Misunderstand line-buffer in Unix

Question

I'm reading Advanced Programming in the UNIX Environment, 3rd Edition and misunderstanding a section in it (page 145, Section 5.4 Buffering, Chapter 5).

Line buffering comes with two caveats. First, the size of the buffer that the standard I/O library uses to collect each line is fixed, so I/O might take place if we fill this buffer before writing a newline. Second, whenever input is requested through the standard I/O library from either (a) an unbuffered stream or (b) a line-buffered stream (that requires data to be requested from the kernel), all line-buffered output streams are flushed. The reason for the qualifier on (b) is that the requested data may already be in the buffer, which doesn’t require data to be read from the kernel. Obviously, any input from an unbuffered stream, item (a), requires data to be obtained from the kernel.

I can't get the bold lines. My English isn't good. So, could you clarify it for me? Maybe in an easier way. Thanks.

Answer 1

The point behind the machinations described is to ensure that prompts appear before the system goes into a mode where it is waiting for input.

If an input stream is unbuffered, every time the standard I/O library needs data, it has to go to the kernel for some information. (That's the last sentence.) That's because the standard I/O library does not buffer any data, so when it needs more data, it has to read from the kernel. (I think that even an unbuffered stream might buffer one character of data, because it would need to read up to a space character, for example, to detect when it has reached the end of a %s format string; it has to put back (ungetc()) the extra character it read so that the next time it needs a character, there is the character it put back. But it never needs more than the one character of buffering.)

If an input stream is line buffered, there may already be some data in its input buffer, in which case it may not need to go to the kernel for more data. In that case, it might not flush anything. This can occur if the scanf() format requested "%s" and you typed hello world; it would read the whole line, but the first scan would stop after hello, and the next scanf() would not need to go to the kernel for the world word because it is already in the buffer.

However, if there isn't any data in the buffer, it has to ask the kernel to read the data, and it ensures that any line-buffered output streams are flushed so that if you write:

printf("Enter name: ");
if (scanf("%63s", name) != 1)
    …handle error or EOF…

then the prompt (Enter name:) appears. However, if you'd previously typed hello world and previously read just hello, then the prompt wouldn't necessarily appear because the world was already waiting in the (line buffered) input stream.