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374

CHAPTER 10

■ ESSENTIAL INPUT AND OUTPUT OPERATIONS

such as a printer, or a device that can have both input and output operations, such as a disk drive.

This is illustrated in Figure 10-1.

Figure 10-1. Standard and nonstandard streams

A disk drive can have multiple input and output streams because it can contain multiple files.

The correspondence is between a stream and a file, not between a stream and the device. A stream

can be associated with a specific file on the disk. The stream that you associate with a particular file

could be an input stream, so you could only read from the file; it could be an output stream, in which

case you could only write to the file; or the stream might allow input and output so reading and writing

the file would both be possible. Obviously, if the stream that is associated with a file is an input stream,

the file must have been written at some time so it contained some data. You could also associate a

stream with a file on a CD-ROM drive. Because this device is typically read-only, the stream would,

of necessity, be an input stream.

There are two further kinds of streams: character streams, which are also referred to as text

streams, and binary streams. The main difference between these is that data transferred to or from

character streams is treated as a sequence of characters and may be modified by the library routine

concerned, according to a format specification. Data that’s transferred to or from binary streams is

just a sequence of bytes that isn’t modified in any way. I discuss binary streams in Chapter 12 when

I cover reading and writing disk files.

Standard Streams

C has three predefined standard streams that are automatically available in any program, provided,

of course, that you’ve included the <stdio.h> header file, which contains their definitions, into your

program. These standard streams are stdin, stdout, and stderr. Two other streams that are available

Horton_735-4C10.fm Page 374 Wednesday, September 20, 2006 9:48 AM

CHAPTER 10 ■ ESSENTIAL INPUT AND OUTPUT OPERATIONS

375

with some systems are identified by the names stdprn and stdaux, but these are not part of the C

language standard so your compiler may not support them.

No initialization or preparation is necessary to use these streams. You just have to apply the

appropriate library function that sends data to them. They are each preassigned to a specific physical

device, as shown in Table 10-1.

In this chapter I concentrate on how you can use the standard input stream, stdin, the standard

output stream, stdout, and the printer stream, stdprn.

The stderr stream is simply the stream to which error messages from the C library are sent, and

you can direct your own error messages to stderr if you wish. The main difference between stdout

and stderr is that output to stdout is buffered in memory so the data that you write to it won’t necessarily

be transferred immediately to the device, whereas stderr is unbuffered so any data you write to it is

always transferred immediately to the device. With a buffered stream your program transfers data to

or from a buffer area in memory, and the actual data transfer to or from the physical device can occur

asynchronously. This makes the input and output operations much more efficient. The advantage of

using an unbuffered stream for error messages is that you can be sure that they will actually be displayed

but the output operations will be inefficient; a buffered stream is efficient but may not get flushed

when a program fails for some reason, so the output may never be seen. I won’t discuss this further,

other than to say stderr points to the display screen and can’t be redirected to another device. Output to

the stream stdaux is directed to the serial port and is outside the scope of this book for reasons of

space rather than complexity.

Both stdin and stdout can be reassigned to files, instead of the default of keyboard and screen,

by using operating system commands. This offers you a lot of flexibility. If you want to run your

program several times with the same data, during testing for example, you could prepare the data as

a text file and redirect stdin to the file. This enables you to rerun the program with the same data

without having to re-enter it each time. By redirecting the output from your program to a file, you

can easily retain it for future reference, and you could use a text editor to access it or search it.

Input from the Keyboard

There are two forms of input from the keyboard on stdin that you’ve already seen in previous chapters:

formatted input, which is provided primarily by the scanf() function and unformatted input, in

which you receive the raw character data from a function such as getchar(). There’s rather more to

both of these possibilities, so let’s look at them in detail.

Table 10-1. Standard Streams

Stream Device

stdin Keyboard

stdout Display screen

stderr Display screen

stdprn Printer

stdaux Serial port

Horton_735-4C10.fm Page 375 Wednesday, September 20, 2006 9:48 AM

376

CHAPTER 10

■ ESSENTIAL INPUT AND OUTPUT OPERATIONS

Formatted Keyboard Input

As you know, the function scanf() reads characters from the stream stdin and converts them to one

or more values according to the format specifiers in a format control string. The prototype of the

scanf() function is as follows:

int scanf(char *format, ... );

The format control string parameter is of type char *, a pointer to a character string as shown

here. However, this usually appears as an explicit argument in the function call, such as

scanf("%lf", &variable);

But there’s nothing to prevent you from writing this:

char str[] = "%lf";

scanf(str, &variable);

The scanf() function makes use of the facility of handling a variable number of arguments that

you learned about in Chapter 9. The format control string is basically a coded description of how

scanf() should convert the incoming character stream to the values required. Following the format

control string, you can have one or more optional arguments, each of which is an address in which

a corresponding converted input value is to be stored. As you’ve seen, this implies that each of these

arguments must be a pointer or a variable name prefixed by & to define the address of the variable

rather than its value.

The scanf() function reads from stdin until it comes to the end of the format control string, or

until an error condition stops the input process. This sort of error is the result of input that doesn’t

correspond to what is expected with the current format specifier, as you’ll see. Something that I

haven’t previously noted is that scanf() returns a value that is the count of the number of input

values read. This provides a way for you to detect when an error occurs by comparing the value

returned by scanf() with the number of input values you are expecting.

