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Chapter 9 - Standard Input/Output
THE STDIO.H HEADER FILE
Load the file SIMPLEIO.C for our first look at a file
with standard I/O. Standard I/O refers to the most usual
places where data is either read from, the keyboard, or
written to, 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 lets look
at the file in front of you.
The first thing you will notice is the first line of
the file, the #include "stdio.h" line. This is very much
like the #define we have already studied, 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. Obviously then, 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 compiler. Each of the header files has a
specific purpose and any or all of them can be included in
any program.
Most C compilers use the double quote marks to indicate
that the "include" file will be found in the current
directory. A few use the "less than" and "greater than"
signs to indicate that the file will be found in a standard
header file. Nearly all MSDOS C compilers use the double
quotes, and most require the "include" file to be in the
default directory. All of the programs in this tutorial
have the double quotes in the "include" statements. If your
compiler uses the other notation, you will have to change
them before compiling.
INPUT/OUTPUT OPERATIONS IN C
Actually the C programming language has no input or
output operations defined as part of the language, they must
be user defined. Since everybody does not want to reinvent
his own input and output operations, the compiler writers
have done a lot of this for us and supplied us with several
input functions and several output functions to aid in our
program development. The functions have become a standard,
and you will find the same functions available in nearly
every compiler. In fact, the industry standard of the C
language definition has become the book written by Kernigan
and Ritchie, and they have included these functions in their
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Chapter 9 - Standard Input/Output
definition. You will often, when reading literature about
C, find a reference to K & R. This refers to the book
written by Kernigan and Ritchie. You would be advised to
purchase a copy for reference.
You should print out the file named "stdio.h" and spend
some time studying it. There will be a lot that you will
not understand about it, but parts of it will look familiar.
The name "stdio.h" is sort of cryptic for "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.
OTHER INCLUDE FILES
When you begin writing larger programs and splitting
them up into separately compiled portions, you will have
occasion to use some statements common to each of the
portions. It would be to your advantage to make a separate
file containing the statements and use the #include to
insert it into each of the files. If you want to change any
of the common statements, you will only need to change one
file and you will be assured of having all of the common
statements agree. This is getting a little ahead of
ourselves but you now have an idea how the #include
directive can be used.
BACK TO THE FILE NAMED "SIMPLEIO.C"
Lets continue our tour of the file in question. The
one variable "c" is defined and a message is printed out
with the familiar "printf" function. We then find ourselves
in a continuous loop 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 chapter 3 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 being assumed
because that is the standard input device, 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
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Chapter 9 - Standard Input/Output
output at the current cursor location and the cursor is
advanced one space for the next character. The system is
therefore taking care of a lot of the overhead for us. The
loop continues reading and displaying characters until we
type a capital X 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, and when you hit the
return key, the entire line is repeated. In fact, we only
told it to output each character once but it seems to be
saving the characters up and redisplaying them. A short
explanation is in order.
DOS IS HELPING US OUT (OR GETTING IN THE WAY)
We need to understand a little bit about how DOS works
to understand what is happening here. When data is read
from the keyboard, under DOS control, the characters are
stored in a buffer until a carriage return is entered at
which time the entire string of characters is given to the
program. When the characters are being typed, however, the
characters are displayed one at a time on the monitor. This
is called echo, and happens in many of the applications you
run.
With the above paragraph in mind, it should be clear
that when you are typing a line of data into "SIMPLEIO", the
characters are being echoed by DOS, and when you return the
carriage, the characters are given to the program. 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
display because the characters are being read in under DOS,
echoed to the monitor, and placed in the DOS input buffer.
DOS doesn't think there is anything 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 "SIMPLEIO.C". After running the
program several times and feeling confidant that you
understand the above explanation, we will go on to another
program.
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Chapter 9 - Standard Input/Output
Don't get discouraged by the above seemingly weird
behavior of the I/O system. It is strange, but there are
other ways to get data into the computer. You will actually
find the above method useful for many applications, and you
will probably find some of the following useful also.
ANOTHER STRANGE I/O METHOD
Load the file named SINGLEIO.C and display it on your
monitor for another method of character I/O. Once again, we
start with the standard I/O header file, we define a
variable named "c", and we print a welcoming message. Like
the last program, we are in a loop that will continue to
execute until we type a capital X, but the action is a
little different here.
The "getch()" is a new function that is 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 program where it
is operated on immediately. This function then 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 hit a carriage
return, and when you hit the capital X, the program
terminates immediately. No carriage return is needed to get
it to accept the line with the X in it. We do have another
problem here, there is no linefeed with the carriage return.
NOW WE NEED A LINE FEED
It is not apparent to you in most application programs
but when you hit the enter key, the program supplies a
linefeed to go with the carriage return. You need to return
to the left side of the monitor and you also need to drop
down a line. The linefeed is not automatic. We need to
improve our program to do this also. If you will load and
display the program named BETTERIN.C, you will find a change
to incorporate this feature.
In BETTERIN.C, we have two additional statements at the
beginning that will define the character codes for the
linefeed (LF), and the carriage return (CR). If you look at
any ASCII table you will find that the codes 10 and 13 are
exactly as defined here. In the main program, after
outputting the character, we compare it to CR, and if it is
equal to CR, we also output a linefeed which is the LF. We
could have just as well have left out the two #define
statements and used "if (c == 13) putchar(10);" but it would
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Chapter 9 - Standard Input/Output
not be very descriptive of what we are doing here. The
method used in the program represents better programming
practice.
