Input and Output in C

M. Campbell © 1993 Deakin University
Module 814
Input and Output in CModule 814 Input and Output in C
Page 813-1
After working through this module you should be able to create C
programs that fully utilise the input/output libraries to read data from
the keyboard and to display data on the computer monitor.
Learning objectives
After working through this module you should be able to:
1. Identify and describe the purpose of the C standard input/output
2. Use the standard input operations in C programs.
3. Use the standard output operations in C programs.
4. Convert data values to formatted text using standard output
5. State the significance of standard error output.
6. Use other input/output libraries, for example the Turbo C console
library, to perform input and output operations.
The standard input/output library.
Standard input operations.
Standard output operations.
Standard error output.
Other input/output libraries.
Screen output using CONIO.
Learning Strategy
Read the printed module and the assigned readings and complete the
exercises as requested.
Completion of exercises and the CML test at the end of the module.Module 814 Input and Output in C
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References & resources
The C Programming Language. 2nd. edition
Brian W. Kernighan and Dennis M. Ritchie
Prentice-Hall, 1988
Turbo C/C++ Manuals.
Turbo C/C++ MS DOS compiler.Module 814 Input and Output in C
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Introduction to Module 814
In this module you will be introduced to the methods and practices used
by C programmers to perform input and output. In the previous module
(Module 813: Fundamental Programming Structures in C) you
discovered the fundamental elements of a C program. This module
describes in more detail the standard input/output library that is
common to all C compilers.
C is not a language rich in input and output facilities and the functions
that are provided are very primitive. Programmers therefore rely
extensively on program, and in particular input/output, libraries for
support in developing the user interface for a program. The standard
input/output library is, by its very nature, standard and must therefore
be suited to any hardware platform. Consequently the functions that it
provides do not support colour monitors because you cannot guarantee
that the hardware platform has a colour monitor. Similarly, the
functions do not support mouse input, text-based or high resolution
graphics output, or other “non-standard” input and output devices.
For this reason many programmers create, or purchase from third-party
vendors, input/output libraries that are specific to the hardware
platform and interface tasks required by the program. Many libraries
exist to perform functions such as the creation and management of
menus, high-resolution graphics, windows, forms and so on. One library
that will be looked at in this module is the console input/output
(CONIO) library that is provided with the Turbo C development
system. This library extends the function provided in STDIO to include
screen management operations for text-based output.
These notes make use of a number of program examples. Each example
is provided on a floppy disk which is part of the unit materials. You
should load, compile and run these programs to discover how they
work. You can also use these examples as prototypes for the design of
your own programs. In order to conserve paper, the programs have
been written with a minimum of documentation. Your programs would
normally be developed with more substantial documentation.Module 814 Input and Output in C
Page 813-4
Objective 1 After working through this module you should be able to identify and
describe the purpose of the C standard input/output library.
Each hardware platform interacts with the environment in its own way.
Different peripheral devices have different modes of communication.
The operating system may, or may not, perform a variety of input and
output functions. The designers of C were faced with the dilemma of
including input and output facilities, and therefore writing a separate
compiler for each hardware platform, or creating a universal language
that does not include the hardware specific input and output operations.
The designers chose the latter approach and opted to define, but not
implement, a standard set of input and output operations. These
operations form part of the standard C library. Any C program, as long
as it uses these standard operations, can be compiled and run on any
hardware platform as long as the library routines are available on that
The header file STDIO.H and its associated object code file define the
standard input and output operations. This file forms part of the larger
C standard library defined in the ANSI standard for the language.The
standard library consists of 12 library files, each designated to a
particular purpose. The standard library includes data structures and
functions to:
· handle strings, as defined in STRING.H
· perform character tests and conversions, as defined in CTYPE.H
· perform mathematical operations, as defined in MATH.H
· perform number conversions, storage allocation and similar tasks,
as defined in STDLIB.H
· manipulate date and time data, as defined in TIME.H
The standard input and output library implements a simple model of
text input and output. A text stream consists of a sequence of lines;
each line ends with a newline character. If the hardware and operating
system platform does not operate in this way, the library does whatever
necessary to make it appear as if it does. For instance, the library might
convert carriage return and linefeed to newline on input and back again
on output. Standard input and output are special cases of text file input
and output. As such, many of the functions used for standard input and
output have equivalent functions that are used with files. This will be
explored further in Module 816: File Access in C.Module 814 Input and Output in C
Page 813-5
In many environments, a file may be substituted for the keyboard by
using the < convention for input redirection. For example, the command
prog < infile
will run the executable file called prog with all input coming from the
text file called infile. Similarly, a file may be substituted for monitor
output by using the > convention for output redirection. For example,
the command line:
prog > outfile
will run the executable file called prog with all output going to the text
file called outfile. It is possible to redirect both input and output
simultaneously, as shown by the example:
prog < infile > outfile
The facility for redirection comes from two components. The C
language has defined input and output to be stream based and therefore
the substitution of one file input with another is transparent to the
library system. The second component is the operating system itself.
