11. Debugging

GPS is also a graphical front-end for text-based debuggers such as GDB. A knowledge of the basics of the underlying debugger used by GPS will help understanding how GPS works and what kind of functionalities it provides.

Please refer to the debugger-specific documentation - e.g. the GDB documentation (gdb.html) - for more details.

Debugging is tightly integrated with the other components of GPS. For example, it is possible to edit files and navigate through your sources while debugging.

To start a debug session, go to the menu Debug->Initialize, and choose either the name of your executable, if you have specified the name of your main program(s) in the project properties, or start an empty debug session using the <no main file> item. It is then possible to load any file to debug, by using the menu Debug->Debug->Load File...

After the debugger has been initialized, you have access to two new windows: the data window (in the top of the working area), and the debugger console (in a new page, after the Messages and Shell windows). All the menus under Debugger are now also accessible, and you also have access to additional contextual menus, in particular in the source editor where it is possible to easily display variables, set breakpoints, and get automatic display (via tool tips) of object values.

When you want to quit the debugger without quitting GPS, go to the menu Debug->Terminate, which will terminate your debug session.

11.1 The Debug Menu
11.2 The Call Stack Window
11.3 The Data Window
11.4 The Breakpoint Editor
11.5 The Memory Window
11.6 Using the Source Editor when Debugging
11.7 The Assembly Window
11.8 The Debugger Console

11.1 The Debug Menu

The Debug entry in the menu bar provides operations that act at a global level. Key shortcuts are available for the most common operations, and are displayed in the menus themselves. Here is a detailed list of the menu items that can be found in the menu bar:

Run...

Opens a dialog window allowing you to specify the arguments to pass to the program to be debugged, and whether this program should be stopped at the beginning of the main subprogram. If you confirm by clicking on the OK button, the program will be launched according to the arguments entered.

Step

Execute the program until it reaches a different source line.

Step Instruction

Execute the program for one machine instruction only.

Next

Execute the program until it reaches the next source line, stepping over subroutine calls.

Next Instruction

Execute the program until it reaches the next machine instruction, stepping over subroutine calls.

Finish

Continue execution until selected stack frame returns.

Continue

Continue execution of the program being debugged.

Interrupt

Asynchronously interrupt the program being debugged. Note that depending on the state of the program, you may stop it in low-level system code that does not have debug information, or in some cases, not even a coherent state. Use of breakpoints is preferable to interrupting programs. Interrupting programs is nevertheless indispensable in some situations, for example when the program appears to be in an infinite (or at least very time-consuming) loop.

Terminate

Terminate your debug session, by closing the data window and the debugger console, as well as terminating the underlying debugger (e.g gdb) used to handle the low level debugging.

11.1.1 Debug

Connect to Board...

Opens a simple dialog to connect to a remote board. This option is only relevant to cross debuggers.

Load File...

Opens a file selection dialog that allows you to choose a program to debug. The program to debug is either an executable for native debugging, or a partially linked module for cross environments (e.g VxWorks).

Add Symbols...

Add the symbols from a given file/module. This corresponds to the gdb command add-symbol-file. This menu is particularly useful under VxWorks targets, where the modules can be loaded independently of the debugger. For instance, if a module is independently loaded on the target (e.g. using windshell), it is absolutely required to use this functionality, otherwise the debugger won't work properly.

Attach...

Instead of starting a program to debug, you can instead attach to an already running process. To do so, you need to specify the process id of the process you want to debug. The process might be busy in an infinite loop, or waiting for event processing. Note that as for core files, you need to specify an executable before attaching to a process.

Detach

Detaches the currently debugged process from the underlying debugger. This means that the executable will continue to run independently. You can use the Attach To Process menu later to re-attach to this process.

Debug Core File...

This will open a file selection dialog that allows you to debug a core file instead of debugging a running process. Note that you must first specify an executable to debug before loading a core file.

Kill

Kills the process being debugged.

11.1.2 Data

Call Stack

Displays the Call Stack window. See 11.2 The Call Stack Window for more details.

