Debugging Programs with gdb

The GNU debugger is gdb . It was written and is maintained by the Free Software Foundation. gdb is a powerful tool for looking inside a running program and trying to figure out why that program is not behaving as expected. Apple has modified gdb to be aware of Objective-C syntax and objects, and to work together with the PB and Terminal applications.

The gdb tool is located in the directory /usr/bin. If you click the Computer icon in the Finder’s toolbar and then select Macintosh HD, you will not see /usr listed. Mac OS X and the Finder hide many system details from the user, including Unix system directories such as /usr/bin, /bin, and /etc. You can view these directories in the Finder using the Go to Folder sheet, but you cannot see all directories in the Finder (e.g., the .app directories are hidden). You can, however, see all the filesystem directories in a Terminal shell — your vehicle for exploring the guts of Mac OS X.

Using gdb in Project Builder, Step by Step

The easiest way to use the gdb debugger is in PB. We’ll show you how to get started with that in this section. First, we must have an application to work with, so we’ll use a copy of the CircleView example application that is bundled with the Mac OS X developer system.

When you debug a running program within PB, you can access some of the most useful gdb commands graphically. You can set breakpoints by clicking the mouse next to a line of code (a breakpoint is a place where a running program stops executing and control is returned to the debugger). When a breakpoint is reached in a running program, the stack frame and the variables on the stack will be displayed in the debugger window. You can also use the buttons on the upper-right side of the PB window to control execution. The up and down arrows will step you up and down the call stack. The arrow over the parenthesis will execute a gdb step command. The pause button will pause execution, and the button with the triangle will continue program execution.

Let’s try a few of these commands on a real example:

  1. Open the Go to Folder sheet in the Finder and enter “/Developer/Examples/AppKit”.

  2. Copy the CircleView folder into your Home folder by Option-dragging the folder and dropping it on your Home icon. (If CircleView is not available, choose another example application in the same folder.)

  3. Click your Home icon and then click the CircleView folder. You’ll see the files in the folder in column view, as shown in Figure 2-13.

CircleView folder with files for the CircleView project

Figure 2-13. CircleView folder with files for the CircleView project

  1. Open the CircleView project in PB by double-clicking on the CircleView.pbproj project file in the CircleView folder.

  2. Click the build and run button above the Groups & Files pane in PB.

    After a few seconds, the application will be built (compiled) and will run. The build and run button will turn into a stop button. The running CircleView application is shown in Figure 2-14.

The CircleView application running

Figure 2-14. The CircleView application running

  1. You can play with this fun little application by dragging the sliders, changing the text (try entering “Mac OS X”), changing the color, and clicking the nondescript button below the color well to animate the text.

  2. Quit the CircleView application by clicking the stop button in PB’s toolbar (or quit CircleView directly).

  3. Back in PB, click the disclosure triangle next to Classes in the Groups & Files pane to see the CircleView.h and CircleView.m class files.

  4. Single-click the CircleView.m file to open it in the lower-right pane in PB’s main window, as shown in Figure 2-15.

The CircleView project in PB

Figure 2-15. The CircleView project in PB

  1. Press the up-down (“stepper”) arrows to the right of “<No selected symbol>” in the middle of the PB window, drag to “-setColor:”, and release the mouse button. This action takes you to the “-setColor:” method (like a function) in the CircleView.m file.

  2. Set a breakpoint by clicking the mouse in the white column to the left of “-(void)setColor:”. An arrow should appear, as in Figure 2-16.

Stopped at a CircleView breakpoint in PB

Figure 2-16. Stopped at a CircleView breakpoint in PB

In a moment we’ll run the CircleView program in debug mode within PB. PB will run gdb in the background and provide us with a nice graphical interface to many of its commands. (If you want to see that gdb is running in the background, look at your running processes in ProcessViewer). We’ll test some of the graphical commands available in PB, but first we’ll clean out (remove) all of the derived build files in the project to ensure that we start from scratch:

  1. Clean the CircleView build files by clicking the clean active target (whiskbroom) button in PB’s toolbar. Click the clean active target (blue) button in the resulting sheet.

