ptg10805159Section 5.4 Buffering 145
5.3 StandardInput, StandardOutput, and StandardError
Three streams arepredefined and automatically available to a process: standardinput,
standardoutput, and standarderror.These streams refer to the same files as the file
descriptors STDIN_FILENO, STDOUT_FILENO,and STDERR_FILENO,respectively,
which we mentioned in Section 3.2.
These three standardI/O streams arereferenced through the predefined file
pointersstdin,stdout,andstderr.The file pointers aredefined in the<stdio.h>
header.5.4 Buffer ing
The goal of the buffering provided by the standardI/O library is to use the minimum
number ofreadandwritecalls. (Recall Figure3.6, which showed the amount of CPU
time required to perform I/O using various buffer sizes.) Also, this library tries to do
its buffering automatically for each I/O stream, obviating the need for the application to
worry about it. Unfortunately,the single aspect of the standardI/O library that
generates the most confusion is its buffering.
Three types of buffering areprovided:- Fully buffered. In this case, actual I/O takes place when the standardI/O
buffer is filled. Files residing on disk arenormally fully buffered by the
standardI/O library.The buffer used is usually obtained by one of the standard
I/O functions callingmalloc(Section 7.8) the first time I/O is performed on a
stream.
The termflushdescribes the writing of a standardI/O buffer.Abuffer can be
flushed automatically by the standardI/O routines, such as when a buffer fills,
or we can call the functionfflushto flush a stream. Unfortunately, in the
UNIX environment,flushmeans two different things. In terms of the standard
I/O library,itmeans writing out the contents of a buffer,which may be partially
filled. In terms of the terminal driver,such as the tcflush function in
Chapter 18, it means to discardthe data that’s already stored in a buffer. - Line buffered. In this case, the standardI/O library performs I/O when a
newline character is encountered on input or output. This allows us to output a
single character at a time (with the standardI/Ofputcfunction), knowing that
actual I/O will take place only when we finish writing each line. Line buffering
is typically used on a stream when it refers to a terminal—standardinput and
standardoutput, for example.
Line buffering comes with two caveats. First, the size of the buffer that the
standardI/O library uses to collect each line is fixed, so I/O might take place if
we fill this buffer beforewriting a newline. Second, whenever input is
requested through the standardI/O library from either (a) an unbuffered stream