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Adam Fedor
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<?xml version="1.0"?>
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<!DOCTYPE gsdoc PUBLIC "-//GNUstep//DTD gsdoc 1.0.0//EN" "http://www.gnustep.org/gsdoc-1_0_0.xml">
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<gsdoc base="Debugging" up="Tools">
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<head>
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<title>Debugging with GDB</title>
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<author name="Adam Fedor"></author>
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</head>
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<body>
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<chapter>
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<heading>Debugging</heading>
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<p>
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GDB is the GNU debugger and is the main method used for debugging
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Objective-C programs. Full support for debugging Objective-C with
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GDB was added in version 6.0. This document will describe the
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various features of GDB that help with debugging Objective-C programs.
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However, GDB is a very complex program, and not everything that it
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can do will be described here.
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</p>
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<section>
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<heading>Basic GDB usage</heading>
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<p>
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To start the debugger, specify the program you want to debug:
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</p>
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<example>
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gdb myprogram
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</example>
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<p>
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With GNUstep you can also use the debugtool and debugapp scripts
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to begin a debugging session:
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</p>
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<example>
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debugapp MyApp.app
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</example>
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<p>
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Following is a short list of important commands that gdb accepts.
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After this list, a more detailed explaination of each command is
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given.
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</p>
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<list>
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<item>run <var>args</var> - Run the program</item>
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<item>break <var>func/method</var> - Stop at a function or method</item>
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<item>print <var>var/func</var> - Print value of a variable/function</item>
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<item>backtrace - List the fuction stack</item>
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<item>frame <var>value</var> - Move up and down the fuction stack</item>
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<item>help - Get help on gdb commands</item>
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<item>quit - Quit the program</item>
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</list>
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</section>
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<section>
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<heading>The <em>run</em> command</heading>
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<p>
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This command starts the program inside the debugger. You can optionally
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add arguments to the run command and these arguments will get passed
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directly to the program as normal command-line arguments. For instance,
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you might want to start an application and open a file:
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</p>
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<example>
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run -NSOpen=afile.txt
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</example>
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</section>
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<section>
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<heading>The <em>break</em> command</heading>
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<p>
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This command instructs the debugger to stop when it reaches a
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certain location in the program. The syntax for break can be very
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complex. However we will only cover some simple examples. One instance
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is to break on a particular line number.
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</p>
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<example>
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break afile.m:345
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</example>
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<p>
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will stop the debugger at line 345 in the file <file>afile.m</file>.
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</p>
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<example>
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break a_function
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</example>
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<p>
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will tell the debugger to stop at the beggining of the
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<code>a_function</code> function. Finally, and most importantly
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for Objective-C programs, you can enter a fully-qualified or
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partially-qualified method name to stop at.
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A fully qualified Objective-C method name is specified as
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<p>
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<example>
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-[<var>Class</var> <var>methodName</var>]
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</example>
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<p>
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where the minus sign is used to indicate an instance method and
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a plus sign (not shown) is used to indicate a class method. The
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class name <var>Class</var> and method name <var>methoName</var> are
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enclosed in brackets, similar to the way messages are specified
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in Objective-C source code. For example, to set a breakpoint at
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the <code>create</code> instance method of class <code>Fruit</code>
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in the program currently being debugged, enter:
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<p>
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<example>
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break -[Fruit create]
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</example>
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<p>
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One can also specify just a method name:
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</p>
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<example>
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break initWithValue:
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</example>
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<p>
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gdb will automatically determine what class this method belongs to. If
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there is more than one class that implements this method, you will
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be presented with a list of classes that implement the method from which
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you must chose one.
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</p>
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</section>
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<section>
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<heading>The <em>print</em> command</heading>
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<p>
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The print command can be used to display a wide variety of information
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that gdb knows about the program. As with the <var>break</var> command,
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the variety of things you can do is very large, but we will discuss only
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a few of the things that can be done. Below are several simple examples
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of printing the value of a variable.
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</p>
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<example>
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print aVariable
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print anIvar
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print self->anIvar
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print anArray[4]
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print aStruct.subvalue
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print *(int *)pointerValue
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</example>
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<p>
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Note that you can specify variables in the same way you specify them
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in source code, using array subscripts, pointer dereferences, etc.
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You can also set the value of a variable using print:
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</p>
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<example>
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print aVariable = 4
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</example>
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<p>
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One can also print the value of a function. Here gdb will actually
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call the function you specify and return the output:
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</p>
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<example>
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print my_function(4, "hithere")
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</example>
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<p>
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When debugging Objective-C programs, the same thing can be done
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with methods.
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</p>
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<example>
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print -[object hash]
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</example>
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<p>
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A special command has been added to gdb to print the
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<var>description</var> of an object (based on the method of the
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same name). This is the <var>print-object</var> (or <var>po</var>)
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command:
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</p>
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<example>
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po anObject
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</example>
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<p>
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Which is the same as typing
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</p>
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<example>
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print -[myObject desciption]
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</example>
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</section>
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<section>
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<heading>Other command</heading>
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<p>
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The <var>clear</var>, <var>info line</var>, <var>jump</var>,
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and <var>list</var> commands also accept Objective-C method
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syntax for specifying locations.
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</section>
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</chapter>
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</body>
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</gsdoc>
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