📄 0468-0471.html
字号:
<HTML>
<HEAD>
<TITLE>Developer.com - Online Reference Library - 0672311739:RED HAT LINUX 2ND EDITION:C and C++ Programming</TITLE>
<META NAME="ROBOTS" CONTENT="NOINDEX, NOFOLLOW">
<SCRIPT>
<!--
function displayWindow(url, width, height) {
var Win = window.open(url,"displayWindow",'width=' + width +
',height=' + height + ',resizable=1,scrollbars=yes');
}
//-->
</SCRIPT>
</HEAD>
-->
<!-- ISBN=0672311739 //-->
<!-- TITLE=RED HAT LINUX 2ND EDITION //-->
<!-- AUTHOR=DAVID PITTS ET AL //-->
<!-- PUBLISHER=MACMILLAN //-->
<!-- IMPRINT=SAMS PUBLISHING //-->
<!-- PUBLICATION DATE=1998 //-->
<!-- CHAPTER=23 //-->
<!-- PAGES=0455-0486 //-->
<!-- UNASSIGNED1 //-->
<!-- UNASSIGNED2 //-->
<P><CENTER>
<a href="0465-0467.html">Previous</A> | <a href="../ewtoc.html">Table of Contents</A> | <a href="0472-0474.html">Next</A>
</CENTER></P>
<A NAME="PAGENUM-468"><P>Page 468</P></A>
<P>Interestingly, if the structure pointer is incremented, the address is increased not by
1, but by the size of the structure.
</P>
<H4><A NAME="ch23_ 12">
Functions
</A></H4>
<P>Functions are an easy way to group statements and to give them a name. These are
usually related statements that perform repetitive tasks such as I/O.
printf, described earlier, is a function. It is provided with the standard C library. Listing 23.4 illustrates a function definition,
a function call, and a function.
</P>
<P>
<CENTER>
<TABLE BGCOLOR="#FFFF99">
<TR><TD><B>
NOTE
</B></TD></TR>
<TR><TD>
<BLOCKQUOTE>
The three-dot ellipsis simply means that some lines of sample code are not shown here
in order to save space.
</BLOCKQUOTE></TD></TR>
</TABLE></CENTER>
</P>
<P>Listing 23.4. An example of a function.
</P>
<!-- CODE //-->
<PRE>
int swapandmin( int *, int *); /* Function declaration */
...
int i,j,lower;
i=2; j=4;
lower=swapandmin(&i, &j); /* Function call */
...
int swapandmin(int *a,int *b) /* Function definition */
{
int tmp;
tmp=(*a);
(*a)=(*b);
(*b)=tmp;
if ((*a)<(*b)) return(*a);
return(*b);
}
</PRE>
<!-- END CODE //-->
<P>ANSI C and K&R differ most in function declarations and calls. ANSI C requires that
function arguments be prototyped when the function is declared. K&R
required only the name and the type of the returned value. The declaration in Listing 23.4 states that a
function swapandmin will take two pointers to integers as arguments and that it will return an
integer. The function call takes the addresses of two integers and sets the variable named
lower to the return value of the function.
</P>
<A NAME="PAGENUM-469"><P>Page 469</P></A>
<P>When a function is called from a C program, the values of the arguments are passed to
the function. Therefore, if any of the arguments will be changed for the calling function, you
can't pass only the variable—you must pass the address, too. Likewise, to change the value of
the argument in the calling routine of the function, you must assign the new value to the address.
</P>
<P>In the function in Listing 23.4, the value pointed to by
a is assigned to the tmp variable. b is assigned to
a, and tmp is assigned to b. *a is used instead of
a to ensure that the change is reflected in the calling routine. Finally, the values of
*a and *b are compared, and the lower of the two is returned.
</P>
<P>If you include the line
</P>
<!-- CODE SNIP //-->
<PRE>
printf("%d %d %d",lower,i,j);
</PRE>
<!-- END CODE SNIP //-->
<P>after the function call, you will see 2 4 2 as output.
</P>
<P>This sample function is quite simple, and it is ideal for a macro. A macro is a technique used
to replace a token with different text. You can use macros to make code more readable. For
example, you might use EOF instead of (-1) to indicate the end of a file. You can also use
macros to replace code. Listing 23.5 is the same as Listing 23.4 except that it uses macros.
</P>
<P>Listing 23.5. An example of macros.
</P>
<!-- CODE //-->
<PRE>
#define SWAP(X,Y) {int tmp; tmp=X; X=Y; Y=tmp; }
#define MIN(X,Y) ((X<Y) ? X : Y )
...
int i,j,lower;
i=2; j=4;
SWAP(i,j);
lower=MIN(i,j);
</PRE>
<!-- END CODE //-->
<P>When a C program is compiled, macro replacement is one of the first steps performed.
Listing 23.6 illustrates the result of the replacement.
</P>
<P>Listing 23.6. An example of macro replacement.
</P>
<!-- CODE SNIP //-->
<PRE>
int i,j,lower;
i=2; j=4;
{int tmp; tmp=i; i=j; j=tmp; };
lower= ((i<j) ? i : j );
</PRE>
<!-- END CODE SNIP //-->
<P>The macros make the code easier to read and understand.
