4 F I L E I / O : T H E U N I V E R S A L I / O M O D E L

Summary

In order to perform I/O on a regular file, we must first obtain a file descriptor using open().

I/O is then performed using read() and write().

After performing all I/O, we should free the file descriptor and its associated resources using close().

These system calls can be used to perform I/O on all types of files.

The fact that all file types and device drivers implement the same I/O interface allows for universality of I/O, meaning that a program can typically be used with any type of file without requiring code that is specific to the file type.

For each open file, the kernel maintains a file offset, which determines the location at which the next read or write will occur.

The file offset is implicitly updated by reads and writes.

Using lseek(), we can explicitly reposition the file offset to any location within the file or past the end of the file. Writing data at a position beyond the previous end of the file creates a hole in the file.

Reads from a file hole return bytes containing zeros.

The ioctl() system call is a catchall for device and file operations that don’t fit into the standard file I/O model.