Most operating systems also define input and output as text streams and
therefore substituting files for keyboards, or files for monitors, is a
possible and desirable characteristic.
One consequence of the text stream input and output model is that the
facilities provided are plain, and fundamentally provide only terminallike facilities. The use of characteristics such as colour, screen
positioning, mouse or scanner input, graphics, and sound are not part of
this stream based model. These facilities are not found in the standard
library. If the user interface requires these features then another library
– either user defined or third party – must be used.
Spend some time studying the file named STDIO.H. There will be a lot
in it that is difficult to understand, but parts of it will look familiar. The
name STDIO.H is a cryptic form of the name standard input/output
header, because that is exactly what it does. It defines the standard
input and output functions in the form of #defines and macros. Don’t
worry too much about the details of this now – you can always return
to this topic later for more study if it interests you – but you will really
have no need to completely understand the STDIO.H file. You will
have a tremendous need to use it however, so these comments on its
use and purpose are necessary.
The first program in this section [SIMPLEIO.C] is also the first formal
study of a program with standard I/O. Standard I/O refers to the most
usual places where data is either read from, the keyboard, or written to,Module 814 Input and Output in C
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the video monitor. Since they are used so much, they are used as the
default I/O devices and do not need to be named in the Input/Output
instructions. This will make more sense when we actually start to use
them so let’s look at the program.
#include “stdio.h” /* standard header for input/output */
void main( )
char c;
printf(“Enter any characters, X = halt program.\n”);
do {
c = getchar( ); /* get a single character from the kb */
putchar(c); /* display the character on the monitor */
} while (c != ‘X’); /* until an X is hit */
printf(“\nEnd of program.\n”);
The first line of the program uses an #include statement. This is very
much like the #define studied in the previous modeule, except that
instead of a simple substitution, an entire file is read in at this point. The
system will find the file named stdio.h and read its entire contents in,
replacing this statement. The file named stdio.h must contain valid C
source statements that can be compiled as part of a program. This
particular file is composed of several standard #defines to define some
of the standard I/O operations. The file is called a header file and you
will find several different header files on the source disks that came with
your C compiler. Each of the header files has a specific purpose and any
or all of them can be included in any program.
A header file contains only the definitions and prototypes of data
structures and functions provided by the library. The source code for
the library will be contained elsewhere or, in the case of third-party
libraries, may not be provided at all. The library must, however, be
available as an object file that will be linked with the program during
the linking phase of the compilation process. A program may include
many header files thus referencing a large number of program libraries.
The C compiler uses the double quote marks to indicate that the search
for the include file will begin in the current directory, and if it is not
found there, the search will continue in the directory specified for
program libraries set up in the Turbo C environment.
It is also possible to use the less than (<) and greater than (>) symbols
to indicate that the search for the header file should begin in the
directory specified in the environment. In these circumstances the
current directory is not searched at all thereby increasing the
compilation speed slightly. For standard libraries it is possible to use
either form of notation since the standard libraries are not normally
found in the current directory. For user-defined libraries it is preferable
to use the quotation symbol notation since the libraries may reside with
the program.Module 814 Input and Output in C
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Most of the programs in this unit use the quotation symbol notation in
the include statements. The next program uses < and > to illustrate the
usage. Note that this will result is a slightly faster (but probably
unnoticeable) compilation because the system will not bother to search
the current directory.
Input/Output programming in C
C was not designed to be used as a language for lots of input and
output, but as a systems language where a lot of internal operations are
required. You would do well to use another language for I/O intensive
programming, but C could be used if you desire. The keyboard input is
very flexible, allowing you to get at the data in a very low level way,
but very little help is given you. It is therefore up to you to take care of
all of the bookkeeping chores associated with your required I/O
operations. This may seem like a real pain in the neck, but in any given
program, you only need to define your input routines once and then use
them as needed. Don’t let this worry you; as you gain experience with
C, you will easily handle your I/O requirements.
Exercise 1
Modify the SIMPLEIO.C program to terminate input when the user
enters a ‘$’ symbol.Module 814 Input and Output in C
Page 813-8
Objective 2 After working through this module you should be able to use the
standard input operations in C programs.
Character Input
The simplest input mechanism is to read one character at a time from
the standard input, normally the ketboard, with getchar.
int getchar(void)
Returning to the SIMPLEIO.C program introduced in the previous
section. Just one variable – c – is defined and a message is printed out
with the familiar printf function. A continuous loop follows which
continues as long as c is not equal to (capital) X. If there is any question
in your mind about the loop control, you should review Module 813
(Fundamental Programming Structures in C) before continuing. The
two new functions within the loop are of paramount interest in this
program since they are the new functions. These are functions to read a
character from the keyboard and display it on the monitor one character
at a time.
The function getchar( ) reads a single character from the standard input
device, the keyboard, and assigns it to the variable c. The next function
putchar(c), uses the standard output device, the video monitor, and
outputs the character contained in the variable c. The character is
output at the current cursor location and the cursor is advanced one
space for the next character. The loop continues reading and displaying
characters until a capital X is entered which terminates the loop.
