C

1
Streams and files
CSC 298
2
Overview

• Topics
• Streams communicating with the outside world
• Data representation bits and bytes
• Basic C streams

3
Streams

• C model of communication streams
• Sequence of data flowing from a source to a
  program, or from a program to a destination
  (sink)
• Huge variety of stream classes in System.IO
• Highly modular
• Some are data sources/sinks
• Others are wrapper classes that take data from a
  stream and transform it somehow to produce a
  stream with different characteristics

4
Stream Processing

• Basic idea the same for input output

5
Opening Closing Streams

• Before a stream can be used it must be opened
• Create a stream object and connect it to source
  or destination of the stream data
• Often done implicitly as part of creating stream
  objects
• When were done with a stream, it should be
  closed
• Cleans up any unfinished operations, then breaks
  the connection between the program and the data
  source/destination

6
Data Representation

• Underneath its all bits (binary digits 0/1)
• Byte group of 8 binary digits
• Smallest addressable unit of memory
• Meaning depends on interpretation
• 01000001 int 65 char A
• 0011111 int 63 char ?
• 00111100 int 54 char 6
• But its still just bits

7
Some C types (1)

• bool 1 byte (0 false 1 true)
• Integer types
• sbyte 1 byte (-128 to 127)
• byte 1 byte (0 to 255)
• short 2 bytes (-32768 to 32767)
• ushort 2 bytes (0 to 65535)
• int 4 bytes (-231 to 231 - 1)
• uint 4 bytes (0 to 232 - 1)
• long 8 bytes (-263 to 263 - 1)
• ulong 8 bytes (0 to 264 1)
• char 2 bytes Unicode characters w/decimal
  values 0 to 65535

8
C types (2)

• Floating-point (decimal) types
• float 4 bytes approx. 6 decimal digits
  precision
• double 8 bytes approx. 15 decimal digits
  precision
• Other struct and class types
• depends on the struct or class definition (could
  be huge).

9
C Streams

• 2 major sets of stream classes
• Byte streams read/write individual bytes
• Corresponds to physical data network and disk
  I/O streams
• Low-level
• Character streams 2-byte Unicode characters
• Primary text input/output stream classes

10
Basic Abstract stream classes

• Byte stream Stream
• Character streams TextReader, TextWriter

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Basic Reader/Writer Operations

• Reader
• int Read()// return Unicode value of next
  // character
• string ReadLine()// return a line of text
• Convention returns -1 if no data available (end
  of file)
• Writer
• void Write(char c) // write character c
• void WriteLine(string s)// write string s
• All can throw IOExceptions
• Normally throws a specific subclass of
  IOException depending on the actual error

12
File I/O

• Idea Create a data object that manages the file
  stream
• Member functions support input and output
  operations
• Files normally opened when these objects are
  created
• Close files explicitly when done to be sure
  things terminate cleanly
• Basic classes
• FileStream derived from Stream
• StreamReader derived from TextReader
• StreamWriter derived from TextWriter

13
Copy File One byte at a Time

• class FileCopy
• public static void Main()
• FileStream inFile new FileStream(_at_c\sourc
  e.txt, FileMode.Open, FileAccess.Read)
• FileStream outFile new FileStream(_at_c\sink
  .txt, FileMode.OpenOrCreate, FileAccess.Write)
• int b//current byte (an int since it can be
  -1 for EOF)
• b inFile.ReadByte()
• while (b ! -1) // end of file?
• outFile.WriteByte(b)
• b inFile.ReadByte()
• inFile.Close()
• outFile.Close()

14
Critique

• Problems with this?
• Very low-level would like to work with more
  convenient chunks like lines of text
• Not very useful to hard-wire file names into the
  code
• What about possible exceptions?
• See full code on the class web site

15
Command Line Arguments

• Arguments can be supplied on command line when
  program is executed
• FileCopy c\input.txt c\output.txt
• These values are found in the String array
  parameter to main
• public static void Main(String args)
• if (args.length ! 2)
• Console.WriteLine
• (Must have 2 file names for arguments)
• return
• // open files using names in args0 and//
  args1, etc...

16
Higher-Level Input StreamReader

• Can be wrapped around a Stream, or can be created
  directly from a file
• Provides ReadLine() method to read next line and
  return it as a single stream
• Returns null if end-of-file reached
• Handles newline portability problems dont try
  to handle newlines yourself take advantage of
  this
• FileStream inFile new FileStream(fileName,
  FileMode.Open, FileAccess.Read)
• StreamReader sr new StreamReader(inFile)
• String line sr.readLine()
• // Could do new StreamReader(fileName)

17
StreamWriter

• Same as StreamReader but for output
• WriteLine(s) // write a string on one line

18
Exception Handling

• Most I/O functions can throw subclasses of
  IOException
• Good practice handle common cases and generate
  a user-friendly error message
• Input file not found
• Unable to open output file
• etc.

19
BinaryReader and Writer

• A wrapper of FileStream able to deal directly
  with basic types (int, double, etc)
• Use the methods ReadBoolean, ReadInt, etc for
  input
• Use the method Write() (with many overloads)
  for output
• Useful to read and write objects from and to
  files. Serialization is another approach to read
  and write objects from and to files.