A watchpoint is a location or region of memory that you want the debugger to observe for you. Whenever a new value is written to that area of memory, the debugger suspends execution of the program and notifies you with an alert message on your screen (Figure 11.58). After dismissing the alert, you can examine the call chain, inspect or change variables, step through your code, or use any of the debugger's other facilities. (In particular, from the debugger level, you can change the contents of the location that triggered the watchpoint without triggering it again.) Use the Run command (or the Run button on the debugger toolbar) to continue execution from the watchpoint.

The following sections discuss watchpoints in more detail:

• Setting and clearing watchpoints
• Viewing watchpoints

(Mac OS) Watchpoints require System 7.5 or later, and will not work on 68K computers unless you enable virtual memory. Watchpoints are also known to be incompatible with the Speed Emulator portion of Speed Doubler, and possibly with RAM Doubler as well.

If you have a small program with a small set of global variables for 68K, the global data area is placed against the end of the stack. Since the debugger cannot detect watchpoints for variables on the stack, the global variables cannot be watched. One way to avoid this problem is to declare a global array of type char, about four kilobytes in size, to move the global variables away from the stack.

You can set a watchpoint in any of the following ways:

• Select a variable, in a Variable window or in the Symbolics window, and choose Debug > Set Watchpoint.
• Drag a variable from another window into the Watchpoints window.
• Select a range of bytes in a Memory window and choose Debug > Set Watchpoint.
• Invoke the debugger contextual menu (Figure 11.18) for an appropriate variable or memory range and choose the Set Watchpoint command.

There are some significant restrictions on where in memory you can place a watchpoint. You can use watchpoints only on global variables or on objects allocated from your application heap. You cannot set a watchpoint on a stack-based local variable or on a variable being held in a register

You cannot set a watchpoint anywhere in low memory or the system heap. If you attempt to set a watchpoint in this area of memory, an alert box displays with the following text: "Could not set a watchpoint because the page containing that memory location overlaps low memory or the system heap."

You can clear a watchpoint in any of the following ways:

• After triggering the watchpoint, choose Debug > Clear Watchpoint.
• Select a variable, in a Variable window or in the Symbolics window, and choose Debug > Clear Watchpoint.
• Select a range of bytes in a Memory window and choose Debug > Clear Watchpoint.
• Select an existing watchpoint in the Watchpoints window and

¯ (Windows, Solaris, and Linux) Press Backspace, or choose Edit > Delete

¯ (Mac OS) Press Delete, or choose Edit > Clear

All watchpoints are automatically cleared after the debugger kills the program, or after the program's execution terminates.

You can manipulate watchpoints with the following commands in the Debug menu:

In addition, you can use the debugger contextual menu in source-code views and the Watchpoints window to choose the same menu commands. For more information, see "Debugger Contextual Menu".

To see a list of all watchpoints currently set in your program, open the Watchpoints window (Figure 11.59). Use one of the following techniques:

• (Windows menu bar layout) Choose View > Watchpoints.
• (Macintosh menu bar layout) Choose Window > Watchpoints Window.