</P>
<A NAME="PAGENUM-470"><P>Page 470</P></A>
<P>
<CENTER>
<TABLE BGCOLOR="#FFFF99">
<TR><TD><B>
CAUTION
</B></TD></TR>
<TR><TD>
<BLOCKQUOTE>
Macros can have side effects. Side effects occur because the programmer expects
a variable to be evaluated once when it is actually evaluated more than once. Replacing
the variable i with i++ changes things dramatically:
<!-- CODE SNIP //-->
<PRE>
lower=MIN(i++,j);
is converted to
lower= ((i++ < j) ? i++ : j );
</PRE>
<!-- END CODE SNIP //-->
<BR>As a result, the variable i can be incremented twice instead of once as the
programmer expects.
</BLOCKQUOTE></TD></TR>
</TABLE></CENTER>
<H4>
Creating a Simple Program
</H4>
<P>For the next example, you'll write a program that prints a chart of the first ten integers
and their squares, cubes, and square roots.
</P>
<H4><A NAME="ch23_ 13">
Writing the Code
</A></H4>
<P>Using the text editor of your choice, enter all the code in Listing 23.7 and save it in a file
called sample.c.
</P>
<P>Listing 23.7. Source code for sample.c.
</P>
<!-- CODE //-->
<PRE>
#include <stdio.h>
#include <math.h>
main()
{
int i;
double a;
for(i=1;i<11;i++)
{
a=i*1.0;
printf("%2d. %3d %4d %7.5f\n",i,i*i,i*i*i,sqrt(a));
}
}
</PRE>
<!-- END CODE //-->
<P>The first two lines are header files. The
stdio.h file provides the function definitions and
structures associated with the C input and output libraries. The
math.h file includes the definitions of mathematical library functions.
You need it for the square root function.
</P>
<P>The main loop is the only function that you need to write for this example. It takes no
arguments. You define two variables: One is the integer
i, and the other is a double-precision floating-point number calleda.
You don't have to use a, but you can for the sake of convenience.
</P>
<A NAME="PAGENUM-471"><P>Page 471</P></A>
<P>The program is a simple for loop that starts at 1 and ends at 11. It increments
i by 1 each time through. When i equals 11, the
for loop stops executing. You also could have written
i<=10 because the expressions have the same meaning.
</P>
<P>First, you multiply i by 1.0 and assign the product to
a. A simple assignment would also work, but the multiplication reminds you that you are converting the value to a
double-precision floating-point number.
</P>
<P>Next, you call the print function. The format string includes three integers of widths
2, 3, and 4. After the first integer is printed, you print a period. After the next integer is printed,
you print a floating-point number that is seven characters wide with five digits following the
decimal point. The arguments after the format string show that you print the integer, the square
of the integer, the cube of the integer, and the square root of
the integer.
</P>
<H4><A NAME="ch23_ 14">
Compiling the Program
</A></H4>
<P>To compile this program by using the GNU C compiler, enter the following command:
</P>
<!-- CODE SNIP //-->
<PRE>
gcc sample.c -lm
</PRE>
<!-- END CODE SNIP //-->
<P>This command produces an output file called
a.out. This is the simplest use of the C compiler.
gcc is one of the most powerful and flexible commands of a UNIX system.
</P>
<P>A number of different flags can change the compiler's output. These flags are often
dependent on the system or compiler. Some flags are common to all C compilers. These are described
in the following paragraphs.
</P>
<P>The -o flag tells the compiler to write the output to the file named after the flag. The
gcc -o sample sample.c command puts the program in a file named
sample.
</P>
<P>
<CENTER>
<TABLE BGCOLOR="#FFFF99">
<TR><TD><B>
NOTE
</B></TD></TR>
<TR><TD>
<BLOCKQUOTE>
The output discussed here is the compiler's output, not the sample program.
Compiler output is usually the program, and in every example here, it is an executable program.
</BLOCKQUOTE></TD></TR>
</TABLE></CENTER>
</P>
<P>The -g flag tells the compiler to save the symbol table (the data used by a program to
associate variable names with memory locations) in the executable, which is necessary for debuggers.
Its opposite is the -O flag, which tells the compiler to optimize the code—that is, to make it
more efficient. You can change the search path for header files with the
-I flag, and you can add libraries with the -l and
-L flags.
</P>
<P>The compilation process takes place in several steps:
</P>
<OL>
<LI> First, the C preprocessor parses the file. To parse the file, it sequentially reads
the lines, includes header files, and performs macro replacement.
</OL>
<P><CENTER>
<a href="0465-0467.html">Previous</A> | <a href="../ewtoc.html">Table of Contents</A> | <a href="0472-0474.html">Next</A>
</CENTER></P>
</td>
</tr>
</table>
<!-- begin footer information -->
</body></html>⌨️ 快捷键说明
复制代码
Ctrl + C
搜索代码
Ctrl + F
全屏模式
F11
切换主题
Ctrl + Shift + D
显示快捷键
?
增大字号
Ctrl + =
减小字号
Ctrl + -