Compile and run this program for a few surprises. When you type on
the keyboard, you will notice that what you type is displayed faithfully
on the screen. When you hit the Enter key, the entire line is repeated. In
fact, should only output each character once but it seems to be saving
the characters up and redisplaying the line; a short explanation is
DOS is helping us (or getting in the way)
A little bit about how DOS works is needed to understand what is
happening here. When a keypress is read from the keyboard, under
DOS control, the characters are stored in a buffer. When the Enter key
is detected, the entire string of characters is given to the program.
When the characters are being typed, however, DOS will display the
characters on the monitor. This is called echo, and happens in many of
the applications you run.Module 814 Input and Output in C
Page 813-9
With the above paragraph in mind, it should be clear that when you are
typing a line of data into the program, the characters are being echoed
by DOS, and when you press the Enter key, the characters are given to
the program one at a time. As each character is given to the program, it
displays it on the screen resulting in a repeat of the line typed in. To
better illustrate this, type a line with a capital X somewhere in the
middle of the line. You can type as many characters as you like
following the X and they will all be displayed because the characters are
being read in, echoed to the monitor, and placed in the DOS input
buffer by the operating system.
To DOS of course, there is nothing special about a capital X. When the
string is given to the program, however, the characters are accepted by
the program one at a time and sent to the monitor one at a time, until a
capital X is encountered. After the capital X is displayed, the loop is
terminated, and the program is terminated. The characters on the input
line following the capital X are not displayed because the capital X
signalled program termination.
Compile and run the program making sure that you test the program
completely. Don’t get discouraged by this seemingly weird behaviour of
the I/O system. These functions form the base level of input and output
operations provided by STDIO. Other input and output functions are
available but sometimes you will need to build your own high-level
functions from these primitive operations.
A strange I/O method
Let us turn to another method of character I/O. The [SINGLEIO.C]
program performs exactly the same purpose as the last and so the
structure of the two programs are almost identical. Once again, we start
with the standard I/O header file, this time using the < > method of
locating it. A character variable named c is defined, and finally a
welcoming message is printed. Like the last program, a loop is initiated
that will continue to execute until a capital X is entered. The output of
this program, however, is a little different to the previous example.
#include <stdio.h>
void main( )
char c;
printf(“Enter any characters, terminate program with X\n”);
do {
c = getch( ); /* get a character */
putchar(c); /* display the hit key */
} while (c != ‘X’);
printf(“\nEnd of program.\n”);
The getch( ) is a new function that is also a get character function. It
differs from getchar( ) in that it does not get tied up in DOS. It reads
the character in without echo, and puts it directly into the programModule 814 Input and Output in C
Page 813-10
where it is operated on immediately. This function therefore reads a
character, immediately displays it on the screen, and continues the
operation until a capital X is typed.
When you compile and run this program, you will find that there is no
repeat of the lines when you press the Enter key, and when you hit the
capital X, the program terminates immediately. No Enter key is needed
to get the program to accept the line with the X in it. If you look
closely at the output, however, you will notice another problem. No
linefeed is sent to the monitor when the Enter key is pressed and so no
new line is started.
It is not apparent when using in most application programs, but when
the Enter key is pressed, the program supplies a linefeed to go with the
carriage return. A carriage return means that the output needs to return
to the left side of the monitor and at the same time, it also needs to drop
down a line. The linefeed is not automatic. The example
[BETTERIN.C] improves the program by accomplishing this task.
#include “stdio.h”
#define CR 13 /* this defines CR to be 13 */
#define LF 10 /* this defines LF to be 10 */
void main( )
char c;
printf(“Input any characters, hit X to stop.\n”);
do {
c = getch( ); /* get a character */
putchar(c); /* display the hit key */
if (c == CR) putchar(LF);
/* if it is a carriage return put out a linefeed too*/
} while (c != ‘X’);
printf(“\nEnd of program.\n”);
Two additional statements have been placed at the beginning of the
program that define the character codes for the linefeed (LF), and the
carriage return (CR). In the main program, after outputting the
character, it is compared to CR, and if it is equal to CR, a linefeed
character is also sent to the output. Note that the two #define
statements can be omitted from the program and the character codes
used directly, for example,
if (c = = 13) putchar(10);
This technique, however is not very informative and makes it difficult
for someone else to read. The method used in the program represents
better programming practice.
Compiling and running this program should display exactly what you
type in, including a linefeed with each carriage return, and should stop
immediately when you type a capital X.Module 814 Input and Output in C
Page 813-11
Which method?
Two methods of reading characters into a C program have been
examined. Programmers are faced with a choice of which one they
should use. It really depends on the application because each method
has advantages and disadvantages.
When using the first method, the operating system is actually doing all
of the work by storing the characters in an input buffer and signalling
when a full line has been entered. The advantage of allowing DOS to
perform the actual input is that the computer could be performing other
tasks whilst input is occuring. However, single keystrokes cannot be
entered with this technique because DOS would not report a buffer of
characters to us until it recognised a carriage return.
