However, most applications will autoflush STDOUT when it's connected to a tty.

GNU libc by default opens stdout in line buffered mode if it's tty, and stderr is opened in unbuffered mode. The following code will demonstrate behavior:

#include <stdio.h>
#include <unistd.h>

main() {
    printf("This is stdio output");
    fprintf(stderr, "This is stderror output");
    sleep(5);
}
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On my Ubuntu it outputs "This is stderror output" and 5 seconds later "This is stdio output"