Standard Input and Output

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Standard Input and Output
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Overview

• Data communication with a C program and the
  outside world is performed through files
• Files are a non-volatile way to store data by
  means of such media as tape, CD-ROM, floppy disk,
  ZIP disk, hard drive, etc.
• C (just like the UNIX operating system) considers
  all process communication media to be files
• An ordinary file on a disk is considered to be a
  file
• So is the keyboard, the screen, parallel ports,
  and serial ports

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Communication through Files

• Programs access files through basic file
  operations
• Open a file
• Read data from a file
• Write data to a file
• Close a file
• Text files store all data types as character
  bytes the way a program reads the data (either
  text or binary mode) determines how the data is
  interpreted
• Example 12 z rti 456.79 room
• character?, character string?, integer?, floating
  point number?

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stdin, stdout, stderr

• Three files are automatically opened each time a
  C program is run and automatically closed when a
  C program ends stdin, stdout, and stderr
• stdin
• Standard input, default source is the keyboard
• stdout
• Standard output, default destination is the
  screen
• stderr
• Standard error, default destination is the screen
• These three variable names are referred to as
  file descriptors each is a handle (or pointer)
  to a record (i.e., struct) describing the actual
  file

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C Program Entrances and Exits
Constants and variables
main
stdout (Screen)
A
B
C
D
stderr (Screen)
stdin (Keyboard)
X
Y
Z
Instructions and operating procedures
Operations and functions
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Files as Pipes for Data
Files act as pipes to bring a stream of data into
the program and send a stream of data out of the
program
Program to compute average
45 27 15 52
Average is 34.75
stdin
stdout
The program reads data from stdin and writes data
to stdout as if they were ordinary files
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One Byte At a Time
Data is read or written one byte at a time from a
file until the end of the file is reached or
until the file is closed. The file system uses
a pointer to keep track of the next byte to read
or to write
Smith Jack 1045.76 Manager 15 Hanson
Susan 98.62 Operator 7 Jones Nancy
790.25 Administrator 10 Doe Carl 526.71
Technician 12
In this file, the program has read the data as
far as the letter 'J'. The next character to be
read is 'a'
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Functions Using stdin and stdout-1

• Some standard C functions implicitly use stdin
  and stdout
• scanf("d", aNumber)
• aSymbol getchar()
• gets(theBuffer) // High security risk
• printf("Average 5.2f", theAverage)
• putchar(aCharacter)
• puts(aPhrase)

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Functions Using stdin and stdout-2

• Other functions need to have stdin and stdout
  specified as the file descriptor
• fscanf(stdin, "d", aNumber)
• aSymbol fgetc(stdin)
• fgets(theBuffer, sizeof(theBuffer), stdin)
• fprintf(stdout, "Average 5.2f",
  theAverage)
• fputc(aCharacter, stdout)
• puts(aPhrase, stdout)

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Input and Output Redirection

• Because a C program considers stdin and stdout to
  be ordinary files, a user can redirect the input
  and output to another source or destination
• That source or destination can be an I/O device
  or an ordinary file
• This redirection can be done when a program is
  run from the DOS or UNIX command line
• The 'lt' sign redirects input from a file
• The 'gt' sign redirects output to a new file
• The "gtgt" sign redirects output and appends the
  data to a current file or creates the file if it
  doesn't yet exist

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Example of I/O Redirection

• Input comes from a file, output goes to the
  screen
• C\myprogram ltnumbers.dat
• Input comes from the keyboard, output goes to a
  file
• C\myprogram gtresults.txt
• Input comes from a file output goes to a file
• C\myprogram ltlab.txt gtfindings.dat
• Input comes from the keyboard and the output is
  appended to the contents in a file
• C\myprogram gtgtfindings.dat

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Some Redirection to Try Outin MS-DOS

• Example 1
• C\dir gtdirwords.txt
• C\type dirwords.txt
• Example 2
• C\dir gtdir.dat
• C\find ".c" ltdir.dat
• C\find ".c" ltdir.dat gtfindings.txt
• C\find ".exe" ltdir.dat gtgtfindings.txt
• C\type findings.txt

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printf Function

• The printf function writes formatted output to
  stdout
• Ex. printf("Average 5.2f \n", theAverage)
• The first argument is a character string
  containing ordinary text, escape characters, and
  format specifiers
• Syntax for format specifier ltfieldwidthgt.ltprecis
  iongtltconversion specifiergt
• Each escape character is a combination of a '\'
  and a character, which represents a nonviewable
  ASCII character. An example is \n for newline
• The order, number, and type of the format
  specifiers corresponds to the order, number, and
  type of the arguments in the printf call

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printf Examples

• No format specifiers, one escape character, no
  other arguments
• printf("Name Age Address Distance\n")
• One format specifier, two arguments
• printf("Results is d\n", theResult)
• Four format specifiers, five arguments
• printf("c d f s", aCharacter, anInteger,
  aFloat, aString)
• Same as above, but with field width and precision
  specified
• printf("4c 3d 5.3f 8s", aCharacter,
  anInteger, aFloat, aString)

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scanf Function

• The scanf function reads formatted input from
  stdin
• scanf("d", aNumber)
• The first argument is a character string
  containing one or more conversion specifiers
• Syntax for format specifier ltconversion
  specifiergt
• The order, number, and type of the conversion
  specifiers corresponds to the order, number, and
  type of the other arguments in the scanf call
• Each argument value must by the address of the
  variable, NOT the variable itself, where the
  value should be stored
• To designate the address for a character,
  integer, structure, or floating point variable,
  precede the variable name by the operator,
  which means "address of
• Array names are a special case when passed as an
  argument

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scanf Examples

• One format specifier, two arguments
• scanf("d", theResult)
• Three format specifiers, four arguments
• scanf("dfs", anInteger, aFloat, aString)
• Same as above, but in a different order
• scanf("s f d", aString, aFloat, anInteger)

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