The second method allows programs to get a single character, and act
on it immediately. The program does not have to wait until DOS
delivers a line of characters. The program must wait for the input
keystroke thus requiring a loop. The computer cannot perform any
other tasks while it is waiting for input. This method is useful for highly
interactive types of program interfaces.
It should be mentioned at this point that there is also an ungetch
function that works with the getch function. If a character is obtained
using the getch( ) function and it is found that too many characters have
been read, then ungetch( ) can be used to place the character back on
the input device. This simplifies some programs because you often
don’t know that you don’t want the character until you get it. You can
only ungetch one character back to the input device, but that is
sufficient to accomplish the task this function was designed for. It is
difficult to demonstrate this function in a simple program so its use will
be up to you to study when you need it.
The discussion so far in this section should be a good indication that,
while the C programming language is very flexible, it does put a lot of
responsibility on you as the programmer to keep many details in mind.
Reading integers
Most input required by programs is not single character input. Usually
programs require numeric or string based input. This type of input is
called formatted input and the C library provides a mechanism to
achieve this kind of input.
The program [INTIN.C] is an example of how to read in some
formatted data. The structure of this program is very similar to the last
three except that it defines an int type variable and the loop continues
until the variable somehow acquires a value of 100.Module 814 Input and Output in C
Page 813-12
#include “stdio.h”
void main( )
int valin;
printf(“Input a number from 0 to 32767, stop with 100.\n”);
do {
scanf(“%d”,&valin); /* read a single integer value in */
printf(“The value is %d\n”, valin);
} while (valin != 100);
printf(“End of program\n”);
Instead of reading in a character at a time, as has occurred in the last
three programs, an entire integer value is read with one call using the
function named scanf. This function is very similar to the printf that has
been used previously, except that it is used for input instead of output.
Examine the line with the scanf and you will notice that it does not ask
for the variable valin directly, but gives the address of the variable since
it expects to have a value returned from the function. Remember that a
function must have the address of a variable in order to return a value
to that variable in the calling program. Failing to supply a pointer in the
scanf function is probably the most common problem encountered in
using this function.
The function scanf reads the input line until it finds the first data field. It
ignores leading blanks, and in this case it reads integer characters until it
finds a blank or an invalid decimal character, at which time it stops
reading and returns the value.
Remembering the discussion about the way the DOS input buffer
works, it should be clear that nothing is actually acted on until a
complete line is entered and it is terminated by a carriage return. At this
time, the buffer becomes the input line, and the program will search the
line reading all integer values it can find until the line is completely
scanned. This is because the program is in a loop and the program is
instructed to find a value, print it, find another, print it, etc. If you enter
several values on one line, it will read each one in succession and
display the values. Entering the value of 100 will cause the program to
terminate, and entering the value 100 with other values following, will
cause termination before the following values are considered.
Occasional errors
If you enter a number up to and including 32767 it will display
correctly, but if you enter a larger number, it will appear to make an
error. For example, if you enter the value 32768, it will display the
value of -32768, entering the value 65536 will display as a zero. These
are not errors but are caused by the way an integer is defined. The most
significant bit of the 16 bit pattern available for the integer variable is
the sign bit, so there are only 15 bits left for the value. The variable canModule 814 Input and Output in C
Page 813-13
therefore only have the values from -32768 to 32767, any other values
are outside the range of integer variables. This is for you to take care of
in your programs. It is another example of the increased responsibility
you must assume using C.
Compile and run this program, entering several numbers on a line to see
the results, and with varying numbers of blanks between the numbers.
Try entering numbers that are too big to see what happens, and finally
enter some invalid characters to see what the system does with
nondecimal characters.
Character string input
The next program [STRINGIN.C] is an example of reading a string
variable. This program is identical to the last one except that instead of
an integer variable, we have defined a string variable with an upper limit
of 24 characters (remember that a string variable must have a null
character at the end).
#include “stdio.h”
void main( )
char big[25];
printf(“Input a character string, up to 25 characters.\n”);
printf(“An X in column 1 causes the program to stop.\n”);
do {
printf(“The string is -> %s\n”, big);
} while (big[0] != ‘X’);
printf(“End of program.\n”);
The variable in the scanf does not need an & (pointer declaration)
because big is an array variable and by definition it is already a pointer.
This program should require no additional explanation; compile and run
it to see if it works the way you expect.
You probably got a surprise when you ran it because it separated your
sentence into separate words. When used in the string mode of input,
scanf reads characters into the string until it comes to either the end of a
line or a blank character. Therefore, it reads a word, finds the blank
following it, and displays the result. Since we are in a loop, this
program continues to read words until it exhausts the DOS input buffer.
We have written this program to stop whenever it finds a capital X in
column 1, but since the sentence is split up into individual words, it will
stop anytime a word begins with capital X. Try entering a 5 word
sentence with a capital X as the first character in the third word. You
should get the first three words displayed, and the last two simply
ignored when the program stops.
