63.1. Overview

Most of the programs that we have presented so far in this book employ an I/O model under which a process performs I/O on just one file descriptor at a time, and each I/O system call blocks until the data is transferred. For example, when reading from a pipe, a read() call normally blocks if no data is currently present in the pipe, and a write() call blocks if there is insufficient space in the pipe to hold the data to be written. Similar behavior occurs when performing I/O on various other types of files, including FIFOs and sockets.

Note:

Disk files are a special case. As described in Chapter 13, the kernel employs the buffer cache to speed disk I/O requests. Thus, a write() to a disk returns as soon as the requested data has been transferred to the kernel buffer cache, rather than waiting until the data is written to disk (unless the O_SYNC flag was specified when opening the file). Correspondingly, a read() transfers data from the buffer cache to a user buffer, and if the required data is not in the buffer cache, then the kernel puts the process to sleep while a disk read is performed.