Threads

Opens a new window containing the list of threads currently present in the executable as reported by the underlying debugger. For each thread, it will give information such as internal identifier, name and status. This information is language- and debugger-dependent. You should refer to the underlying debugger's documentation for more details. As indicated above, the process being debugged needs to be stopped before using this command, otherwise a blank list will be displayed.

When supported by the underlying debugger, clicking on a thread will change the context (variables, call stack, source file) displayed, allowing you to inspect the stack of the selected thread.

Tasks

For GDB only, this will open a new window containing the list of Ada tasks currently present in the executable. Similarly to the thread window, you can switch to a selected task context by clicking on it, if supported by GDB. See the GDB documentation for the list of items displayed for each task.

As for the thread window, the process being debugged needs to be stopped before using this window.

Protection Domains

For VxWorks AE only, this will open a new window containing the list of available protection domains in the target. To change to a different protection domain, simply click on it. A indicates the current protection domain.

Assembly

Opens a new window displaying an assembly dump of the current code being executed. See 11.7 The Assembly Window for more details.

Edit Breakpoints

Opens an advanced window to create and modify any kind of breakpoint (see 11.4 The Breakpoint Editor). For simple breakpoint creation, see the description of the source window.

Examine Memory

Opens a memory viewer/editor. See 11.5 The Memory Window for more details.

Command History

Opens a dialog with the list of commands executed in the current session. You can select any number of items in this list and replay the selection automatically.

Display Local Variables

Opens an item in the Data Window containing all the local variables for the current frame.

Display Arguments

Opens an item in the Data Window containing the arguments for the current frame.

Display Registers

Opens an item in the Data Window containing the machine registers for the current frame.

Display Any Expression...

Opens a small dialog letting you specify an arbitrary expression in the Data Window. This expression can be a variable name, or a more complex expression, following the syntax of the underlying debugger. See the documentation of e.g gdb for more details on the syntax. The check button Expression is a subprogram call should be enabled if the expression is actually a debugger command (e.g p/x var) or a procedure call in the program being debugged (e.g call my_proc).

Refresh

Refreshes all the items displayed in the Data Window.

11.2 The Call Stack Window

The call stack window gives a list of frames corresponding to the current execution stack for the current thread/task.

The bottom frame corresponds to the outermost frame where the thread is currently stopped. This frame corresponds to the first function executed by the current thread (e.g main if the main thread is in C). You can click on any frame to switch to the caller's context, this will update the display in the source window. See also the up and down buttons in the tool bar to go up and down one frame in the call stack.

The contextual menu (right mouse button) allows you to choose which information you want to display in the call stack window (via check buttons):

• Frame number: the debugger frame number (usually starts at 0 or 1)
• Program Counter: the low level address corresponding to the function's entry point.
• Subprogram Name: the name of the subprogram in a given frame
• Parameters: the parameters of the subprogram
• File Location: the filename and line number information.

By default, only the subprogram name is displayed. You can hide the call stack window by closing it, as for other windows, and show it again using the menu Data->Call Stack.

11.3 The Data Window

11.3.1 Description

The data window contains all the graphic boxes that can be accessed using the Data->Display menu items, or the data window Display Expression... contextual menu, or the source window Display contextual menu items, or finally the graph command in the debugger console.

For each of these commands, a box is displayed in the data window with the following information:

• A title bar containing:
  • The number of this expression: this is a positive number starting from 1 and incremented for each new box displayed. It represents the internal identifier of the box.

  • The name of the expression: this is the expression or variable specified when creating the box.

  • An icon representing either a flash light, or a lock. This is a click-able icon that will change the state of the box from automatically updated (the flash light icon) to frozen (the lock icon). When frozen, the value is grayed, and will not change until you change the state. When updated, the value of the box will be recomputed each time an execution command is sent to the debugger (e.g step, next).

  • An icon representing an 'X'. You can click on this icon to close/delete any box.

• A main area. The main area will display the data value hierarchically in a language-sensitive manner. The canvas knows about data structures of various languages (e.g C, Ada, C++) and will organize them accordingly. For example, each field of a record/struct/class, or each item of an array will be displayed separately. For each subcomponent, a thin box is displayed to distinguish it from the other components.

A contextual menu, that takes into account the current component selected by the mouse, gives access to the following capabilities:

Close component

Closes the selected item.