Try entering more than 24 characters to see what the program does. In
an actual program, it is your responsibility to count characters and stop
when the input buffer is full. You may be getting the feeling that a lot ofModule 814 Input and Output in C
Page 813-14
responsibility is placed on you when writing in C. It is, but you also get
a lot of flexibility in the bargain too.
Line Input
It is possible to read a line of characters without dividing the line into
individual words. The program [LINEIN.C] provides an example of
how this can be accomplished.
#include “stdio.h”
void main( )
char line[25];
printf(“Input a character string, up to 25 characters.\n”);
printf(“The string is -> %s\n”, line);
printf(“End of program.\n”);
The gets function is a simple but dangerous function that achieves the
purpose of reading in a line of characters. It is simple to use and works
quite well except that it does not check to see if the user has entered
more characters than can be stored by the character string. If the user
does enter more than the designated number of characters then end
result cannot be predicted. At the very least, other data values may be
corrupted. At worst the program may crash. Clearly it is not desirable
to allow users to enter data that could destroy the program. Although
effective, the gets function should be used with caution.
Better Line Input
The C library does not provide a standard input function which
validates the user’s string input and checks that data remains within the
length of the character string. The text file function fgets can be used
with standard input to achieve this purpose. This function, however,
will not be discussed here – it will be discussed in more detail in
Module 816: File Input/Output in C.
A better line input function can be created from the character functions
presented earlier. The program [GETLINE.C] illustrates how a function
can read a line of text and simultaneously check that the data entry does
not exceed the length of the character string.
The heart of the solution lies with the user-defined function called
getline. This function is defined in a prototype on line 3 of the program.
Its definition follows the main program. The getline function is passed
two arguments. The first is the character string that will receive the data
input. The character string, s, is an array variable, and therefore by
definition, it is a pointer variable. No pointer notation is required. The
second argument, lim, will determine the maximum number of
characters that can be stored by the character string, s.Module 814 Input and Output in C
Page 813-15
#include “stdio.h”
void getline(char s[], int lim);
void main() {
char line[80];
printf(“Please enter your line\n”);
getline1(line, 30);
printf(“The line entered was \n%s\n\n”,line);
/*============================================================== */
/* GETLINE function definition */
/*============================================================== */
void getline(char s[], int lim)
/* local variables */
int c, i;
/* create a loop that terminates under three conditions */
/* 1. lim – 1 characters have been entered */
/* 2. an EOF character is entered at the keyboard */
/* 3. a carriage return is entered at the keyboard */
for (i=0; i<lim-1 && (c=getchar())!=EOF && c!=’\n’; ++i)
s[i] = c;
/* if the limit has been reached, then skip over the */
/* remaining characters. */
while (c!=’\n’)
/* Place the string termination character in the string. */
s[i] = ‘\0’;
The function consists essentially of two loops. The first collects
characters from the standard input using the getchar function. This loop
terminates when the limit has been reached or when the user terminates
the string using an end-of-file character (Ctrl-Z on most systems), or a
carriage return. The second loop will continue to read characters from
the standard input – remember that DOS has collected a whole line of
characters – until all characters have been removed. This step is
necessary to prevent the next getline receiving part of this line as input.
Finally, the function appends the C end-of-string character to the
character string.
Compile and run this program. Test it to see if it correctly reads a
specified number of characters from the standard input. Does it
correctly handle carriage returns and extraneous input?
Exercise 2
Write a program that will prompt the user and accept as input an
integer, a character, a string, and a floating point number.
Write a program to read in a character using a loop, and display the
character in its normal char form. Also display it as a decimal number.
Check for a dollar sign to use as the stop character. Use the getch form
of input so it will print immediately. Hit some of the special keys, such
as function keys, when you run the program for some surprises. You
will get two inputs from the special keys, the first being a zero which is
the indication to the system that a special key was hit.Module 814 Input and Output in C
Page 813-16
Objective 3 After working through this module you should be able to use the
standard output operations in C programs.
Character output
The function
int putchar(int c)
is the fundamental output mechanism in C. putchar(c) puts the character
c on the standard output device, which by default is the screen. Notice
that putchar returns an integer value. This return value is used to test
whether the output was successful or not. If the output was successful,
then putchar returns the character that was displayed. If, for some
reason, an error occurred during output then putchar will return the
EOF value (-1 on most systems).
Like character input, character is not useful in most circumstances.
Programmers usually are required to output numeric data, character
strings and other structures. This kind of output is termed formatted
output and the standard input/output library provides a powerful
function that deals with this problem.
Formatted output
An example of formatted output [LOTTYPES.C] was provided in
Module 813: Fundamental Programming Structures in C. The program
is duplicated here for further study.
