gnat-gps-doc 5.3dfsg-1ubuntu1 / usr / share / doc / gnat-gps / users_guide / _sources / debugging.txt

.. _Debugging:

*********
Debugging
*********

.. index:: debugger
.. index:: 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 GNAT User's
Guide (chapter *Running and Debugging Ada Programs*) or the GDB documentation
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.

.. index:: menu

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...`

Note that you first need to build your executable with debug information (`-g`
switch), either explicitly as part of your project properties, or via the
`Debug` build mode (see :ref:`The_Build_Mode` for more details).

Note that you can create multiple debuggers by using the `Initialize` menu
several times: this will create a new debugger each time.  All the
debugger-related actions (e.g. stepping, running) are performed on the current
debugger, which is represented by the current debugger console.  To switch
between debuggers, simply select its corresponding console.

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.

.. index:: menu

When you want to quit the debugger without quitting GPS, go to the menu
`Debug->Terminate Current`, that will terminate your current debug session, or
the menu `Debug->Terminate` that will terminate all your debug sessions at
once.

.. _The_Debug_Menu:

The Debug Menu
==============

.. index:: menu
.. index:: debug
.. index:: debugger

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...*
  .. index:: 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*
  .. index:: step

  Execute the program until it reaches a different source line.

*Step Instruction*
  .. index:: stepi

  Execute the program for one machine instruction only.

*Next*
  .. index:: next

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

*Next Instruction*
  .. index:: nexti

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

*Finish*
  .. index:: finish

  Continue execution until selected stack frame returns.

*Continue*
  .. index:: continue

  Continue execution of the program being debugged.

*Interrupt*
  .. index:: 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 required in some situations, for example when the program
  appears to be in an infinite (or at least very time-consuming) loop.

*Terminate Current*
  .. index:: terminate

  Terminate the current debug session by terminating the underlying debugger
  (e.g `gdb`) used to handle the low level debugging. You can control what
  happens to the windows through the `Debugger/Debugger Windows` preference.

*Terminate*
  .. index:: terminate

  Terminate all your debug sessions. Same as `Terminate Current` if there is
  only one debugger open.

Initialize
----------

This menu contains one entry per main unit defined in your project, which
will start a debug session and load the executable associated with the main
unit selected and if relevant, all corresponding settings: a debug session
will open the debug perspective and associated debug properties (e.g.
saved breakpoints, and data display).

*<No Main File>*

  Will initialize the debugger with no executable. You can then use one of
  the menu items in the `Debug` menu (e.g. `Load File...` or `Attach...`)
  if needed.

Debug
-----

*Connect to Board...*
  .. index:: connect
  .. index:: board
  .. index:: target
  .. index:: cross debugger

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

*Load File...*
  .. index:: load

  .. _open_program_menu:

  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...*
  .. index:: 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...*
  .. index:: 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 :ref:`Core Files <core_files>`, you
  need to specify an executable before attaching to a process.

*Detach*
  .. index:: 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...*
  .. index:: core file

  .. _core_files:

  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*
  .. index:: kill

  Kills the process being debugged.

Data
----

.. index:: menu
.. index:: data

Note that most items in this menu need to access the underlying debugger when
the process is stopped, not when it is running. This means that you first need
to stop the process on a breakpoint or interrupt it, before using the following
commands. Failing to do so will result in blank windows.

*Data Window*

  Displays the Data window. If this window already exists, it is raised so that
  it becomes visible

*Call Stack*
  .. index:: call stack

  Displays the Call Stack window.
  See :ref:`The_Call_Stack_Window` for more details.

*Threads*
  .. index:: thread

  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*
  .. index:: task

  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.

  .. index:: screen shot
  .. image:: tasks.jpg

*Protection Domains*
  .. index:: protection domain

  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 @c{*} character indicates the
  current protection domain.

*Assembly*
  .. index:: assembly

  Opens a new window displaying an assembly dump of the current code being
  executed.  See :ref:`The_Assembly_Window` for more details.

*Edit Breakpoints*
  .. index:: breakpoint

  Opens an advanced window to create and modify any kind of breakpoint,
  including watchpoints (see :ref:`The_Breakpoint_Editor`).  For simple
  breakpoint creation, see the description of the source window.

*Examine Memory*
  .. index:: memory view

  Opens a memory viewer/editor. See :ref:`The_Memory_Window` for more details.

*Command History*
  .. index:: command
  .. index:: 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*
  .. index:: local variables

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

*Display Arguments*
  .. index:: arguments

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

*Display Registers*
  .. index:: registers

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

*Display Any Expression...*
  .. index:: display 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`).

*Recompute*
  .. index:: recompute

  Recomputes and refreshes all the items displayed in the Data Window.

.. _The_Call_Stack_Window:

The Call Stack Window
=====================

.. index:: call stack

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

.. index:: screen shot
.. image:: call-stack.jpg

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.

.. index:: contextual menu

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`.

.. _The_Data_Window:

The Data Window
===============

.. index:: data
.. index:: data window

Description
-----------

The Data Window is the area in which various information about the debugged
process can be displayed. This includes the value of selected variables, the
current contents of the registeres, the local variables, ...

