March 9th, 2008 by admin | No Comments »
How to write a C program without a main function.Is it possible to do that.. OK.. heres the code
#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 “);
}
Yes the above program runs perfectly fine.But how,whats the logic behind it?
Here we are using preprocessor directive #define with arguments.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 here we are using the proprocessor directive to intelligently replace the word begin” by “main” .
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