Hide all component

Hides all subcomponents of the selected item. To select a particular field or item in a record/array, move your mouse over the name of this component, not over the box containing the values for this item.

Show all component

Shows all subcomponents of the selected item.

Clone component

Clones the selected component into a new, independent item.

View memory at address of component

Brings up the memory view dialog and explore memory at the address of the component.

Set value of component

Sets the value of a selected component. This will open an entry box where you can enter the new value of a variable/component. Note that GDB does not perform any type or range checking on the value entered.

Update Value

Refreshes the value displayed in the selected item.

Show Value

Shows only the value of the item.

Show Type

Shows only the type of each field for the item.

Show Value+Type

Shows both the value and the type of the item.

Auto refresh

Enables or disables the automatic refreshing of the item upon program execution (e.g step, next).

A contextual menu can be accessed in the canvas itself (point the mouse to an empty area in the canvas, and click on the right mouse button) with the following entries:

Display Expression...

Open a small dialog letting you specify an arbitrary expression in the Data Window. This expression can be a variable name, or a more complex expression, following the syntax of the current language and underlying debugger. See the documentation of e.g gdb for more details on the syntax. The check button Expression is a subprogram call should be enabled if the expression is actually not an expression but rather a debugger command (e.g p/x var) or a procedure call in the program being debugged (e.g call my_proc).

Align On Grid

Enables or disables alignment of items on the grid.

Detect Aliases

Enables or disables the automatic detection of shared data structures. Each time you display an item or dereference a pointer, all the items already displayed on the canvas are considered and their addresses are compared with the address of the new item to display. If they match, (for example if you tried to dereference a pointer to an object already displayed) instead of creating a new item a link will be displayed.

Zoom in

Redisplays the items in the data window with a bigger font

Zoom out

Displays the items in the data window with smaller fonts and pixmaps. This can be used when you have several items in the window and you can't see all of them at the same time (for instance if you are displaying a tree and want to clearly see its structure).

Zoom

Allows you to choose the zoom level directly from a menu.

11.3.2 Manipulating items

11.3.2.1 Moving items

Automatic scrolling is also provided if you move the mouse while dragging an item near the borders of the data window. As long as the mouse remains close to the border and the button is pressed on the item, the data window is scrolled and the item is moved. This provides an easy way to move an item a long distance from its initial position.

11.3.2.2 Colors

black

This is the default color used to print the value of variables or expressions.

blue

This color is used for C pointers (or Ada access values), i.e. all the variables and fields that are memory addresses that denote some other value in memory.

You can easily dereference these (that is to say see the value pointed to) by double-clicking on the blue text itself.

red

This color is used for variables and fields whose value has changed since the data window was last displayed. For instance, if you display an array in the data window and then select the Next button in the tool bar, then the elements of the array whose value has just changed will appear in red.

As another example, if you choose to display the value of local variables in the data window (Display->Display Local Variables), then only the variables whose value has changed are highlighted, the others are left in black.

11.3.2.3 Icons

trash bin icon

This icon indicates that the debugger could not get the value of the variable or expression. There might be several reasons, for instance the variable is currently not in scope (and thus does not exist), or it might have been optimized away by the compiler. In all cases, the display will be updated as soon as the variable becomes visible again.

package icon

This icon indicates that part of a complex structure is currently hidden. Manipulating huge items in the data window (for instance if the variable is an array of hundreds of complex elements) might not be very helpful. As a result, you can shrink part of the value to save some screen space and make it easier to visualize the interesting parts of these variables.

Double-clicking on this icon will expand the hidden part, and clicking on any sub-rectangle in the display of the variable will hide that part and replace it with that icon.

See also the description of the contextual menu to automatically show or hide all the contents of an item. Note also that one alternative to hiding subcomponents is to clone them in a separate item (see the contextual menu again).

11.4 The Breakpoint Editor

The breakpoint editor can be accessed from the menu Data->Edit Breakpoints. It gives access to the different kind of breakpoints: on source location, subprogram, address, variables, exceptions.

The top area provides an interface to easily create the different kinds of breakpoints, while the bottom area lists the existing breakpoints and their characteristics.

