Why does passing the result of printf to another printf work? [closed]

Answer 1

%d is expecting an integer to print it. printf returns the number of printed chars, and you're printing a 5 char string.

It evaluates first the inner print to find out how many character were printed and then it evaluates the outer one printing 5.

Answer 2

This is absolutely fine :-)

The print("earth") outputs earth and return 5 (the number of characters printed).

The other printf gets the 5 as a parameter and outputs it as an integer (because of the %d)

Answer 3

The return value of printf is the number of characters printed. The inner printf is called first. Equivalent to:

int rc = printf("earth");
printf("%d", rc);