C Program

Question

#include<stdio.h>
#define CUBE(x) (x*x*x)
int main()
{
int a, b=3;
a = CUBE(b++);
printf("%d, %d ", a, b);
return 0;
}

Why this give 27,6 as output?

Answer 1

Undefined Behavior.

Compile with gcc with the following command :

gcc filename.c -save-temps

Now look intro your directory where you have saved this c file. Open the filename.i file. It would be large preprocessed file. Go to the end of the file and there you can see how your macros are being replaced with b++.

a = (b++*b++*b++);

This creates undefined behaviour. 

You can execute your program here:

http://codepad.org/

Output:27,6

http://ideone.com/

Output:60,6

https://gateoverflow.in/62411/what-is-the-output