It is possible to access to advanced breakpoint characteristics by clicking on the Advanced button, which will display a new area on the right of the window, where you can specify commands to run automatically after a breakpoint is hit, or specify how many times a selected breakpoint will be ignored. If running VxWorks AE, you can also change the Scope and Action of breakpoints.

11.4.1 Scope/Action Settings for VxWorks AE

In VxWorks AE breakpoints have two extra properties:

• Scope: which task(s) can hit a given breakpoint. Possible Scope values are:
  • task: the breakpoint can only be hit by the task that was active when the breakpoint was set. If the breakpoint is set before the program is run, the breakpoint will affect the environment task
  • pd: any task in the current protection domain can hit that breakpoint
  • any: any task in any protection domain can hit that breakpoint. This setting is only allowed for tasks in the Kernel domain.

• Action: when a task hits a breakpoints, which tasks are stopped:
  • task: stop only the task that hit the breakpoint.
  • pd: stop all tasks in the current protection domain
  • all: stop all breakable tasks in the system

These two properties can be set/changed through the advanced breakpoints characteristics by clicking on the Advanced button. There are two ways of setting these properties:

• Per breakpoint settings: after setting a breakpoint (the default Scope/Action values will be task/task), select the Scope/Action tab in the Advanced settings. To change these settings on a given breakpoint, select it from the breakpoints list, select the desired values of Scope and Action and click on the Update button.

• Default session settings: select the Scope/Action tab in the Advanced settings. Select the desired Scope and Action settings, check the Set as session defaults check box below and click the Close button. From now on, every new breakpoint will have the selected values for Scope and Action.

11.5 The Memory Window

The memory window allows you to display the contents of memory by specifying either an address, or a variable name.

To display memory contents, enter the address using the C hexadecimal notation: 0xabcd, or the name of a variable, e.g foo, in the Location text entry. In the latter case, its address is computed automatically. Then either press Enter or click on the View button. This will display the memory with the corresponding addresses in the bottom text area.

You can also specify the unit size (Byte, Halfword or Word), the format (Hexadecimal, Decimal, Octal or ASCII), and you can display the corresponding ASCII value at the same time.

The up and down arrows as well as the Page up and Page down keys in the memory text area allows you to walk through the memory in order of ascending/descending addresses respectively.

Finally, you can modify a memory area by simply clicking on the location you want to modify, and by entering the new values. Modified values will appear in a different color (red by default) and will only be taken into account (i.e written to the target) when you click on the Submit changes button. Clicking on the Undo changes or going up/down in the memory will undo your editing.

Clicking on Close will close the memory window, canceling your last pending changes, if any.

11.6 Using the Source Editor when Debugging

When debugging, the left area of each source editor provides the following information:

Lines with code

In this area, blue dots are present next to lines for which the debugger has debug information, in other words, lines that have been compiled with debug information and for which the compiler has generated some code. Currently, there is no check when you try to set a breakpoint on a non dotted line: this will simply send the breakpoint command to the underlying debugger, and usually (e.g in the case of gdb) result in setting a breakpoint at the closest location that matches the file and line that you specified.

Current line executed

This is a green arrow showing the line about to be executed.

Lines with breakpoints

For lines where breakpoints have been set, a red mark is displayed on top of the blue dot for the line. You can add and delete breakpoints by clicking on this area (the first click will set a breakpoint, the second click will remove it).

The second area in the source window is a text window on the right that displays the source files, with syntax highlighting. If you leave the cursor over a variable, a tooltip will appear showing the value of this variable. Automatic tooltips can be disabled in the preferences menu. See preferences dialog.

When the debugger is active, the contextual menu of the source window contains a sub menu called Debug providing the following entries.

Note that these entries are dynamic: they will apply to the entity found under the cursor when the menu is displayed (depending on the current language). In addition, if a selection has been made in the source window the text of the selection will be used instead. This allows you to display more complex expressions easily (for example by adding some comments to your code with the complex expressions you want to be able to display in the debugger).

Print selection

Prints the selection (or by default the name under the cursor) in the debugger console.

