Input Output

1
Input / Output
2
General Input and Output

• I/O support is support is provided through
  collections of function libraries.
• Low level I/O
• open( )
• close( )
• read( )
• write( )
• lseek( )
• ioctl( )

3
General Input and Output

• I/O support is support is provided through
  collections of function libraries.
• Standard library I/O
• fopen( ), fclose( ) - opening and closing
  files
• fscanf( ), fprintf( ) - field at a time
  input/output with
  data conversion
• fgetc( ), fputc( ) - character (byte) at a
  time input/output
• fgets( ), fputs( ) - line at a time
  input/output
• fread( ), fwrite, - physical block at a
  time i/o fseek( )

4
Files

• The I/O library functions operate on ADT's of
  type FILE (which is defined in stdio.h).

5
Files

• A collection of related data treated as a unit.
• Two types
• Text
• Binary
• Stored in secondary storage devices.
• Buffer
• Temporary storage area that holds data while they
  are being transferred to or from memory.

6
Text Files

• Data are stored as human-readable characters.
• Each line of data ends with a newline character.

7
Standard Files

• Three special FILE 's are automatically opened
  when any program starts
• stdin standard input
• Normally associated with the keyboard (but may be
  redirected with lt)
• stdout (standard output)
• Normally associated with terminal output (but may
  be redirected with gt)
• Accessed using stdout
• stderr (standard error)
• Normally terminal output

8
Standard Files

• stdin, stdout, and stderr are predeclared and
  preopened files.
• They must not be declared nor opened in your
  program.

9
User Files

• External files defined by the user
• Must be explicitly opened in the program.
• C assigns a logical name to each external file.
• fopen function
• Prepares a file for processing
• Makes the connection between the external file
  and the program.
• Creates a program file table to store the
  information needed to process the file.

10
fopen

• fopen("filename", "mode")
• Filename string that supplies the name of the
  file as known to the external world. e.g.
  scores.dat

11
Example

• / inOut.c /
• include ltstdio.hgt
• int main(int argc, char argv )
• FILE inFile
• FILE outFile
• int number
• inFile fopen("prog1.dat", "r")
• if(inFile NULL)
• fprintf(stderr, r"input failure. exiting
  program\n")
• exit(1)
• outFile foen("prog1.out", "w")
• if(outFile NULL)
• fprintf(stderr, "output failure. exiting
  program")
• exit(2)

12
Example

• fscanf(inFile, "d", number)
• while (!feof(inFile)
• fprintf(stdout, "d\n", number)
• fprintf(outFile, "d\n", number)
• fscanf(inFile, "d", number)
• fclose(inFile)
• fclose(outFile)
• return 0

13
fopen

• Always check to make sure the open was
  successful. If not, print an error message and
  exit. For example,
• FILE input fopen("data.dat", "r")
• if(input null)
• fprintf(stdout, "Failure to open file.
  Exiting\n")
• exit(1)

14
fclose

• Used to close a file when no longer needed.
• Prevents associated file from being accessed
  again.
• Guarantees that data stored in the stream buffer
  is written to the file.
• Releases the FILE structure so that it can be
  used with another file.
• Frees system resources, such as buffer space.

15
fclose

• Examples
• fclose(inFile)
• fclose (outFile)

16
Formatted Input Functions

• Used to read and convert a stream of characters
  and store the converted values in a list of
  variables found in the address list.
• scanf
• scanf ("format string", address list)
• Reads text data from standard input
• fscanf
• fscanf(fp, "format string", address list)
• Reads input from the specified file.
• fp can be stdin

17
Formatted Output Functions

• Display output in human readable form
• printf
• printf ("format string", value list)
• Writes to standard output or standard error file
• fprintf
• fprintf (fp, "format string", value list)
• Writes to the specified file.
• fp can be stdout

18
Formatting Input/Output Functions

• Format strings
• Consist of three types of data
• Whitespace
• Text characters
• Field specification
• These may be repeated

19
Field Specification

• Consists of character, a conversion code, and
  other formatting instructions.
• With one exception (), each field specification
  must have a matching parameter in the parameter
  list that follows the string.
• The type in the field specification and the type
  of the parameter must match.
• Can have up to six elements for output.
• Can have up to five elements for input precision
  is not allowed.

20
Field Specification

• Elements of field specification
• scanf / fscanf

do not store data
h short l long int l double (scan only) L long
double
d, f, c, s, e, E, x, X, i, u, g, G

• left justify
• sign ( or -)
• space if positive
• 0 zero padding

printf / fprintf
21
Character Input Functions

• Read one character at a time from a text stream.
• getchar
• int getchar ( )
• Reads the next character from the standard input
  stream and returns its value.
• Return type is int

22
Character Input Functions

• getc and fgetc
• int getc (FILE fpIn)
• int fgetc (FILE fpIn)
• Reads the next character (byte) from the file
  stream.
• Return type is int

23
Character Output Functions

• Write one character (byte) at a time
• putchar
• int putchar (int outChar)
• Writes the parameter to standard output.
• If successful, returns the character written.

24
Character Output Functions

• putc and fputc
• int putc (int oneChar, FILE fpOut)
• int fputc (int oneChar, FILE fpOut)
• Write the parameter to the specified file stream.
• If the character is successfully written, the
  function returns it.

25
Character Input/Output Functions

• While fgetc() and fputc() are fine for building
  interactive applications, they are very
  inefficient and should never be used for reading
  or writing a large volume of data such as a
  photographic image file.

