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<title>Debugging with GDB</title>
</head>
<body>
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<a href="Tools.html">Up</a>
<br />
<h1><a name="title$Debugging">Debugging with GDB</a></h1>
<h3>Authors</h3>
<dl>
<dt>Adam Fedor</dt>
<dd>
</dd>
</dl>
<p><b>Copyright:</b> (C) Copyright (C) 2005 Free Software Foundation, Inc.</p>
<h1><a name="001000000000">Debugging</a></h1>
<p>
GDB is the GNU debugger and is the main method used for debugging
Objective-C programs. Full support for debugging Objective-C with
GDB was added in version 6.0. This document will describe the
various features of GDB that help with debugging Objective-C programs.
However, GDB is a very complex program, and not everything that it
can do will be described here.
</p>
<h2><a name="001001000000">Basic GDB usage</a></h2>
<p>
To start the debugger, specify the program you want to debug:
</p>
<pre>
gdb myprogram
</pre>
<p>
With GNUstep you can also use the debugtool and debugapp scripts
to begin a debugging session:
</p>
<pre>
debugapp MyApp.app
</pre>
<p>
Following is a short list of important commands that gdb accepts.
After this list, a more detailed explaination of each command is
given.
</p>
<ul>
<li>
run <var>args</var> - Run the program
</li>
<li>
break <var>func/method</var> - Stop at a function or method
</li>
<li>
print <var>var/func</var> - Print value of a variable/function
</li>
<li>
backtrace - List the fuction stack
</li>
<li>
frame <var>value</var> - Move up and down the fuction stack
</li>
<li>
help - Get help on gdb commands
</li>
<li>
quit - Quit the program
</li>
</ul>
<h2><a name="001002000000">The <em>run</em> command</a></h2>
<p>
This command starts the program inside the debugger. You can optionally
add arguments to the run command and these arguments will get passed
directly to the program as normal command-line arguments. For instance,
you might want to start an application and open a file:
</p>
<pre>
run -NSOpen=afile.txt
</pre>
<h2><a name="001003000000">The <em>break</em> command</a></h2>
<p>
This command instructs the debugger to stop when it reaches a
certain location in the program. The syntax for break can be very
complex. However we will only cover some simple examples. One instance
is to break on a particular line number.
</p>
<pre>
break afile.m:345
</pre>
<p>
will stop the debugger at line 345 in the file <code>afile.m</code>.
</p>
<pre>
break a_function
</pre>
<p>
will tell the debugger to stop at the beggining of the
<code>a_function</code> function. Finally, and most importantly
for Objective-C programs, you can enter a fully-qualified or
partially-qualified method name to stop at.
A fully qualified Objective-C method name is specified as
</p>
<pre>
-[Class methodName]
</pre>
<p>
where the minus sign is used to indicate an instance method and
a plus sign (not shown) is used to indicate a class method. The
class name <var>Class</var> and method name <var>methodName</var> are
enclosed in brackets, similar to the way messages are specified
in Objective-C source code. For example, to set a breakpoint at
the <code>create</code> instance method of class <code>Fruit</code>
in the program currently being debugged, enter:
</p>
<pre>
break -[Fruit create]
</pre>
<p>
One can also specify just a method name:
</p>
<pre>
break initWithValue:
</pre>
<p>
gdb will automatically determine what class this method belongs to. If
there is more than one class that implements this method, you will
be presented with a list of classes that implement the method from which
you must chose one.
</p>
<h2><a name="001004000000">The <em>print</em> command</a></h2>
<p>
The print command can be used to display a wide variety of information
that gdb knows about the program. As with the <var>break</var> command,
the variety of things you can do is very large, but we will discuss only
a few of the things that can be done. Below are several simple examples
of printing the value of a variable.
</p>
<pre>
print aVariable
print anIvar
print self->anIvar
print anArray[4]
print aStruct.subvalue
print *(int *)pointerValue
</pre>
<p>
Note that you can specify variables in the same way you specify them
in source code, using array subscripts, pointer dereferences, etc.
You can also set the value of a variable using print:
</p>
<pre>
print aVariable = 4
</pre>
<p>
One can also print the value of a function. Here gdb will actually
call the function you specify and return the output:
</p>
<pre>
print my_function(4, "hithere")
</pre>
<p>
When debugging Objective-C programs, the same thing can be done
with methods.
</p>
<pre>
print -[object hash]
</pre>
<p>
A special command has been added to gdb to print the
<var>description</var> of an object (based on the method of the
same name). This is the <var>print-object</var> (or <var>po</var>)
command:
</p>
<pre>
po anObject
</pre>
<p>
Which is the same as typing
</p>
<pre>
print -[myObject desciption]
</pre>
<h2><a name="001005000000">Other command</a></h2>
<p>
The <var>clear</var>, <var>info line</var>, <var>jump</var>,
and <var>list</var> commands also accept Objective-C method
syntax for specifying locations.
</p>
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