Bufferring Behaviour of stdout(Stardard Out)

Recently, I encounter a problem of capturing stdout of a program. The program is expected to run for a long time. When the output of the program is on the console, everything works fine. However, when the output of the program is redirected to a pipe or a file, the output is buffered for a long time. This means the output of the program is not appeared immediately. This behaviour is not desired. To illustrate the problem, I write the following simple program:

int main(int argc, char** argv)
{
    while (1)
    {
            sleep(1);
            printf("hello!\n");
    }
}

The program prints "hello!" for every second. However, the output will be buffered if it is
redirected to a pipe, like:
a.out | tee tmp.log
After some investigation, I found the behaviour is documented in setbuf(3) man page:

The three types of buffering available are unbuffered, block buffered, and line buffered. When an output stream is unbuffered, information appears on the destination file or terminal as soon as written; when it is block buffered many characters are saved up and written as a block; when it is line buffered characters are saved up until a newline is output or input is read from any stream attached to a terminal device (typically stdin).... Normally all files are block buffered. When the first I/O operation occurs on a file, malloc(3) is called, and a buffer is obtained. If a stream refers to a terminal (as stdout normally does) it is line buffered. The standard error stream stderr is always unbuffered by default....

There are two ways to solve the problem. First, call fflush(stdout) after printf.
Second, call setlinebuf(stdout) at the startup of the program.