Programmers zone

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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

#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);
}

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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<stdio.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 “);
}

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.

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This program can be used to set or change the system time

#include <stdio.h>
#include <dos.h>

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;
}

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