c - Why is the line following printf(), a call to sleep(), executed before anything is printed?

Question

I thought I was doing something simple here, but C decided to go asynchronous on me. I'm not sure what's going on. Here's my code:

#include <stdio.h>
int main() {
    printf("start");
    sleep(5);
    printf("stop");
}

When I compile and run, I notice that sleep(5) works like a charm. But the compiler decided it was a good idea to skip the first printf() and go out of order, so when running, the program waits for 5 seconds and then prints startstop.

What's the deal? My theory is that the program initiates the print operation with the shell, then continues with the program, leaving Bash to wait until the program is no longer busy to actually render the strings. But I really don't know.

Thanks

Answer 1

printf uses buffered output. This means that data first accumulates in a memory buffer before it is flushed to the output source, which in this case is stdout (which generally defaults to console output). Use fflush after your first printf statement to force it to flush the buffered data to the output source.

#include <stdio.h>
int main() {
    printf("start");
    fflush(stdout);
    sleep(5);
    printf("stop");
}

Also see Why does printf not flush after the call unless a newline is in the format string?