Files

1
Chapter 7

• Files

2
File Identifiers

• File Identifier made of name and extension
• Name can usually be at least eight characters
  long, and usually alpha-numeric characters
• Extension is usually up to three characters long,
  and used to group files by type
• Some operating systems are case-sensitive

PROGRAM.C PROGRAM.EXE lab9.c
LAB9.C stdio.h stdlib.h
3
Buffered I/O

• Secondary storage is accessed in blocks
• So when we want to access a small part of it, we
  bring the entire block into memory
• The block is temporarily stored in a buffer
• ANSI-C has functions that handle buffering so
  that it is transparent to us
• We keep track of the byte position in the file
  with a file position indicator

4
Opening Files

• We use the FILE data type to let C know that we
  intend to use a file
• The function fopen() prepares a file for use
• FILE fopen(char file_id, char mode)
• The file_id is the path, file name, and extension
• The mode is how we intend to use it
• The return value is the file pointer
• FILE payfile /Declare a file
  variable/
• payfile fopen("apayroll.txt", "r") /Open
  the file /

5
File Modes

• File mode is a text string
• Mode "r" opens an existing file for reading
• File position indicator placed at beginning
• Mode "w" creates a new file for writing
• File position indicator placed at beginning
• Mode "a" opens an existing file (or creates a new
  one) for appending
• File position indicator placed at end

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File Modes (Cont ..)

• Mode "r" or "w" or "a" opens a file for both
  reading and writing
• Mode "rb" or "rb" or "rb" opens an existing
  binary file for reading
• Mode "wb" or "wb" or "wb" creates a new binary
  file for writing
• Mode "ab" or "ab" or "ab" opens an existing
  binary file (or creates a new one) for appending

7
Closing a File

• Closing the file writes the contents of the
  buffer to secondary storage and de-allocates
  memory for the buffer and file pointer
• The function fclose() closes a file
• int fclose(FILE file)
• Files are automatically closed at normal program
  termination, including exit()
• They are not closed when the program crashes

8
Accessing Files

• Use fprintf() to write to a file
• int fprintf(FILE file, char control,
  arguments)
• Example
• FILE inFile
• inFile fopen(data.txt, r)
• fprintf(inFile, "Pay .2lf\n", pay)
• Use fscanf() to read from a file
• int fscanf(FILE file, char control,
  arguments)
• Example
• fFILE outFile
• outFile fopen(results.txt, w)
• scanf(outFile, "lf", pay)

9
Example of File Access
include ltstdio.hgt include ltstdlib.hgt void
main(void) FILE inFile int i double
one, two inFile fopen("name.txt", "r")
for ( i1 ilt4 i) fscanf(inFile,
"lf lf", one, two) printf("lf lf\n",
one, two) fclose(inFile)
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Single Character Access

• Single character access from the keyboard and to
  the screen with getchar() and putchar()
• With files it is getc() and putc()
• int getc(FILE file)
• int putc(int character, FILE file)
• Both return EOF if an error occurs

11
Function Error Returns

• Function fscanf() returns EOF if it encounters
  the end of the file
• Function getc() returns EOF on any error
  (including end-of-file)

12
The End-of-File Function

• Function feof() checks to see if the end-of-file
  indicator is set
• int feof(FILE file)
• The end-of-file indicator is usually set by an
  attempt to read at or beyond the end of the file
• Returns a nonzero value if the end-of-file
  indicator is set (end-of-file reached), or zero
  if it is not set (end-of-file not reached)

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Example of using EOF with fscanf()

• include ltstdio.hgt
• include ltstdlib.hgt
• void main(void)
• FILE inFile
• int end 1
• double one, two
• inFile fopen("name.txt", "r")
• while ( end ! EOF )
• end fscanf(inFile, "lf lf", one,
  two)
• if (end EOF)
• break
• printf("lf lf\n", one, two)
• fclose(inFile)

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Example of feof()

• include ltstdio.hgt
• include ltstdlib.hgt
• void main(void)
• FILE inFile
• int end 0
• double one, two
• inFile fopen("name.txt", "r")
• while ( end 0 )
• fscanf(inFile, "lf lf", one, two)
• printf("lf lf\n", first, last)
• end feof(inFile)
• if (end ! 0)
• break
• fclose(inFile)

15
My favorite

• include ltstdio.hgt
• include ltstdlib.hgt
• void main(void)
• FILE inFile
• double one, two
• inFile fopen("name.txt", "r")
• while ( fscanf(inFile, lf lf, one, two)
  ! EOF)
• printf("lf lf\n", first, last)
• fclose(inFile)