Mac OS X Leopard - ARCAism

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CHAPTER 25 MAC OS X DEVELOPMENT: THE TOOLS
Breakpoints
Debuggers, like vampires, cannot enter uninvited. To invite GDB into your program, mark your
code with breakpoints. Setting a breakpoint is simple—just click the gutter that runs along the
left side of the editor. This will add a blue pointer, representing the breakpoint. To toggle the
breakpoint on and off, click it. To remove it, drag it from the gutter and it will vanish in a puff
of smoke.
Normally, breakpoints are inactive and the program runs without paying any attention to
them. To activate breakpoints, select Activate Breakpoints from the Run menu, or click the
appropriate icon in the toolbar.
The normal reaction to a breakpoint is for the program to suspend execution before the line
of code that contains the breakpoint. However, this is not the only option. Breakpoints can be set
to only break on certain conditions, or to immediately continue after logging something to the
console, running other GDB commands, making a sound, or even reading something aloud via
text-to-speech. Unlike hard-coded logging, breakpoints can be set, removed, deactivated, moved,
and edited on the fly.
Ctrl+clicking or right-clicking the gutter will bring up the breakpoint contextual menu,
which has several additional options, including a list of predefined breakpoint behaviors. There
is also a Breakpoints window where breakpoints can be set, removed, and edited. To launch the
Breakpoints window, select Breakpoints from the Show submenu of the Run menu.
You can also set breakpoints outside your own code. A common trick among programmers
is to create a breakpoint named objc_exception_throw. This will cause the program to automatically
stop whenever it is about to throw an error. Since unexpected errors lead to crashes, this
allows the programmer to examine the state of the program right before everything falls apart.
Xcode’s Debugger Interface
When a program stops at a breakpoint, Xcode launches its graphical interface to GDB, as shown
in Figure 25-18. From the GDB interface, the program’s state and execution can be examined
and controlled.
Figure 25-18. Xcode’s traditional debugging environment