Buffered and unbuffered stream

Question

In case of buffered stream it said in a book that it wait until the buffer is full to write back to the monitor. For example:

cout << "hi"; 

1. What do they mean by "the buffer is full".

   cerr << "hi";
   

2. It is said in my book that everything sent to cerr is written to the standard error device immediately, what does it mean?

   char *ch;
   cin>> ch; // I typed "hello world";
   

3. In this example ch will be assigned to "hello" and "world" will be ignored does it mean that it still in the buffer and it will affect the results of future statements?

Answer 1

Your book doesn't seem very helpful.

1) The output streams send their bytes to a std::streambuf, which may contain a buffer; the std::filebuf (derived from streambuf) used by and std::ofstream will generally be buffered. That means that when you output a character, it isn't necessarily output immediately; it will be written to a buffer, and output to the OS only when the buffer is full, or you explicitly request it in some way, generally by calling flush() on the stream (directly, or indirectly, by using std::endl). This can vary, however; output to std::cout is synchronized with stdout, and most implementations will more or less follow the rules of stdout for std::cout, changing the buffering strategy if the output is going to an interactive device.

At any rate, if you're unsure, and you want to be sure that the output really does leave your program, just add a call to flush.

2) Your book is wrong here.

One of the buffering strategies is unitbuf; this is a flag in the std::ostream which you can set or reset (std::ios_base::set() and std::ios_base::unset()—std::ios_base is a base class of std::ostream, so you can call these functions on an std::ostream object). When unitbuf is set, std::ostream adds a call to flush() to the end of every output function, so when you write:

std::cerr << "hello, world";

the stream will be flushed after all of the characters in the string are output, provided unitbuf is set. On start-up, unitbuf is set for std::cerr; by default, it is not set on any other file. But you are free to set or unset it as you wish. I would recommend against unsetting it on std::cerr, but if std::cout is outputting to an interactive device, it makes a lot of sense to set it there.

Note that all that is in question here is the buffer in the streambuf. Typically, the OS also buffers. All flushing the buffer does is transfer the characters to the OS; this fact means that you cannot use ofstream directly when transactional integrity is required.

3) When you input to a string or a character buffer using >>, the std::istream first skips leading white space, and then inputs up to but not including the next white space. In the formal terms of the standard, it "extracts" the characters from the stream, so that they will not be seen again (unless you seek, if the stream supports it). The next input will pickup where ever the previous left off. Whether the following characters are in a buffer, or still on disk, is really irrelevant.

Note that the buffering of input is somewhat complex, in that it occurs at several different levels, and at the OS level, it takes different forms depending on the device. Typically, the OS will buffer a file by sectors, often reading several sectors in advance. The OS will always return as many characters as were demanded, unless it encounters end of file. Most OSs will buffer a keyboard by line: not returning from a read request until a complete line has been entered, and never returning characters beyond the end of the current line in a read request.

In the same manner as std::ostream uses a streambuf for output, std::istream uses one to get each individual character. In the case of std::cin, it will normally be a filebuf; when the istream requests a character, the filebuf will return one from its buffer if it has one; if it doesn't, it will attempt to refill the buffer, requesting e.g. 512 (or whatever its buffer size is) characters from the OS. Which will respond according to its buffering policy for the device, as described above.

At any rate, if std::cin is connected to the keyboard, and you've typed "hello world", all of the characters you've typed will be read by the stream eventually. (But if you're using >>, there'll be a lot of whitespace that you won't see.)