  2. Now click the build and debug button in PB’s toolbar. The button displays a hammer and a bug spray can icon that turns into a stop button when the application runs.

The CircleView application should now be running in the foreground. Because we set the breakpoint at the setColor: method, we won’t reach it until we try to change the color.

  1. Click the border of the color well in the CircleView window.

You might expect the Font window to appear, but it doesn’t because of the breakpoint. Your PB window should now look like the one in Figure 2-16. You can see the “Step into method or function call” button at the right of PB’s toolbar. This action executes one step (line of code). You can use it repeatedly to step through your program and debug it line by line.

  1. Click the “Step over method or function call” button. This action steps out of the current method.

  2. Click the “Continue execution” button. This action continues execution of the CircleView program.

  3. Now reactivate CircleView, and the Font window will appear.

  4. Quit CircleView.

PB made it easy for us to use gdb to debug CircleView. Next, we’ll show you the “hard” way, in a Terminal window.

Using gdb in a Terminal Window

You can run gdb directly in a Terminal window, or from within GNU Emacs in a Terminal window. Either way, you must specify the filename of the actual Mach executable file in the /Contents/MacOS directory in the “app wrapper,” rather than the application directory itself (the directory that ends with .app). Thus, to debug the program CircleView.app with gdb at the Terminal shell command line, type the text shown in bold below. 

localhost> cd ~/CircleView/build
localhost> gdb CircleView.app/Contents/MacOS/CircleView
...

gdb now starts up with about 10 lines of output. Next, we’ll set the same breakpoint that we set earlier, in PB (line 160 is where the setColor: method begins in CircleView.m): 

(gdb) b CircleView.m:160
(gdb) help
(gdb) quit

That’s enough to get you started. Now, to run gdb from GNU Emacs, use the Emacs command:[6] 

                  M-X gdb <return>

After this, Emacs will prompt you for:

Run gdb on file: /Users/me

where /me is the user’s home directory.

At this prompt, type the name of the file that you want to debug:

Run gdb on file: ~/CircleView/build/CircleView.app/Contents/MacOS/CircleView

The advantage of using gdb from Emacs is that Emacs will automatically split the screen into two windows, giving you a gdb buffer in one and following the program that you are debugging in the other. Many programmers find this an effective way to work.

gdb Commands

gdb is a complicated program with dozens of commands. Fortunately, to get started you need to know only a few basic commands.

Typically, when you are using gdb, you will set a breakpoint and then run your program until you reach that breakpoint. Your program will then automatically stop running, and you will be free to inspect the contents of the variables. Table 2-1 lists the commands you would use to set breakpoints at a few sample locations.

Table 2-1. gdb commands for setting breakpoints

Use this gdb command

To set a breakpoint at

b myFile.m:53

Line 53 in the file myFile.m 

b printer

The function printer( ) 

b [MyView drawrect:]

The Objective-C method drawRect: in the class MyView

To run your program, enter run in the Terminal window. After your program reaches a breakpoint (or if your program crashes), you will return to the gdb command line. You can also interrupt your program’s execution by typing Control-C. From the command line, you may find the commands in Table 2-2 useful in debugging.

Table 2-2. Useful gdb commands

Command

Purpose

run

Starts your program

where

Shows a stack-trace of your program

list

Lists part of your code

print expr

Displays the value of a local variable (expr); p is a shortcut

You can control program execution using the n command in gdb to execute the next statement and the s command to execute another step of your program. The difference between these two commands has to do with how they handle function calls. If the next statement to be executed is a function call, the s command will step into the function, whereas the n command will cause the entire function to be executed and then stop the program. When you are stopped within a program, the up command will jump you up the call stack, while the down command will take you down the call stack.

That’s enough of gdb for now. Use gdb’s help command to learn more.


[6] To type M-X gdb <return> into the Emacs editor, press the Esc key, press the X key, type gdb, and press the Return or Enter key.

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