UNIX Filesystems: Evolution, Design, and Implementation

Standard I/O Library Buffering

The stdio library buffers data with the goal of minimizing the number of calls to the read() and write() system calls. There are three different types of buffering used:

Fully (block) buffered. As characters are written to the stream, they are buffered up to the point where the buffer is full. At this stage, the data is written to the file referenced by the stream. Similarly, reads will result in a whole buffer of data being read if possible.

Line buffered. As characters are written to a stream, they are buffered up until the point where a newline character is written. At this point the line of data including the newline character is written to the file referenced by the stream. Similarly for reading, characters are read up to the point where a newline character is found.

Unbuffered. When an output stream is unbuffered, any data that is written to the stream is immediately written to the file to which the stream is associated.

The ANSI C standard dictates that standard input and output should be fully buffered while standard error should be unbuffered. Typically, standard input and output are set so that they are line buffered for terminal devices and fully buffered otherwise.

The setbuf() and setvbuf() functions can be used to change the buffering characteristics of a stream as shown:

#include <stdio.h>

void setbuf(FILE *stream, char *buf);
int setvbuf(FILE 'stream, char *buf, int type, size_t size);

The setbuf() function must be called after the ...