How to calculate the time required for a block of code in C/C++?

Here are a small code snippets with which you can calculate the execution time of a certain block of codes in C/C++.

#include <stdio.h>
#include <time.h>
#include <stdlib.h>
clock_t startm, stopm;
#define START if ( (startm = clock()) == -1) {printf("Error calling clock");exit(1);}
#define STOP if ( (stopm = clock()) == -1) {printf("Error calling clock");exit(1);}
#define PRINTTIME printf( "%6.3f seconds used by the processor.", ((double)stopm-startm)/CLOCKS_PER_SEC);
int main() {
     START;//start the timer
     //code blocks which execution time will be counted begins here
     int n;
     for(n = 0; n<=65535; n++)
     {
        printf("%d\n", n);
     }
     //code blocks which execution time will be counted ends here
     STOP;//end the timer
     PRINTTIME;//print the time
     return 0;
}

The program will print total 65536 numbers line by line and at the end it’ll print the total seconds e.g 5.109 seconds, it took to print the numbers
—–courtesy: http://kennethfinnegan.blogspot.com/2008/03/timing-events-in-c.html

Basic Programming Quiz 0010

Examine the code snippets given below—-

#include <stdio.h> 
int main()
{ 
    char ch;
    while((ch = getchar()) != '\0')
    putchar(ch);
    return 0;
}

What does the program display?

Look at another code snippets—-

#include <stdio.h> 
int main()
{ 
    while( putchar(getchar( )) != '\0' ) ;
    return 0;
}

Now, what does it display?
Is there any difference in program execution between these two code snippets?

Basic Programming Quiz 0001

Examine the code snippets given below—-

#include <stdio.h> 
int main()
{ 
  int x;
  x = 10;
  if(x == 10) { 
    int x; 
    x = 99; 
    printf("Inner x: %d\n", x);
  }
  printf("Outer x: %d\n", x); 
  return 0;
}

What does the program displays?