The macro va_arg is used to extract an argument from the fixed micro argument list and advance the pointer to the next argument.

Every actual argument list must be known at compile time. In that sense it is not truly a variable argument list.

memcmp compares the first 2 bytes of the blocks dest and src as unsigned chars. So, the ASCII value of 97 is 'a'.

if((i = memcmp(dest, src, 2))==0) When comparing the array dest and src as unsigned chars, the first 2 bytes are same in both variables.so memcmp returns '0'.
Then, the if(0=0) condition is satisfied. Hence the output is "Got it".

scanf() returns the number of variables to which you are provding the input. i = scanf("%d %d", &i, &i); Here Scanf() returns 2. So i = 2.

printf("%d\n", i); Here it prints 2.

Both ceil() and floor() return the integer found as a double.

floor(2.5) returns the largest integral value(round down) that is not greater than 2.5. So output is 2.000000.

ceil(2.5) returns 3, while converting the double to int it returns '0'.
So, the output is '2.000000, 0'.

The gcvt() function converts a floating-point number to a string. It converts given value to a null-terminated string.

#include <stdlib.h>
#include <stdio.h>

int main(void)
{
	char str[25];
	double num;
	int sig = 5; /* significant digits */

	/* a regular number */
	num = 9.876;
	gcvt(num, sig, str);
	printf("string = %s\n", str);

	/* a negative number */
	num = -123.4567;
	gcvt(num, sig, str);
	printf("string = %s\n", str);

	/* scientific notation */
	num = 0.678e5;
	gcvt(num, sig, str);
	printf("string = %s\n", str);

	return(0);
}

Output:
string = 9.876
string = -123.46
string = 67800

In the program, printf() returns the number of charecters printed on the console i = printf("How r u\n"); This line prints "How r u" with a new line character and returns the length of string printed then assign it to variable i.
So i = 8 (length of '\n' is 1).

i = printf("%d\n", i); In the previous step the value of i is 8. So it prints "8" with a new line character and returns the length of string printed then assign it to variable i. So i = 2 (length of '\n' is 1).

printf("%d\n", i); In the previous step the value of i is 2. So it prints "2".