buffer confusion

Question

Could anyone clarify on the types of buffers used by a program?

For eg:

I have a C program that reads from a stdin to stdout.

What are the buffers involved here? I'm aware that there are 2. One provided by the kernel on which a user don't have any control. One provided with standard streams namely stdout, stdin and stderr. Each having a separate buffer.

Is my understanding correct?

Thanks, John

Answer 1

If you are working on linux/unix then you could more easily understand that there are three streams namely

1.STDIN: FILE DESCRIPTOR VALUE 0 (IN unix)

2.STDOUT :FILE DESCRIPTOR VALUE 1

3.STDERR :FILE DESCRIPTOR VALUE 2

By default these streams correspond to keyboard and monitor.In unix we can change these streams to read input from file instead of keyboard.To display output on a file rather than monitor using close(),dup() system calls.Yes there are 3 buffers involved.To clear the contents of input buffer in c we use fflush() function. If you want to know more about handling these streams in UNIX then let me Know.

Answer 2

The kernel (or other underlying system) could have any number of layers of buffering, depending on what device is being read from and the details of the kernel implementation; in some systems there is no buffering at that level, with the data being read directly into the userspace buffer.

The stdio library allocates a buffer for stdin; the size is implementation-dependent but you can control the size and even use your own buffer with setvbuf. It also allows you to control whether I/O is fully buffered (as much data is read into the buffer as is available), line buffered (data is is only read until a newline is encountered), or unbuffered. The default is line buffering if the system can determine that the input is a terminal, else fully buffered.

The story is similar for stdout. stderr is by default unbuffered.