ch27.htm

linux-unix130.linux.and.unix.ebooks130 linux and unix ebookslinuxLearning Linux - Collection of 12 E

<P><TT>gdb</TT> will return the following acknowledgment:</P>

<PRE><FONT COLOR="#0066FF">Watchpoint 2: string2[size - i]

</FONT></PRE>

<P>The version of <TT>gdb</TT> included on the CD-ROM accompanying this book changes

the prompt to <TT>Hardware Watchpoint</TT>, but this is simply a change in the naming

convention used by <TT>gdb</TT>. Now you can step through the execution of the <TT>for</TT>

loop using the <TT>next</TT> command:</P>

<PRE><FONT COLOR="#0066FF">(gdb) next

</FONT></PRE>

<P>After the first time through the loop, <TT>gdb</TT> tells us that <TT>string2[size

- i]</TT> is <TT>`h'</TT>. <TT>gdb</TT> informs you of this by writing the following

message on the screen:</P>

<PRE><FONT COLOR="#0066FF">Watchpoint 2, string2[size - i]

Old value = 0 `\000'

New value = 104 `h'

my_print2(string = 0xbfffdc4 &quot;hello there&quot;) at greeting.c:23

23 for (i=0; i&lt;size; i++)

</FONT></PRE>



<DL>

	<DT><FONT COLOR="#0066FF"></FONT></DT>

</DL>





<DL>

	<DD>

<HR>

<A NAME="Heading17<FONT COLOR="#000077"><B>NOTE:</B> </FONT>The version of

	<TT>gcc</TT> on the CD-ROM accompanying this book appears to have some bugs in the

	<TT>include</TT> files. Some tasks, including checking the next watchpoint, cause

	error messages. You should check the FTP sites for updates or previous versions which

	do not have this fault if this presents a problem for your C compiler usage. 

<HR>



</DL>



<P>This is the value that you expected. Stepping through the loop several more times

reveals similar results. Everything appears to be functioning normally. When you

get to the point where <TT>i=10</TT>, the value of the <TT>string2[size - i]</TT>

expression is equal to <TT>`e'</TT>, the value of the <TT>size - i</TT> expression

is equal to 1, and the program is at the last character that is to be copied over

into the new string.</P>

<P>If you step through the loop one more time, you see that there was not a value

assigned to <TT>string2[0]</TT>, which is the first character of the string. Because

the <TT>malloc</TT> function initializes the memory it assigns to null, the first

character in <TT>string2</TT> is the null character. This explains why nothing was

being printed when you tried to print <TT>string2</TT>.</P>

<P>Now that you have found the problem, it should be quite easy to fix. You must

write the code so that the first character going into <TT>string2</TT> is being put

into <TT>string2</TT> at offset <TT>size - 1</TT> instead of <TT>string2</TT> at

offset <TT>size</TT>. This is because the size of <TT>string2</TT> is 12, but it

starts numbering at offset zero. The characters in the string should start at offset

0 and go to offset 10, with offset 11 being reserved for the null character.</P>

<P>There are many ways to modify this code so that it will work. One way is to keep

a separate size variable that is one smaller than the real size of the original string.

This solution is shown in the following code:</P>

<PRE><FONT COLOR="#0066FF">#include  &lt;stdio.h&gt;



main ()

{

  char my_string[] = &quot;hello there&quot;;



  my_print (my_string);

  my_print2 (my_string);

}



my_print (char *string)

{

  printf (&quot;The string is %s\n&quot;, string);

}



my_print2 (char *string)

{

  char *string2;

  int size, size2, i;



  size = strlen (string);

  size2 = size -1;

  string2 = (char *) malloc (size + 1);

  for (i = 0; i &lt; size; i++)

    string2[size2 - i] = string[i];

  string2[size] = `\0';

  printf (&quot;The string printed backward is %s\n&quot;, string2);

}

</FONT></PRE>

<CENTER>

<H3><A NAME="Heading18<FONT COLOR="#000077">Additional C Programming Tools</FONT></H3>

</CENTER>

<P>The Slackware Linux distribution includes a number of C development tools that

have not yet been described. This section describes many of these additional tools

and their typical uses.

<CENTER>

<H4><A NAME="Heading19<FONT COLOR="#000077">xxgdb</FONT></H4>

</CENTER>

<P><TT>xxgdb</TT> is an X Window system-based graphical user interface to <TT>gdb</TT>.

