//en.cppreference.com/w/c/io/setbuf.html
#include <stdio.h>
#include <threads.h>
int main(void)
{
setbuf(stdout, NULL); // unbuffered stdout
putchar('a'); // 'a' appears immediately if stdout is unbuffered
thrd_sleep(&(struct timespec){.tv_sec=1}, NULL); // sleep 1 sec
putchar('b');
}