write() is an OS system call, while printf() is a C function call. C libraries can (and do) choose to buffer printf, while read/write are usually unbuffered.

write() is an OS system call, while printf() is a C function call. C libraries can (and do) choose to buffer printf, while read/write are usually unbuffered. Of course, both are mechanisms are modifying some global state (your terminal's state), and cannot be relied to be free of race-conditions.