All of the features that exist in the command-line version of <TT>gdb</TT> are present

in <TT>xxgdb</TT>. <TT>xxgdb</TT> enables you to perform many of the most commonly

used <TT>gdb</TT> commands by pressing buttons instead of typing commands. It also

graphically represents where you have placed breakpoints.</P>

<P>You can start <TT>xxgdb</TT> by typing the following into an <TT>Xterm</TT> window.</P>

<PRE><FONT COLOR="#0066FF">xxgdb

</FONT></PRE>

<P>When you initiate <TT>xxgdb</TT>, you can specify any of the command-line options

that were available with <TT>gdb</TT>. The <TT>xxgdb</TT> program also has some of

its own command-line options. These are described in Table 27.2. </P>

<CENTER>

<P><FONT SIZE="4"><B>Table 27.2. The xxgdb command-line options. </B></FONT>

<TABLE BORDER="0">

	<TR ALIGN="LEFT" rowspan="1">

		<TD WIDTH="70" ALIGN="LEFT"><I>Option</I></TD>

		<TD ALIGN="LEFT"><I>Description</I></TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD WIDTH="70" ALIGN="LEFT"><TT>db_name</TT></TD>

		<TD ALIGN="LEFT">Specifies the name of the debugger to be used. The default is <TT>gdb</TT>.</TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD WIDTH="70" ALIGN="LEFT"><TT>db_prompt</TT></TD>

		<TD ALIGN="LEFT">Specifies the debugger prompt. The default is <TT>gdb</TT>.</TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD WIDTH="70" ALIGN="LEFT"><TT>gdbinit</TT></TD>

		<TD ALIGN="LEFT">

			<P ALIGN="CENTER">Specifies the filename of the initial <TT>gdb</TT> command file.

			The default is <TT>.gdbinit</TT>.

		</TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD WIDTH="70" ALIGN="LEFT"><TT>nx</TT></TD>

		<TD ALIGN="LEFT">Tells <TT>xxgdb</TT> not to execute the <TT>.gdbinit</TT> file.</TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD WIDTH="70" ALIGN="LEFT"><TT>bigicon</TT></TD>

		<TD ALIGN="LEFT">Uses a large icon size for the <TT>xxgdb</TT> icon.</TD>

	</TR>

</TABLE>

</P>

</CENTER>

<P><BR>

When you start <TT>xxgdb</TT>, a window opens on your screen. This window is shown

in Figure 27.1.</P>



<P>The <TT>xxgdb</TT> main window contains a message that is similar to the one displayed

on the screen when you start the command-line version of <TT>gdb</TT>. Near the bottom

of the <TT>xxgdb</TT> main window there are four buttons. The Source Listing, Command

Buttons, and Display Window buttons each bring up an additional window when they

are activated. The Quit button terminates the <TT>xxgdb</TT> program.<BR>

<BR>

<A NAME="Heading20<A HREF="../art/27/27lnx01.jpg"><FONT COLOR="#000077"><B>FIGURE

27.1.</B></FONT></A><I> The <TT>xxgdb</TT> main window.</I><BR>

<BR>

The Source Listing button brings up a window that will display the source code for

the program that is being debugged. This window is shown in Figure 27.2.</P>



<P>The Command Buttons button brings up a window that contains 26 other buttons.

These buttons each execute a <TT>gdb</TT> command. The <TT>gdb</TT> command-button

window is illustrated in Figure 27.3.</P>

<P>The Display Window button brings up a window that is used to display the results

of any display command. You can tell <TT>xxgdb</TT> what variable or expression to

display by selecting it from the source listing and then clicking on the Display

button in the command-button window. The display window is illustrated in Figure

27.4.</P>

<P>For more information on using <TT>xxgdb</TT>, refer to the <TT>xxgdb</TT> manual

page and the <TT>gdb</TT> manual page.<BR>

<BR>

<A NAME="Heading21<A HREF="../art/27/27lnx02.jpg"><FONT COLOR="#000077"><B>FIGURE

27.2.</B></FONT></A><I>The <TT>xxgdb</TT> source listing window</I><FONT COLOR="#000077">.</FONT><BR>

<BR>

<A NAME="Heading22<A HREF="../art/27/27lnx03.jpg"><FONT COLOR="#000077"><B>FIGURE