26
String Input/Output

• gets
• gets (bufferAddr)
• Takes a line (terminated by a newline) from
  standard input.
• Converts newline to \0
• If successful, returns the string

27
String Input

• fgets
• fgets (bufferAddr, int buffSize, FILE fp)
• Takes a line (terminated by a newline) from the
  specified file pointer.
• Puts newline (\n) in the string and appends \0 at
  the end of the string
• If successful, returns the string

28
String Output

• puts
• int puts (buff)
• Takes a null-terminated string from memory and
  writes it to standard output.
• Changes \0 to \n

29
String Output

• fputs
• int fputs (buff, FILE fp)
• Takes a null-terminated string from memory and
  writes it to the specified file pointer.
• Drops \0
• Programmer's responsibility to make sure the
  newline is present at the appropriate place.

30
End of File Controlled Loops

• feof
• Function to check if end of file has been
  reached.
• For an end of file controlled loop
• Read before the loop
• Test for end of file while (!feof(ptr))
• Process
• Read at the bottom of the loop

31
Processing Heterogeneous Input Files

• While scanf( ) provides a convenient way to
  process input files that have a predefined
  format, handling mixed character and numeric
  input that do not have a definite format can be
  more difficult.
• An example of such an input is a .ppm image file.

32
Processing Heterogeneous Input Files

• a .ppm image file begins with a header of the
  following format
• P6
• Program 1
• CpSc 101
• 400 300
• max value for a pixel
• 255

33
.ppm Image Files

• Items of useful information in the header include
• P - this is a .ppm file
• 6 - this is a .ppm file containing a color
  (as opposed to a grayscale (P5)) image
• 400 - the number of columns of pixels in each
  row of the image
• 300 - the number of rows of pixels in the
  image
• 255 - the maximum brightness value for a pixel
  
• Comment lines
• Lines beginning with a are comments
• Any number (including 0) of comment lines
  may be present.

34
.ppm Image Files

• Example headers
• Example 1
• P6 400 300 255
• Example 2
• P6
• width height
• 400 300
• 255

35
.ppm Image Files

• Example headers
• Example 3
• P6
• width
• 400
• 300
• 255

36
Processing Heterogeneous Input Files

• Exercise Write a program that can read general
  ppm headers.

37
Reading .ppm headers with fgets() and sscanf

• Because of the arbitrary location of comment
  lines and the number of comment lines, there is
  no obvious way to read the header data using
  scanf().
• An alternate approach is to use a combination of
  fgets() and sscanf()
• sscanf( ) consumes data from a memory resident
  buffer instead of consuming data from a file.
• sscanf( ) returns the number of items it
  successfully consumed from the buffer.

38
Reading .ppm headers with fgets() and sscanf

• Suppose the header file consists of the
  following
• P6 800 600 255

39
Reading .ppm headers with fgets() and sscanf

• The following program would suffice to read it.
• include ltstdio.hgt
• int main(int argc, char argv )
• char id3 0, 0, 0
• long values3
• char count 0
• char buff256
• FILE inFile fopen("image.ppm", "r")
• fgets(buff, 256, inFile)
• id0 buff0
• id1 buff1
• count sscanf(buf2, "d d d",
  values1, values2, values3)
• fprintf(stderr, "obtained d values\n",
  count)
• fprintf(stderr, "d d d\n", values1,
  values2, values3)
• return 0

40
Reading .ppm headers with fgets() and sscanf

• The preceeding program will not work if the .ppm
  header is formatted as follows
• P6
• comment 1
• 800
• comment 2
• 600 255
• In this case, buff will contain P6\n at line 12
  and sscanf() will return 0

41
Reading .ppm headers with fgets() and sscanf

• We need a loop that will invoke fgets() to read
  a new line of input into buff.
• sscanf() will attempt to consume 3 integers from
  the buffer.
• The loop should end when 3 integers have been
  placed in the values array or when there are no
  more values to read.

42
Block input / output

• fread( ) and fwrite( ) functions are the most
  efficient way to read or write large amounts of
  data.
• fread( ) reads a specified number of bytes from a
  binary file an places them into memory at the
  specified location.
• prototype for fread ( )
• int fread(void pInArea, int elementSize, int
  count, FILE fp)
• pInArea a pointer to the input area in memory.
• elementSize size of a basic data item, often
  specified using the sizeof operator
• count number of data items
• fp file pointer of an open file.
• fread( ) returns the number of items read.

43
Block input / output

• prototype for fwrite ( )
• int fread(void pOutArea, int elementSize, int
  count, FILE fp)
• pOutArea a pointer to the output area in
  memory.
• elementSize size of a basic data item, often
  specified using the sizeof operator
• count number of data items
• fp file pointer of an open file.
• fwrite( ) copies elementSize count bytes from
  the address specified by pOutArea to the file.
• fwrite( ) returns the number of items written.

44
fread( ) / fwrite( ) example

• FILE inFile
• int itemsRead
• int count 500
• int data500
• int iteration 0
• inFile fopen(argv1, "r")
• if(!inFile)
• printf("File s failed\n", argv1)
• exit(1)
• itemsRead fread(data, sizeof(int), count,
  inFile)
• while (itemsRead ! 0)
• fwrite(data, sizeof(int), itemsRead,
  stdout)
• fprintf(stderr, "d d\n, iteration,
  itemsRead)
• iteration 1
• itemsRead fread(data, sizeof(int), count,
  inFile)