#include “stdio.h”
void main()
int a; /* simple integer type */
long int b; /* long integer type */
short int c; /* short integer type */
unsigned int d; /* unsigned integer type */
char e; /* character type */
float f; /* floating point type */
double g; /* double precision floating point */
a = 1023;
b = 2222;
c = 123;
d = 1234;
e = ‘X’;
f = 3.14159;
g = 3.1415926535898;
printf(“a = %d\n”,a); /* decimal output */
printf(“a = %o\n”,a); /* octal output */
printf(“a = %x\n”,a); /* hexadecimal output */
printf(“b = %ld\n”,b); /* decimal long output */
printf(“c = %d\n”,c); /* decimal short output */
printf(“d = %u\n”,d); /* unsigned output */
printf(“e = %c\n”,e); /* character output */Module 814 Input and Output in C
Page 813-17
printf(“f = %f\n”,f); /* floating output */
printf(“g = %f\n”,g); /* double float output */
printf(“a = %d\n”,a); /* simple int output */
printf(“a = %7d\n”,a); /* use a field width of 7 */
printf(“a = %-7d\n”,a); /* left justify width = 7 */
printf(“f = %f\n”,f); /* simple float output */
printf(“f = %12f\n”,f); /* use field width of 12 */
printf(“f = %12.3f\n”,f); /* use 3 decimal places */
printf(“f = %12.5f\n”,f); /* use 5 decimal places */
printf(“f = %-12.5f\n”,f);/* left justify in field */
Printing numbers
Let’s return to the printf statements for a definition of how they work.
Notice that they are all identical and that they all begin just like the
printf statements we have seen before. The first difference occurs when
we come to the % character. This is a special character that signals the
output routine to stop copying characters to the output and do
something different, namely output a variable. The % sign is used to
signal the start of many different types of variables, but we will restrict
ourselves to only one for this example. The character following the %
sign is a d, which signals the output routine to get a decimal value and
output it. Where the decimal value comes from will be covered shortly.
After the d, we find the familiar \n, which is a signal to return the video
‘carriage’, and the closing quotation mark.
All of the characters between the quotation marks define the pattern of
data to be output by this statement, and after the pattern, there is a
comma followed by the variable name index. This is where the printf
statement gets the decimal value which it will output because of the %d
we saw earlier. We could add more %d output field descriptors within
the brackets and more variables following the description to cause more
data to be printed with one statement. Keep in mind, however, that it is
important that the number of field descriptors and the number of
variable definitions must be the same or the runtime system will get
confused and probably quit with a runtime error.
Conversion characters
Here is a list of the conversion characters and the way they are used in
the printf statement:
d decimal notation
o octal notation
x hexadecimal notation
u unsigned notation
c character notation
s string notation
f floating point notationModule 814 Input and Output in C
Page 813-18
Each of these is used following a percent sign to indicate the type of
output conversion, and between those two characters, the following
fields may be added.
– left justification in its field
(n) a number specifying minimum field width
. to separate n from m
(m) significant fractional digits for a float
l to indicate a “long”
These are all used in the examples which are included in the program
presently displayed on your monitor, with the exception of the string
notation which will be covered shortly. Note especially the variable field
width specification in lines 33 to 36. Compile and run this program to
see what effect the various fields have on the output.
Printing Character Strings
The printf can also be used to output character strings. The conversion
character s is used to denote a string variable. The following statements
indicate the different forms of output possible when the printf function
is used with character strings. Assume that the variable line contains the
string “Deakin University” which is 17 characters in length. The output
expected is shown below each statement. The extent of each output is
delimited by colons.
printf(“:%s:”, line);
:Deakin University:
printf(“:%10s:”, line);
:Deakin University:
printf(“:%.10s:”, line);
:Deakin Uni:
printf(“:%-10s:”, line);
:Deakin University:
printf(“:%.20s:”, line);
:Deakin University:
printf(“:%-20s:”, line);
:Deakin University :
printf(“:%20.10s:”, line);
: Deakin Uni:
printf(“:%-20.10s:”, line);
:Deakin Uni :
You should make sure that you can recognise and use each of these
different forms of the printf function.Module 814 Input and Output in C
Page 813-19
printf uses its first argument to decide how many arguments follow and
what their types are. It will get confused, and you will get wrong
answers, if there are not enough arguments or if they are the wrong
You now have the ability to display any of the data fields in the
previous programs and it would be to your advantage to go back and
see if you can display any of the fields anyway you desire.
Exercise 3
Consider the following two output statements:
If the character string s contains the string “Mary received 98% for her
C test” the first statement will fail, whereas the second will work
correctly produce the expected output. Can you explain why?
Write a program to determine if the statement
printf(“%20.2f”, num);
produces 97.77 or 97.78 when num is given the value of 97.7779. What
can you conclude about the rounding or truncating features of the printf
function?Module 814 Input and Output in C
Page 813-20
Objective 4 After working through this module you should be able to convert data
values to formatted text using standard output operations.
