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# 11. Debug

Open the preferences dialog (menu Edit->Preferences) and click on the Debugger item on the left; set the button Break on exceptions to Enabled: this will enable by default a special breakpoint every time an exception is raised. Click on OK to confirm your change.

Now select the menu Debug->Initialize->sdc: GPS automatically switches to the Debug perspective as shown in the menu Window->Perspectives, and new windows have appeared: the debugger data window on top, the call stack window on the right, and the debugger console at the bottom.

You can also look at the various debug menu item and tool bar buttons which are now activated.

On the call stack window (you can use the menu Debug->Data->Call Stack to open it if you do not have it displayed), select the contextual menu: various pieces of information can be displayed or removed in the call stack. From this contextual menu, add the Frame Number info by clicking on it.

Now select the menu Debug->Run... and type input.txt in the text input field. Click on OK: the debugger should stop on an exception (Constraint_Error in the file `stack.adb`, at line 49).

Go up in the call stack by clicking on the tokens.process line (frame number 6 or 7, depending on your GNAT version).

If you move the mouse over the parameter T at line 64, a tool tip is displayed showing the value of T. You have probably noticed that tool tips, like menus, are contextual: depending on the current session and on the entity selected, different information is displayed.

Select the contextual menu Debug->Display T: this will highlight the data window, with a new box displaying graphically the contents of the different fields of T, each clearly separated.

Move your mouse over the 1:T box, select the contextual menu Display->Show Value + Type: this displays for all fields both their type and value.

Special colors are used in the data display: blue for pointers that can be dereferenced by a double-click (double click on T.val); red for fields that have been modified since last step.

From the T box, right-click to display the contextual menu and select View memory at address of T: a memory view is opened on top of the source editors. Use the up and down arrows on the right to visit memory.

Click in the memory dump, and modify it by typing numbers. Notice the red color for modified values; click on Undo Changes to cancel the modifications; then close the memory window by e.g. clicking on the x icon or pressing `Ctrl-W`.

In the call stack, go back to stack.push frame (num 4 or 5). Move the mouse over Last and let the debugger display its value: 0. From the contextual menu, select Goto declaration of Last: this will jump to the line 16 of `stack.adb`, where you can see that Last is a Natural. Now click on the Goto Previous Location button in the tool bar: we’re now back at line 49 where we can see that for a Push procedure, Last should be incremented, and not decremented.

Fix the line to Last := Last + 1;

Save the file (`Ctrl-S`); End the debug session: menu Debug->Terminate; Rebuild (press `F4` key); Rerun (menu Build->Run->sdc, click on Execute): the program now completes as expected. Close the execution window.

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