printf gives unexpected result

From the printf man page

Return value

Upon successful return, these functions return the number of characters printed (excluding the null byte used to end output to strings).

56 is printed for the inner printf
2 characters were printed so the argument to the next %d format specifier is 2
1 character was printed by the middle printf so the argument to the outer %d format specifier is 1
Only the outer printf includes a newline so the preceding calls output one after another on the same line, giving 5621\n.

printf() returns the number of characters printed:

printf("%d",i) outputs the value 56.
printf("%d",printf("%d",i)) outputs 56 and then 2, the number of characters in 56.
printf("%d\n",printf("%d",printf("%d",i))) outputs 56, then 2, then the number of characters in 2, which is 1.

It's equivalent to

#include <stdio.h> main() { int n, i = 56; n = printf("%d",i); n = printf("%d", n); n = printf("%d\n", n); } 

printf returns the number of characters written.

The printf() function returns the number of characters that it prints on console.

For example after the following printf call, num_chars will have the value 10 as string "Hi haccks\n" consistes of 10 non-nul characters that will be print on screen.

num_chars = printf("Hi haccks\n"); // ^^^^^^^^^ ^ // 12345678910 

Note: \n is single 10^th char. So in above code returned value from printf assigned to num_chars variable.

In your code, in the given statement, inner printf() prints the values and then return number of chars that value printed by outer printf as shown below:

// 1 2 3 printf("%d\n", printf("%d", printf("%d",i))); // Here i = 56 ^ ^ ^ print: 1 print: 2 print: 56 returns: 1 returns: 1 returns: 2 // 3 2 1 <--Order of printf called 

So it outputs 5621