FILES

1
FILES

• Files types and operations

2
Files

• Files are used to store data
• Data can be
• Text (ASCII only 0 ?127)
• Binary (Full range 0 ? 256)
• Each file resides in a directory
• The path is the directory to get to the file.
• Each file in a directory has a unique filename.

3
File Actions

• Files can be
• Written
• Read
• Appended (Added)
• Files need to be identified before operating on
  them (path/filename)
• Many files can be opened at the same time!

4
Using Files

• Indicate the file to use and how you want to use
  it
• path/filename or filename
• read/write/append
• Perform the write or read operation
• Indicate you have finished using the file.

5
Book Analogy Choose and open book

• You can think of using files in the same way that
  you would use a book from your bookshelf at home.
• In order to use a book (read from it or write to
  it), you must first open the book. This involves
  using the name of the book to find it amongst the
  other books on the bookshelf. Also the type of
  book you will get will depend upon whether you
  want to read it (printed text only eg text book)
  or write to it (blank pages eg note book).
• - This is the same as opening a file.

6
Book Analogy Bookmark position

• Once you have the book you will want to identify
  the book and where you are in the book using a
  bookmark. If you have more than one book then you
  will use more than one bookmark.
• - This is the same as the file pointer.
• You can read from the book or write to the book
  you have chosen. This means that you must use the
  bookmark to identify both the book and the page
  in the book you are using.
• - This is the same since all file access requires
  a file pointer.

7
Book Analogy Close Book

• Once you have finished using the book, you will
  close the book and put it back on the book. You
  can then put the bookmark away.
• - This is the same as closing the file. Any data
  will be written to the file when it is closed.

8
Text Files

• Text files are used to store ASCII data.
• Text files consist of ASCII data and a special
  end of file marker to show the end of the file.
• ctrl-Z in Windows (Ascii character 26)
• ctrl-D in UNIX (Ascii character 4)

• ASCII Text ltEOLgt
• ASCII Text ltEOLgt
• ASCII TextltEOLgt
• ASCII TextltEOLgt
• Sample of an ASCII file! ltEOLgt
• ASCII Text ltEOLgt
• ltEOFgt

9
Text Files

• File operations require special Input/Output
  routines which can be found in ltstdio.hgt
• More than one file can be used at the same time
  in a program.
• Each file needs to be identified by its own file
  pointer.
• A file pointer is used by the OS to keep track of
  the operations upon the file.

10
File Pointers in C

• Each file is referred to by a special variable
  called a file pointer.
• eg FILE fp
• One file pointer is used per file. Therefore if
  more than one file is to be used, then more than
  one file pointer must be declared.
• eg FILE fp_in, fp_out
• The file pointer is assigned when the file is
  opened for use. This requires the use of the
  fopen() function. The open function requires the
  filename and how the file is to be accessed
  (read, write, read/write etc).

11
Text Files

• The file data can be accessed using fgetch(),
  fprintf(), fscanf(), fgets(). These are same as
  the I/O functions you are familiar with, except
  they require a file pointer. The characters in
  the file are read sequentially.
• The file pointer is deassigned when you close a
  file. If you have written any data to the file,
  the file will be updated when the file is closed.

12
Opening Files fopen()

• Files are opened using fopen()
• fopen(filename, mode)
• filename is a string containing the name of the
  file to be opened.
• mode is a string stating how the file is to be
  used
• r read only text file
• w write only text file
• rw read and write text file
• a appending text file. Writes to the end of the
  file.
• rb read binary file.
• wb write binary file.
• rwb read write binary file
• fopen() returns the file pointer if the file
  could be opened.
• fopen() returns a NULL if the file could not be
  opened. If the happens end the program with
  exit(-1).