27.3.</B></FONT></A><FONT COLOR="#000077"> </FONT><I>The <TT>xxgdb</TT> command-button

window.</I><BR>

<BR>

<A NAME="Heading23<A HREF="../art/27/27lnx04.jpg"><FONT COLOR="#000077"><B>FIGURE

27.4.</B></FONT></A><FONT COLOR="#000077"> </FONT><I>The <TT>xxgdb</TT> display window.</I>

<CENTER>

<H4><A NAME="Heading24<FONT COLOR="#000077">calls</FONT></H4>

</CENTER>

<P><TT>calls</TT> is a program that is not included on the Linux CD-ROM accompanying

this book, but you can obtain a copy from the <TT>sunsite.unc.edu</TT> FTP site under

the directory <TT>/pub/Linux/devel/lang/c/calls.tar.Z</TT>. Some older CD-ROM distributions

of Linux include this file. Because it is a useful tool, we will cover it here. If

you think it will be of use to you, obtain a copy from an FTP or BBS site or another

CD-ROM. <TT>calls</TT> runs the GCC preprocessor on the files that are passed to

it on the command line, and displays a function call tree for the functions that

are in those files.



<DL>

	<DT></DT>

</DL>





<DL>

	<DD>

<HR>

<A NAME="Heading25<FONT COLOR="#000077"><B>NOTE:</B> </FONT>To install <TT>calls</TT>

	on your system, perform the following steps while you are logged in as root: 1. Uncompress

	and <TT>untar</TT> the file. 2. <TT>cd</TT> into the <TT>calls</TT> subdirectory

	that was created by <TT>untar</TT>ring the file. 3. Move the file named <TT>calls</TT>

	to the <TT>/usr/bin</TT> directory. 4. Move the file named <TT>calls.1</TT> to the

	<TT>/usr/man/man1</TT> directory. 5. Remove the <TT>/tmp/calls</TT> directory. This

	will install the <TT>calls</TT> program and man page on your system. 

<HR>



</DL>



<P>When <TT>calls</TT> prints out the call trace, it includes the filename in which

the function was found in brackets after the function name:</P>

<PRE><FONT COLOR="#0066FF">main [test.c]

</FONT></PRE>

<P>If the function was not in one of the files that was passed to <TT>calls</TT>,

it does not know where that function lives and prints only the function name:</P>

<PRE><FONT COLOR="#0066FF">printf

</FONT></PRE>

<P><TT>calls</TT> also makes note of recursive and static functions in its output.

Recursive functions are represented in the following way:</P>

<PRE><FONT COLOR="#0066FF">fact &lt;&lt;&lt; recursive in factorial.c &gt;&gt;&gt;

</FONT></PRE>

<P>Static functions are represented like this:</P>

<PRE><FONT COLOR="#0066FF">total [static in calculate.c]

</FONT></PRE>

<P>As an example, assume that you executed calls with the following program as input:</P>

<PRE><FONT COLOR="#0066FF">##include &lt;stdio.h&gt;



main ()

{

char my_string[] = &quot;hello there&quot;;

my_print (my_string);

my_print2(my_string);

}



my_print (char *string)

{

printf (&quot;The string is %s\n&quot;, string);

}



my_print2 (char *string)

{

  char *string2;

  int size, size2, i;



  size = strlen (string);

  size2 = size -1;

  string2 = (char *) malloc (size + 1);

  for (i = 0; i &lt; size; i++)

    string2[size2 - i] = string[i];

  string2[size] = `\0';

  printf (&quot;The string printed backward is %s\n&quot;, string2);

}

</FONT></PRE>

<P>This would generate the following output:</P>

<PRE><FONT COLOR="#0066FF">    1 main [test.c]

    2       my_print [test.c]

    3             printf

    4       my_print2 [test.c]

    5             strlen

    6             malloc

    7             printf

</FONT></PRE>



<DL>

	<DT><FONT COLOR="#0066FF"></FONT></DT>

</DL>





<DL>

	<DD>

<HR>

<A NAME="Heading26<FONT COLOR="#000077"><B>WARNING:</B> </FONT>The version

	of <TT>gcc</TT> included with this book's CD-ROM may generate error messages running

	this code. Again, you may want to check FTP sites for a more recent version or bug