The next operation may seem a little strange at first, but you will
probably see lots of uses for it as you gain experience. This program
[INMEM.C] displays another type of I/O, one that never accesses the
outside world, but stays in the computer.
void main( )
int numbers[5], result[5], index;
char line[80];
numbers[0] = 74;
numbers[1] = 18;
numbers[2] = 33;
numbers[3] = 30;
numbers[4] = 97;
sprintf(line,”%d %d %d %d %d\n”, numbers[0], numbers[1], numbers[2],
numbers[3], numbers[4]);
sscanf(line,”%d %d %d %d %d”, &result[4], &result[3],
(result+2), (result+1), result);
for (index = 0; index < 5; index++)
printf(“The final result is %d\n”, result[index]);
First we define a few variables, then assign some values to the ones
named numbers for illustrative purposes, and then use a sprintf function.
The function acts just like a normal printf function except that instead
of printing the line of output to a device, it prints the line of formatted
output to a character string in memory. In this case the string goes to
the string variable line, because that is the string name we inserted as
the first argument in the sprintf function. The spaces after the 2nd %d
were put there to illustrate that the next function will search properly
across the line. We print the resulting string and find that the output is
identical to what it would have been by using a printf instead of the
sprintf in the first place. You will see that when you compile and run
the program shortly.
Since the generated string is still in memory, we can now read it with
the function sscanf. We tell the function in its first argument that line is
the string to use for its input, and the remaining parts of the line are
exactly what we would use if we were going to use the scanf function
and read data from outside the computer. Note that it is essential that
we use pointers to the data because we want to return data from a
function. Just to illustrate that there are many ways to declare a pointer
several methods are used, but all are pointers. The first two simply
declare the address of the elements of the array, while the last three use
the fact that result, without the accompanying subscript, is a pointer.Module 814 Input and Output in C
Page 813-21
Just to keep it interesting, the values are read back in reverse order.
Finally the values are displayed on the monitor.
It seems silly to read input data from within the computer but it does
have a real purpose. It is possible to read data from an input device
using any of the standard functions and then do a format conversion in
memory. You could read in a line of data, look at a few significant
characters, then use these formatted input routines to reduce the line of
data to internal representation. That would certainly beat writing your
own data formatting routines!
Exercise 4
Write a program that formats a floating point number into a string that
contains the number in a currency format. For example, the number
23456.78 would be converted into the string “$23,456.78”.
Use the sscanf function to convert a date in the form of “dd/mm/yy”
into the respective integers representing day, month and year.Module 814 Input and Output in C
Page 813-22
Objective 5 After working through this module you should be able to state the
significance of standard error output.
Sometimes it is desirable to redirect the output from the standard
output device to a file. However, you may still want the error messages
to go to the standard output device, in our case the monitor. This next
function allows you to do that, and is shown in the next program
#include “stdio.h”
void main( )
int index;
for (index = 0 index < 6; index++) {
printf(“This line goes to the standard output.\n”);
fprintf(stderr,”This line goes to the error device.\n”);
/* This can be tested with the DOS errorlevel command in a batch
The number returned is used as follows;
(continue here if less than 4)
(continue here if 4 or greater)
:DONE */
The program consists of a loop with two messages output, one to the
standard output device and the other to the standard error device. The
message to the standard error device is output with the function fprintf
and includes the device name stderr as the first argument. Other than
those two small changes, it is the same as our standard printf function.
(You will see more of the fprintf function in the next section, but its
operation fits in better as a part of this section.) Ignore the line with the
exit for the moment, we will return to it.
Compile and run this program, and you will find 12 lines of output on
the monitor. To see the difference, run the program again with
redirected output to a file named STUFF by entering the following line
at the DOS prompt;
special >stuff
More information about I/O redirection can be found in your DOS
manual. This time you will only get the 6 lines output to the standard
error device, and if you look in your directory, you will find the file
named STUFF containing the other 6 lines, those to the standard output
device. You can use I/O redirection with any of the programs we haveModule 814 Input and Output in C
Page 813-23
run so far, and as you may guess, you can also read from a file using
I/O redirection.
Finally, let’s look at ‘exit(4)’, the last statement in the program (before
the multi-line comment). This simply exits the program and returns the
value of 4 to DOS. Any number from 0 to 9 can be used in the
parentheses for DOS communication. If you are operating in a BATCH
file, this number can be tested with the ERRORLEVEL command.
Exercise 5
Test the output expected by the example program [SPECIAL.C].
Create the suggested batch file and execute the batch program to ensure
that the error level value is being sent to DOS correctly.Module 814 Input and Output in C
Page 813-24
Objective 6 After working through this module you should be able to use other
input/output libraries, for example the Turbo C console library, to
perform input and output operations.
Other input/output libraries
The ANSI standard for C does not define any text screen or graphics
functions, mainly because the capabilities of diverse hardware
environments preclude standardisation accross a wide range of
machines. However, Turbo C provides extensive screen (and graphics)
support libraries for the PC environment. The use of these libraries, or
other third-party library systems, can greatly enhance the user interface
created by programmers. Because the libraries are non-standard, the
programs that make use of them are not portable to other platforms.
This, however, is often a low priority consideration since intensive
screen control is often a must for most commercial programs.
