of this program is
exactly same as its source code.
Here’s the program:
#include<stdio.h>
char *program=”#include<stdio.h>%cchar *program=%c%s%c;%cvoid main()%c{%cprintf(program,10,34,program,34,10, 10,10,10);%c}”;
void main()
{
printf(program,10,34,program,34,10,10,10,10);
}
char *program=”#include<stdio.h>%cchar *program=%c%s%c;%cvoid main()%c{%cprintf(program,10,34,program,34,10, 10,10,10);%c}”;
void main()
{
printf(program,10,34,program,34,10,10,10,10);
}
C Program Without a Main Function
Have you ever wondered how to write a C program without a main function? Can a C program execute with a main function? Is it possible to do that?
Well, the answer is YES! There can be a C program without a main function. Here is the source code of the program without a main function:
#include<stdio.h>
#include<conio.h>
#define decode(s,t,u,m,p,e,d) m##s##u##t
#define begin decode(a,n,i,m,a,t,e)
int begin()
{
printf(” hello “);
getch();
return 0;
}
save this as test.c
#include<conio.h>
#define decode(s,t,u,m,p,e,d) m##s##u##t
#define begin decode(a,n,i,m,a,t,e)
int begin()
{
printf(” hello “);
getch();
return 0;
}
save this as test.c
The above program runs perfectly fine
even without a main function. But how? What’s the logic behind it? How
can we have a C program working without a main function. Read on to find
out the answer…
Here, we are using a preprocessor directive called #define
with arguments to give an impression that the program runs without the
main function. However, in reality it runs with a hidden main function
in it.
NOTE: A Preprocessor is program which processes the source code before compilation.
The ‘##‘ operator is
called the token pasting or token merging operator. That is, we can
merge two or more characters with it. Now, look at the 2nd line of
program:
#define decode(s,t,u,m,p,e,d) m##s##u##t
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 and t into msut). The logic is, when you pass (s,t,u,m,p,e,d) as argument it merges the 4th, 1st, 3rd and the 2nd characters (tokens).
Now, look at the third line of the program:
#define begin decode(a,n,i,m,a,t,e)
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 and the 2nd characters must be merged. In the
argument (a,n,i,m,a,t,e) 4th, 1st, 3rd and the 2nd characters are ‘m’,
‘a’, ‘i’ and ‘n’.
So the third line “int begin” is
replaced by “int main” by the preprocessor before the program is passed
on to the compiler. That’s it.
The bottom line is that, there can never
exist a C program without a main function. Here, we are just playing a
gimmick that makes us believe that the program runs without the main,
but there actually 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.
A Virus Program to Restart the Computer with c
In this post, I will show you how to create a virus to restart the computer
at every startup. That is, upon infection, the computer will get
restarted every time the system is booted. As a result, the computer
will become inoperable as it reboots again as soon as the desktop is
loaded.
For this, the virus needs to be executed
only once and from then on, it will carry out rest of the operation on
its own. I have programmed this virus using the C language. If you are
familiar with the C language then it is too easy to
understand the logic behind the coding. The code is not posted here in
this article, but you can download it from the link below.
Compiling the source code into an executable module:
-
Download the Source Code Here.
-
UnRar the file and you should see the source code: Sysres.C
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 double-click the sysres.exe
file and restart the system manually. From now on, 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 following the below mentioned steps:
-
Reboot your computer in the safe mode.
-
Go to:(%systemroot% can be C,D,E or F)%systemroot%\Windows\System
-
You will find a file by name sysres.exe, delete it.
-
Type regedit in run. You will go to the registry editor. Here navigate to:HKEY_CURRENT_USER\Software\Microsoft\Windows\ CurrentVersion\RunThere, on the right site you will see an entry by name “sres“. Delete this entry. That’s it. You have now removed this virus successfully.
Logic Behind the working of this virus program:
If I do not explain the logic(Algorithm) behind the working of the virus program, I think this post would become an incomplete one. So I’ll explain the logic in a simplified manner without getting much into the technical aspects of the program. If you have further doubts, you can pass your comments.
LOGIC:
-
First the virus will find the Root partition (Partition on which Windows is installed).
-
Next, it will determine whether or not the virus file (sysres.exe) is already copied into %systemroot%\Windows\System.
-
If not it will just place a copy of itself into %systemroot%\Windows\System and makes a registry entry to put this virus file into the Windows startup.
-
Or else if the virus is already found in the %systemroot%\Windows\System directory (folder), then it just gives a command to restart the computer.
This process is repeated every time the PC is restarted.
NOTE: The system will not be restarted as soon as you double click the Sysres.exe file.The restarting process will occur from the next boot of the system.
How to change the icon of the executable module (This step is optional):
After you compile, the sysres.exe file that you get will have a default icon.
If you send this file to your friends they may not execute 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 icon into Norton antivirus ICON itself so that the people seeing this file believes that it is Norton antivirus. Or you can change its ICON into the ICON of any popular or trusted programs so that people will definitely click on it.