Display selection

Displays the selection (or by default the name under the cursor) in the data window. The value will be automatically refreshed each time the process state changes (e.g after a step or a next command). To freeze the display in the canvas, you can either click on the corresponding icon in the data window, or use the contextual menu for the specific item (see 11.3 The Data Window for more information).

Print selection.all

Dereferences the selection (or by default the name under the cursor) and prints the value in the debugger console.

Display selection.all

Dereferences the selection (or by default the name under the cursor) and displays the value in the data window.

View memory at address of selection

Brings up the memory view dialog and explores memory at the address of the selection.

Set Breakpoint on Line xx

Sets a breakpoint on the line under the cursor, in the current file.

Set Breakpoint on selection

Sets a breakpoint at the beginning of the subprogram named selection

Continue Until Line xx

Continues execution (the program must have been started previously) until it reaches the specified line.

Show Current Location

Jumps to the current line of execution. This is particularly useful after navigating through your source code.

11.7 The Assembly Window

It is sometimes convenient to look at the assembly code for the subprogram or source line you are currently debugging.

You can open the assembly window by using the menu Debug->Data->Assembly.

The current assembly instruction is highlighted with a green arrow on its left. The instructions corresponding to the current source line are highlighted in red by default. This allows you to easily see where the program counter will point to, once you have pressed the "Next" button on the tool bar.

Moving to the next assembly instruction is done through the "Nexti" (next instruction) button in the tool bar. If you choose "Stepi" instead (step instruction), this will also jump to the subprogram being called.

For efficiency reasons, only a small part of the assembly code around the current instruction is displayed. You can specify in the preferences dialog how many instructions are displayed by default. Also, you can easily display the instructions immediately preceding or following the currently displayed instructions by pressing one of the Page up or Page down keys, or by using the contextual menu in the assembly window.

A convenient complement when debugging at the assembly level is the ability of displaying the contents of machine registers. When the debugger supports it (as gdb does), you can select the Data->Display Registers menu to get an item in the canvas that will show the current contents of each machine register, and that will be updated every time one of them changes.

You might also choose to look at a single register. With gdb, select the Data->Display Any Expression, entering something like

output /x $eax

in the field, and selecting the toggle button "Expression is a subprogram call". This will create a new canvas item that will be refreshed every time the value of the register (in this case eax) changes.

11.8 The Debugger Console

This is the text window located at the bottom of the main window. In this console, you have direct access to the underlying debugger, and can send commands (you need to refer to the underlying debugger's documentation, but usually typing help will give you an overview of the commands available).

If the underlying debugger allows it, pressing Tab in this window will provide completion for the command that is being typed (or for its arguments).

There are also additional commands defined to provide a simple text interface to some graphical features.

Here is the complete list of such commands. The arguments between square brackets are optional and can be omitted.

graph (print|display) expression [dependent on display_num] [link_name name]

This command creates a new item in the canvas, that shows the value of Expression. Expression should be the name of a variable, or one of its fields, that is in the current scope for the debugger.

The command graph print will create a frozen item, that is not automatically refreshed when the debugger stops, whereas graph display displays an automatically refreshed item.

The new item is associated with a number, that is visible in its title bar. These numbers can be used to create links between the items, using the second argument to the command, dependent on. The link itself (i.e. the line) can be given a name that is automatically displayed, using the third argument.

graph (print|display) `command`

This command is similar to the one above, except it should be used to display the result of a debugger command in the canvas.

For instance, if you want to display the value of a variable in hexadecimal rather than the default decimal with gdb, you should use a command like:

graph display `print /x my_variable`

This will evaluate the command between back-quotes every time the debugger stops, and display this in the canvas. The lines that have changed will be automatically highlighted (in red by default).

This command is the one used by default to display the value of registers for instance.

graph (enable|disable) display display_num [display_num ...]

This command will change the refresh status of items in the canvas. As explained above, items are associated with a number visible in their title bar.

Using the graph enable command will force the item to be automatically refreshed every time the debugger stops, whereas the graph disable command will freeze the item.

graph undisplay display_num

This command will remove an item from the canvas

view (source|asm|source_asm)

This command indicates what should be displayed in the source window. The first option indicates that only the source code should be visible, the second one specifies that only the assembly code should be visible, and the last one indicates that both should be displayed.