13
Example of fopen()

• include ltstdio.hgt
• include ltstdlib.hgt
• int main(void)
• FILE in / declare the input file pointer /
• if ( (in fopen("icp.txt","r")) NULL)
• // display message and reason for failure
• perror("Error Cannot open file icp.txt")
• exit(-1)

14
Another example of fopen()

• include ltstdio.hgt
• include ltstdlib.hgt
• int main(void)
• FILE in / declare the input file pointer /
• char filename80
• char error_msg80
• printf("Please enter the file to open")
• scanf("s",filename)
• if ( (in fopen(filename,"r")) NULL)
• // display message and reason for failure with
  filename
• sprintf(error_msg,"Error Cannot open file
  s",filename)
• perror(error_msg)
• exit(-1)

15
Closing Files

• Files are closed using the fclose() function.
• fclose(file_pointer)
• where
• file_pointer is the file_pointer returned from
  fopen().

16
Data Access

• The data is accessed with functions similar to
  the ones you have used before to get data from
  the keyboard.
• The file access functions are fprintf(),
  fscanf(), fgetc(), fputc(), fgets(), fputs().
• The file access functions have a f prefix and
  must have a file pointer as an argument. The file
  pointer is used to identify the file to be
  accessed.
• All access is performed sequentially. The file
  access function automatically take into account
  the position in the file.
• The file access function prototypes are found in
  ltstdio.hgt

17
End of File

• The file is marked by an end of file character.
• EOF is a define in the ltstdio.hgt used to denote
  the End Of File character.
• When the end of file is reached
• character input functions return EOF character
• string input functions return a NULL pointer
• scanf() function returns EOF
• A feof() function can be used to determine
  whether the end of the file has been reached.
  This means that the EOF must have already been
  read by one of the input functions. This must be
  used with binary files.

18
Data Access Functions
19
End of File

• Syntax
• feof(file_pointer)
• where
• file_pointer is used as a marker to indicate the
  file you are working with. For each file there
  must be a new file pointer.
• Returns
• true, if End Of File has been found
  otherwise returns false.

20
Reading Files

• The basic sequence for reading a file is
• declare file pointer
• open the file
• WHILE (NOT the end of the file)
• read data from file
• process the data
• close the file

21
Writing Files

• The basic sequence for writing a file is
• declare file pointer
• open the file
• write the data to the file
• close the file

22
Using the File Pointer with Functions

• The file pointer is declared as FILE name in
  the program.
• When declaring a file pointer in the function
  definition, the declaration for the file pointer
  is
• FILE local_name
• In the prototype use FILE as the data type of
  the file pointer.
• When calling a function with the file pointer,
  just use the name of the file_pointer

23
Example to write 99 to a file

• include ltstdio.hgt
• include ltstdlib.hgt
• void fn(FILE ) / prototype /
• int main(void)
• FILE fp
• / open the file /
• if ( (fp fopen("file.txt","w")) NULL)
• perror("Error Cannot open File file.txt")
• exit(-1)
• fn(fp) / call the function /
• fclose(fp) / close the file /

24
Example (cont)

• / function with the file_pointer passed as an
  argument /
• void fn(FILE file_pointer)
• int value
• value 99
• fprintf(file_pointer, "d", value )

25
Program to print the contents of a text file

• include ltstdio.hgt
• include ltstdlib.hgt
• int main(void)
• char line81 / line to read from file /
• FILE in / declare the input file pointer /
• / open the file /
• if ( (in fopen("file.txt","r")) NULL)
• perror("Error Cannot open File file.txt")
• exit(-1)
• / read and print the contents of the file /
• while (fgets(line,81,in) ! NULL) / NULL means
  EOF /
• printf("s",line)
• / close the file /
• fclose(in)

26
Example to read display a stock list

• This program will read a list of stock from a
  file and display the contents in the required
  format.
• The user must be asked for the filename
• File Format Output Format
• description price\n description price\n
• Eg Eg
• flour 1.25 flour 1.25
• stamps 1.50 stamps 1.50
• dvd 18.23 dvd 18.23

