5.4. Buffering
The goal of the buffering provided by the standard I/O library is to use the minimum number of read and write calls. (Recall Figure 3.5, where we showed the amount of CPU time required to perform I/O using various buffer sizes.) Also, it 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 standard I/O library that generates the most confusion is its buffering.Three types of buffering are provided:Fully buffered. In this case, actual I/O takes place when the standard I/O buffer is filled. Files residing on disk are normally fully buffered by the standard I/O library. The buffer used is usually obtained by one of the standard I/O functions calling malloc (Section 7.8) the first time I/O is performed on a stream.The term flush describes the writing of a standard I/O buffer. A buffer can be flushed automatically by the standard I/O routines, such as when a buffer fills, or we can call the function fflush to flush a stream. Unfortunately, in the UNIX environment, flush means two different things. In terms of the standard I/O library, it means 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 discard the data that's already stored in a buffer.Line buffered. In this case, the standard I/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 standard I/O fputc function), 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: standard input and standard output, for example.Line buffering comes with two caveats. First, the size of the buffer that the standard I/O library is using 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 Section 3.8.The standard error stream, for example, is normally unbuffered. This is so that any error messages are displayed as quickly as possible, regardless of whether they contain a newline.ISO C requires the following buffering characteristics.- Standard input and standard output are fully buffered, if and only if they do not refer to an interactive device.
- Standard error is never fully buffered.
This, however, doesn't tell us whether standard input and standard output can be unbuffered or line buffered if they refer to an interactive device and whether standard error should be unbuffered or line buffered. Most implementations default to the following types of buffering.- Standard error is always unbuffered.
- All other streams are line buffered if they refer to a terminal device; otherwise, they are fully buffered.The four platforms discussed in this book follow these conventions for standard I/O buffering: standard error is unbuffered, streams open to terminal devices are line buffered, and all other streams are fully buffered.
We explore standard I/O buffering in more detail in Section 5.12 and Figure 5.11.If we don't like these defaults for any given stream, we can change the buffering by calling either of the following two functions.[View full width]#include <stdio.h>
void setbuf(FILE *restrict fp , char *restrict buf );
int setvbuf(FILE *restrict fp , char *restrict buf ,
 int mode ,
size_t size );
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