Compile and run BETTERIN.C to see if it does what we
have said it should do. It should display exactly what you
type in, including a linefeed with each carriage return, and
should stop immediately when you type a capital X.
If you are using a nonstandard compiler, it may not
find a "CR" because your system returns a "LF" character to
indicate end-of-line. It will be up to you to determine
what method your compiler uses. The quickest way is to add
a "printf" statement that prints the input character in
decimal format.
WHICH METHOD IS BEST?
We have examined two methods of reading characters into
a C program, and are faced with a choice of which one we
should use. It really depends on the application because
each method has advantages and disadvantages. Lets take a
look at each.
When using the first method, DOS is actually doing all
of the work for us by storing the characters in an input
buffer and signalling us when a full line has been entered.
We could write a program that, for example, did a lot of
calculations, then went to get some input. While we were
doing the calculations, DOS would be accumulating a line of
characters for us, and they would be there when we were
ready for them. However, we could not read in single
keystrokes because DOS would not report a buffer of
characters to us until it recognized a carriage return.
The second method, used in BETTERIN.C, allows us to get
a single character, and act on it immediately. We do not
have to wait until DOS decides we can have a line of
characters. We cannot do anything else while we are waiting
for a character because we are waiting for the input
keystroke and tying up the entire machine. This method is
useful for highly interactive types of program interfaces.
It is up to you as the programmer to decide which is best
for your needs.
I should mention at this point that there is also an
"ungetch" function that works with the "getch" function. If
you "getch" a character and find that you have gone one too
far, you can "ungetch" it back to the input device. This
simplifies some programs because you don't know that you
don't want the character until you get it. You can only
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Chapter 9 - Standard Input/Output
"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 chapter, 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.
NOW TO READ IN SOME INTEGERS
Load and display the file named INTIN.C for an example
of reading in some formatted data. The structure of this
program is very similar to the last three except that we
define an "int" type variable and loop until the variable
somehow acquires the value of 100.
Instead of reading in a character at a time, as we have
in the last three files, we read in an entire integer value
with one call using the function named "scanf". This
function is very similar to the "printf" that you have been
using for quite some time by now 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.
Recall that a function must have the address of a variable
in order to return the value to 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" scans 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 our discussion above 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
is input, and our program will search across the line
reading all integer values it can find until the line is
completely scanned. This is because we are in a loop and we
tell it 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
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Chapter 9 - Standard Input/Output
entering the value 100 with other values following, will
cause termination before the following values are
considered.
IT MAKES WRONG ANSWERS SOMETIMES
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 can therefore only have
the values from -32768 to 32767, any other values are
outside the range of integer variables. This is up to you
to take care of in your programs. It is another example of
the increased responsibility you must assume using C rather
than a higher level language such as Pascal, Modula-2, etc.
The above paragraph is true for most MS-DOS C
compilers. There is a very small possibility that your
compiler uses an integer value other than 16 bits. If that
is the case, the same principles will be true but at
different limits than those given above.
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
Load and display the file named STRINGIN.C for 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). The variable in the
"scanf" does not need an & 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
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Chapter 9 - Standard Input/Output
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. It should generate an error, but that will be
highly dependent on the system you are using. 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 of responsibility is placed on you when
writing in C. It is, but you also get a lot of flexibility
in the bargain too.
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.
One final point must be made about these I/O functions.
It is perfectly permissible to intermix "scanf" and
"getchar" functions during read operations. In the same
manner, it is also fine to intermix the output functions,
"printf" and "putchar".
IN MEMORY I/O
The next operation may seem a little strange at first,
but you will probably see lots of uses for it as you gain
experience. Load the file named INMEM.C and display it for
another type of I/O, one that never accesses the outside
world, but stays in the computer.
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Chapter 9 - Standard Input/Output
In INMEM.C, 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. Just to keep it
interesting, the values are read back in reverse order.
Finally the values are displayed on the monitor.
IS THAT REALLY USEFUL?
It seems sort of silly to read input data from within
the computer but it does have a real purpose. It is
possible to read data in 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 sure beat
writing your own data formatting routines.
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. Load and display SPECIAL.C for an example of
this new function.
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Chapter 9 - Standard Input/Output
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 chapter, but its operation
fit in better as a part of this chapter.) 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;
A> 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 have run
so far, and as you may guess, you can also read from a file
using I/O redirection but we will study a better way to read
from a file in the next chapter.
WHAT ABOUT THE exit(4) STATEMENT?
Now to keep our promise about the exit(4) statement.
Redisplay the file named SPECIAL.C on your monitor. The
last statement 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.
Most compilers that operate in several passes return a
1 with this mechanism to indicate that a fatal error has
occurred and it would be a waste of time to go on to another
pass resulting in even more errors.
It is therefore wise to use a batch file for compiling
programs and testing the returned value for errors. A check
of the documentation for my COMPAQ, resulted in a minimal
and confusing documentation of the "errorlevel" command, so
a brief description of it is given in this file.
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Chapter 9 - Standard Input/Output
PROGRAMMING EXERCISE
1. 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.
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