27
Example (cont)

• include ltstdio.hgt
• include ltstdlib.hgt
• int main(void)
• char line81 / line to read from file /
• char fname40 / file name /
• char descript10 / description of item /
• float price / cost of item /
• FILE in / declare the input file pointer
  /
• / get the filename from the user /
• printf("Enter the filename ")
• scanf("s",fname)

28
Example (cont)

• / open the file /
• if ( (in fopen(fname,"r")) NULL)
• perror("Error Cannot open File")
• exit(-1)
• / print the contents of the file /
• while (fscanf(in,"s g",descript,price)! EOF)
  /check the EOF reached /
• printf("s .2g\n",descript,price)
• / close the file /
• fclose(in)

29
Example to display file contents in Ucase

• include ltstdio.hgt
• include ltctype.hgt
• include ltstdlib.hgt
• int main(void)
• char fname40 / file name /
• char ch / character to read/
• FILE in / declare the input file pointer /
• printf("Enter the filename ") / get
  filename from user /
• scanf("s",fname)
• / open the file /
• if ( (in fopen(fname,"r")) NULL)
• perror("Error Cannot open File")
• exit(-1)

30
Example (cont)

• / print the contents of the file in uppercase/
• while ( (ch fgetc(in)) ! EOF) /check for
  EOF /
• printf("c",toupper(ch))
• / close the file /
• fclose(in)

31
Example to write a price list to a file

• include ltstdio.hgt
• include ltstdlib.hgt
• int main(void)
• char fname40 / file name /
• int j / loop counter /
• float price 39.95, 3.22,1.03 / list of
  prices /
• FILE out /declare the output file pointer
  /
• / get the filename from the user /
• printf("Enter the filename ")
• scanf("s",fname)

32
Example (cont)

• / open the file /
• if ( (out fopen(fname,"w")) NULL)
• perror("Error Cannot open File")
• exit(-1)
• / write the prices to the file /
• for (j 0 j lt 3 j)
• fprintf(out, "g",pricej)
• / close the file /
• fclose(out)

33
Binary Files

• Text files contain bytes that contains ASCII
  Values. Text files contain human readable
  information.
• Binary files contain bytes that are between 0 and
  255. Binary files typically are executables, wav
  files, picture formats, mp3, mpegs etc. There
  area a lot of proprietary formats that are
  binary.
• Binary files do not have an EOF character
  (marker) since the EOF is equally likely to occur
  as part of the binary data as any other value.
• The OS stores the filesize as part of its File
  Allocation Tables (FAT / inode).
• Therefore you must use feof() to determine when
  the end of file has been reached.

34
Binary Files

• For binary files, you should only use fgetc() to
  read the binary character.
• Other reading functions eg fscanf() are designed
  only to work with text files.
• For binary files, you should only use fputc() to
  write the binary character.
• Other writing functions eg fprintf() are designed
  only to work with text files.

35
Reading Files

• The basic sequence for reading a binary file is
• declare file pointer
• open the file
• WHILE (NOT feof() )
• read the byte from the file using fgetc(fp)
• process the data
• close the file

36
Writing Files

• The basic sequence for writing a binary file is
• declare file pointer
• open the file
• write the byte to the file using fputc()
• close the file

37
Example to read binary and display in hex

• include ltstdio.hgt
• include ltctype.hgt
• include ltstdlib.hgt
• int main(void)
• char fname40 / file name /
• unsigned char ch / character to read/
• FILE in / declare the input file pointer
  /
• printf("Enter the filename ") / get the
  filename from the user /
• scanf("s",fname)
• / open the binary file for read/
• if ( (in fopen(fname,"rb")) NULL)
• perror("Error Cannot open File")
• exit(-1)

38
Example

• / print the contents of the file in hexadecimal
  space separated/
• while ( !feof(in) ) / check for EOF /
• ch fgetc(in)
• printf("x ",ch)
• / new line /
• printf("\n")
• / close the file /
• fclose(in)