The wscanf() function provides exactly the same capabilities as scanf() except that the first

argument to the function, which is the format control string, must be a wide character string of type

wchar_t *.

Thus you could use wscanf() to read a floating-point value from the keyboard like this:

wscanf(L"%lf", &variable);

The first argument is a wide character string constant and in all other respects the function

works like scanf(). If you omit the L for the wide character string literal, you will get an error message

from the compiler because your argument does not match the type of the first parameter.

Of course, you could also write this:

wchar_t wstr[] = L"%lf";

wscanf(wstr, &variable);

Input Format Control Strings

The format control string that you use with scanf() or wscanf() isn’t precisely the same as that used

with printf(). For one thing, putting one or more whitespace characters—blank ' ', tab '\t', or

newline '\n'—in the format control string causes scanf() to read and ignore whitespace characters

up to the next nonwhitespace character in the input. A single whitespace character in the format

control string causes any number of consecutive whitespace characters to be ignored. You can

therefore include as many whitespace characters as you wish in the format string to make it more

readable. Note that whitespace characters are ignored by scanf() by default except when you are

reading data using %c, %[], or %n specifications (see Table 10-2).

Horton_735-4C10.fm Page 376 Wednesday, September 20, 2006 9:48 AM

CHAPTER 10 ■ ESSENTIAL INPUT AND OUTPUT OPERATIONS

377

Any nonwhitespace character other than % will cause scanf() to read but not store successive

occurrences of the character. If you want scanf() to ignore commas separating values in the input

for instance, just precede each format specifier by a comma. There are other differences too, as

you’ll see when I discuss formatted output in the section “Output to the Screen” a bit later in this

chapter.

The most general form of a format specifier is shown in Figure 10-2.

Figure 10-2. The general form of an output specifier

Let’s take a look at what the various parts of this general form mean:

•The % simply indicates the start of the format specifier. It must always be present.

•The next * is optional. If you include it, it indicates that the next input value is to be ignored.

This isn’t normally used with input from the keyboard. It does become useful, however, when

stdin has been reassigned to a file and you don’t want to process all the values that appear

within the file in your program.

• The field width is optional. It’s an integer specifying the number of characters that scanf()

should assume makes up the current value being input. This allows you to input a sequence

of values without spaces between them. This is also often quite useful when reading files.

• The next character is also optional, and it can be h, L, l (the lowercase letter L), or ll (two

lowercase Ls). If it’s h, it can only be included with an integer conversion specifier (d, I, o, u, or x)

and indicates that the input is to be converted as short. If it’s l, it indicates long when preceding

an int conversion specifier, and double when preceding a float conversion specifier. Prefixing

the c specification with l specifies a wide character conversion so the input is read as wchar_t. The

prefix L applied to e, E, f, g, or G specifies the value is of type long double. The ll prefix applies to

integer conversions and specifies that the input is to be stored as type long long.

• The conversion character specifies the type of conversion to be carried out on the input

stream and therefore must be included. The possible characters and their meanings are

shown in Table 10-2.

Horton_735-4C10.fm Page 377 Wednesday, September 20, 2006 9:48 AM

378

CHAPTER 10

■ ESSENTIAL INPUT AND OUTPUT OPERATIONS

You can also read a string that consists of specific characters by placing all the possible charac-

ters between square brackets in the specification. For example, the specification %[0123456789.-]

will read a numerical value as a string, so if the input is –1.25 it will be read as "-1.25". To read a string

consisting of the lowercase letters a to z, you could use the specification %[abcdefghijklmnopqrstuvwxyz].

This will read any sequence of the characters that appear between the square brackets as a string,

and the first character in the input that isn’t in the set between the square brackets marks the end of

the input. Although it isn’t required by the standard, many C library implementations support the

form %[a-z] to read a string consisting of any lowercase letters.

A specification using square brackets is also very useful for reading strings that are delimited by

characters other than whitespace. In this case, you can specify the characters that are not in the string by

using the ^ character as the first character in the set. Thus, the specification %[^,] will include every-

thing in the string except a comma, so this form will enable you to read a series of strings separated

by commas.

Table 10-3 shows a few examples of applying the various options.

Table 10-2. Conversion Characters and Their Meanings

Conversion

Character

Meaning

d Convert input to int.

i Convert input to int. If preceded by 0, then assume octal digits input. If

preceded by 0x or 0X, then assume hexadecimal digits input.

o Convert input to int and assume all digits are octal.

u Convert input to unsigned int.

x Convert to int and assume all digits are hexadecimal.

c Read the next character as char (including whitespace). If you want to

ignore whitespace when reading a single character, just precede the

format specification by a whitespace character.

s Input a string of successive nonwhitespace characters, starting with the

next nonwhitespace character.

e, f, or g Convert input to type float. A decimal point and an exponent in the

input are optional.

n No input is read but the number characters that have been read from the

input source up to this point are stored in the corresponding argument,

which should be of type int*.

Table 10-3. Examples of Options in Conversion Specifications

Specification Description

%lf Read the next value as type double

%*d Read the next integer value but don’t store it

%lc Read the next character as type wchar_t

%\nc Read the next character as type char ignoring whitespace characters

%10lld Reads the next ten characters as an integer value of type long long

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