The prototypes and header information for the text-screen handling
functions are stored in the conio.h file. The major functions defined by
this library will be discussed shortly. The prototypes and related
information for the graphics system are held in the graphics.h file. The
graphics system will not be discussed here. Interested students are
directed to the Turbo C Reference Guide for further details about the
use of this library.
A key factor in text screen manipulation is the window. A window is the
active part of the screen within which output is displayed. A window
can be as large as the entire screen (as it is by default), or it can be as
small as you require. The upper-left corner of a window is located at
the x,y position of 1,1. x refers to the column number and y refers to the
row number. When a window occupies the entire screen the bottomright corner of the window will be located at position 80,25.
You must be careful not to confuse a Turbo C window with windows
produced by other systems, e.g. Microsoft Windows. Turbo C windows
do not have borders, scroll bars, maximise or minimise buttons, or
menus. A Turbo C window simply refers to an area of the screen that is
to be used for output. Further, it is important to understand that most
screen functions are window relative. This means that the position of
the output is relative to the upper-left corner of the current window,
not the screen.
CONIO Functions
The following definitions describe 12 functions that can be used to
improve the standard output produced by C programs. Bear in mindModule 814 Input and Output in C
Page 813-25
that because these functions are not standard they are not supported
outside the Turbo C environment. Further, because they do not directly
extend the functions provided by the standard library, they cannot be
used with programs that redirect standard input or output.
Syntax: void clreol(void)
Description: The clreol() function clears the screen from the current
cursor position to the end of the line in the active text
window. The cursor position remains unchanged.
Syntax: void clrscr(void)
Description: The clrscr() function clears the entire active text
window and locates the cursor in the upper-left corner
(1,1). The colour of the window is set to the current
textbackground colour.
Syntax: void delline(void)
Description: The delline() function deletes the line in the active
window that contains the cursor. All lines below the
deleted line are moved up to fill the gap and a blank
line is inserted at the bottom of the window. Only the
text inside the cureent window is affected.
Syntax: int gettext(int left, int top,
int right, int bottom,
void *buf)
Description: The gettext() function copies into the buffer pointed to
by buf the text from the rectangle defined by left,top
and right,bottom. The coordinates are screen
coordinates, not window relative. buf needs to point to
an amount of memory equal to 2 ´ rows ´ columns.
The function returns 1 if the action was successful, 0
on failure.
Syntax: void gotoxy(int x, int y)Module 814 Input and Output in C
Page 813-26
Description: The gotoxy() function sends the text screen cursor to
the location specified by x,y. If either or both of the
coordinates are invalid, no action takes place.
Syntax: void insline(void)
Description: The insline() function inserts a blank line at the current
cursor position, and all the lines below the cursor move
down. The function only affects the active text
Syntax: int movetext(int left, int top,
int right, int bottom,
int newleft, int newtop)
Description: The movetext() function moves the portion of the
screen defined by the rectangle left,top and
right,bottom to the region of the screen that has its
upper-left corner defines by newleft,newtop. The
coordinates are screen coordinates, not window
relative. The function returns 1 if the action was
successful, 0 on failure.
Syntax: int putttext(int left, int top,
int right, int bottom,
void *buf)
Description: The gettext() function copies the text previously saved
in the buffer pointed to by buf to the region defined by
left,top and right,bottom. The coordinates are screen
coordinates, not window relative. The function returns
1 if the action was successful, 0 on failure.
Syntax: void clreol(int attr)
Description: The textattr() function sets both the foreground and
background colours in the text screen at one time. The
value of attr represents an encoded form of the colour
information. For example, to set the colours to be
white text on a green background, attr would be
defined as GREEN*16 | WHITE.Module 814 Input and Output in C
Page 813-27
Syntax: void clreol(int colour)
Description: The textbackground() function sets the background
colour of a text screen. A call to textbackground() only
affects subsequent write operations. The existing
background colour(s) remain unchanged. The valid
values for colour are shown below:
Syntax: void textcolor(int colour)
Description: The textcolor() function sets the colour of the text
displayed on the screen. The valid values for colour
Syntax: void window(int left, int top,
int right, int bottom)
Description: The window() function is used to create a rectangular
text window whose upper-left and lower-right
coordinates are specified by left,top and right,bottom
respectively. If any coordinate is invalid, window()
takes no action.
No programming examples are provided in this section as the function
definitions should be self-explanatory. Experiment with the functions to
provide different screen effects.Module 814 Input and Output in C
Page 813-28
Exercise 6
Write a function that allows a text string to be written to the screen
with the programmer controlling the coordinates of the first character,
and the colour of the text and background.
Write a function that centres a given text string on the screen giving the
programmer control over the line that the string will be drawn on, and
the colour of the foreground and background. You may wish to
investigate the LENGTH function in Turbo Pascal.
Write a function that writes a given text string vertically down the
screen. Give the programmer control over then usual parameters. You
will have to decide what to do if the string goes off the bottom of the

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