[游戏开发]CMU Computer Systems: System-Level I/O

I/O

• Unix I/O
• RIO (robust I/O) package
• Metadata, sharing, and redirection
• Standard I/O
• Closing remarks

Unix I/O Overview

• A Linux file is a sequence of m bytes
• Cool fact: All I/O devices are represented as files
• Even the kernel is represented as a file
• Elegant mapping of files to devices allows kernel to export simple interface called Unix I/O

File Types

• Each file has a type indicating its role in the system
  • Regular file: Contains arbitrary data
  • Directory: Index of a related group of files
  • Socket: For communicating with a process on another machine
• Other file types beyond our scope
  • Named pipes
  • Symbolic links
  • Character and block devices

Regular Files

• A regular file contains arbitrary
• Applications often distinguish between text files and binary files
  • Text files are regular files with only ASCII or Unicode characters
  • Binary files are everything else
  • Kernel doesn’t know the difference
• Text file is sequence of text lines
  • Text line is sequence of chars terminated by newline char(’\n’)
• End of line indicators in other systems
  • Linux and Mac OS: ‘\n’
  • Windows and Internet protocols: ‘\r\n’

Directories

• Directory consists of an array of links
  • Each link maps a filename to a file
• Each directory contains at least two entries
  • . is a link to itself
  • … is a link to the parent directory in the directory hierarchy
• Commands for manipulating directories
  • mkdir: create empty directory
  • ls: view directory contents
  • rmdir: delete empty directory

Directory Hierarchy

• All files are organized as a hierarchy anchored by root directory named /
• Kernel maintains current working directory for each command

Opening Files

• Opening a file informs the kernel that you are getting ready to access that file
• Returns a small identifying integer file descriptor
  • fd == -1 indicates that an error occurred
• Each process created by a Linux shell begins life with three open files associated with a terminal
  • 0: standard input
  • 1: standard output
  • 2: standard error

Closing Files

• Closing a file informs the kernel that you are finished accessing that file
• Closing an already closed file is a recipe for disaster in threaded programs
• Moral: Always check return codes, even for seemingly benign functions such as close()

Reading Files

• Reading a file copies bytes from the current file position to memory, and then updates file position
• Returns number of bytes read from file rd into buf
  • Return type ssize_t is signed integer
  • nbytes < 0 indicates that an error occurred
  • Short counts (nbytes < sizeof (buf) ) are possible and are not errors

Writing Files

• Writing a file copies bytes from memory to the current file position, and then updates current file position
• Returns number of bytes written from buf to file fd
  • nbytes < 0 indicates that an error occurred
  • As with reads, short counts are possible and are not errors

On Short Counts

• Short counts can occur in these situations
  • Encountering EOF on reads
  • Reading text lines from a terminal
  • Reading and writing network sockets
• Short counts never occur in these situations
  • Reading from disk files (except for EOF)
  • Writing to disk files
• Best practice is to always allow for short counts

The RIO Package

• RIO is a set of wrappers that provide efficient and robust I/O in apps, such as network programs that are subject to short counts
• RIO provides two different kinds of function
  • Unbuffered input and output of binary data
    • rio_readn and rio_writen
  • Buffered input of text lines and binary data
    • rio_realineb and rio_readnb
    • Buffered RIO routines are thread-safe and can be interleaved arbitrarily on the same descriptor

File Metadata

• Metadata is data about data, in this case file data
• Per-file metadata maintained by kernel
  • accessed by users with the stat and fstat functions

Pros and Cons of Unix I/O

• Pros
  • Unix I/O is the most general and lowest overhead form of I/O
    • All other I/O packages are implemented using Unix I/O functions
  • Unix I/O provides functions for accessing file metadata
  • Unix I/O functions are async-signal-safe and can be used safely in signal handlers
• Cons
  • Dealing with short counts is tricky and error prone
  • Efficient reading of text lines requires some form of buffering, also tricky and error prone
  • Both of these issues are addresses by the standard I/O and RIO

Pros and Cons of Standard I/O

• Pros
  • Buffering increases efficiency by decreasing the number of read and writes system calls
  • Short counts are handled automatically
• Cons
  • Provides no function for accessing file metadata
  • Standard I/O functions are not async-signal-safe, and not appropriate for signal handlers
  • Standard I/O is not appropriate for input and output on network sockets
    • There are poorly documented restrictions on streams that interact badly with restrictions on sockets

Choosing I/O Functions

• General rule: use the highest-level I/O functions you can
  • Many C programmers are able to do all of their work using the standard I/O functions
• When to use standard I/O
  • When working with disk or terminal files
• When to use raw Unix I/O
  • Inside signal handlers, because Unix I/P is async-signal-safe
  • In rare cases when you need absolute highest performance
• When to use RIO
  • When you are reading and writing network sockets
  • Avoid using standard I/O on sockets