.. index:: Data Window

This window is not open by default when you start the debugger. It will be
created automatically when needed (e.g. when using the Debug constextual menu
to display a variable). You can also force its display through the menu
`Debug->Data->Data Window`.

However, if you save the desktop through the menu `File->Save More->Desktop`
while the data window is open, it will be automatically reopen the next time
the desktop is loaded, for instance when restarting GPS.

The contents of the data window is preserved by default whenever you close it.
Thus, if you reopen the data window either during the same debugger session, or
automatically when you start a debugger on the same executable, it will display
the same items again. This behavior is controlled by the `Preserve State on
Exit` preference.

.. index:: menu
.. index:: contextual menu

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:

.. index:: screen shot
.. image:: canvas.jpg

* A title bar containing:

  .. index:: title bar

  * 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.
    .. index:: icon

    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'.
    .. index:: icon

    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.

.. index:: contextual menu

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**
  .. index:: hide

  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**
  .. index:: show

  Shows all subcomponents of the selected item.

*Clone *component**
  .. index:: clone

  Clones the selected component into a new, independent item.

*View memory at address of *component**
  .. index:: memory view

  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*
  .. index:: update value

  Refreshes the value displayed in the selected item.

*Show Value*
  .. index:: show value

  Shows only the value of the item.

*Show Type*
  .. index:: 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*
  .. index:: auto refresh

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

.. index:: contextual menu

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...*
  .. index:: 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*
  .. index:: align

  Enables or disables alignment of items on the grid.

*Detect Aliases*
  .. index:: 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*
  .. index:: zoom in

  Redisplays the items in the data window with a bigger font

*Zoom out*
  .. index:: 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*
  .. index:: zoom

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

*Clear*
  .. index:: clear

  When this item is selected, all the boxes currently displayed are removed.

Manipulating items
------------------

Moving items
^^^^^^^^^^^^

All the items on the canvas have some common behavior and can be fully
manipulated with the mouse.  They can be moved freely anywhere on the canvas,
simply by clicking on them and then dragging the mouse. Note that if you are
trying to move an item outside of the visible area of the data window, the
latter will be scrolled so as to make the new position visible.

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.

Colors
^^^^^^

Most of the items are displayed using several colors, each conveying a special
meaning. Here is the meaning assigned to all colors (note that the exact color
can be changed through the preferences dialog; these are the default colors):

.. index:: screen shot
.. image:: colors.jpg

*black*

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

*blue*
  .. index:: C
  .. index:: Ada

  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.

  .. index:: menu

  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.

Icons
^^^^^

.. index:: icon

Several different icons can be used in the display of items. They also convey
special meanings.

*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).

.. _The_Breakpoint_Editor:

The Breakpoint Editor
=====================

.. index:: breakpoint editor
.. index:: breakpoint
.. index:: screen shot
.. index:: menu
.. image:: breakpoints.jpg

The breakpoint editor can be accessed from the menu *Data->Edit Breakpoints*.
It allows manipulation of different kinds of breakpoints: at a source location,
on a subprogram, at an executable address, on memory access (watchpoints), and
on Ada exceptions.

You can double-click on any breakpoint in the list to open the corresponding
source editor at the right location. Alternatively, you can select the
breakpoint and then click on the `View` button.

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

It is possible to access advanced breakpoint characteristics for a given
breakpoint.  First, select a breakpoint in the list.  Then, click on the
*Advanced* button, which will display a new dialog window.  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 settings for breakpoints.

.. index:: screen shot
.. image:: bp-advanced.jpg

Scope/Action Settings for VxWorks AE
------------------------------------

.. index:: VxWorks AE

In VxWorks AE breakpoints have two extra properties:

* Scope:
  .. index:: scope

  which task(s) can hit a given breakpoint. Possible Scope values are:

  * task:
    .. index:: 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:
    .. index:: protection domain

    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:
  .. index:: action

  when a task hits a breakpoints, which tasks are stopped:

  * task:
    .. index:: task

    stop only the task that hit the breakpoint.
  * pd:
    .. index:: protection domain

    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.

.. index:: saving breakpoints
.. index:: breakpoints, saving

If you have enabled the preference `Preserve state on exit`, GPS will
automatically save the currently set breakpoints, and restore them the next
time you debug the same executable. This allows you to immediately start
debugging your application again, without reseting the breakpoints every time.

.. _The_Memory_Window:

The Memory Window
=================

.. index:: memory view
.. index:: screen shot
.. image:: memory-view.jpg

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

.. index:: C
.. index:: hexadecimal

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.

.. index:: ASCII

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.

.. index:: key

The *up* and *down* arrows as well as the :kbd:`Page up` and :kbd:`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.

.. _Using_the_Source_Editor_when_Debugging:

Using the Source Editor when Debugging
======================================

.. index:: source file
.. index:: editing
.. index:: debug

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*
  .. index:: current line

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


*Lines with breakpoints*
  .. index:: breakpoint

  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).

.. index:: screen shot
.. image:: tooltips.jpg
.. index:: syntax highlighting
.. index:: tooltip

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.

.. index:: preferences

See :ref:`Preferences Dialog <preferences_dialog>`.

.. index:: contextual menu

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**
  .. index:: print

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

*Display *selection**
  .. index:: display

  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 :ref:`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**
  .. index:: memory view

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

*Set Breakpoint on Line *xx**
  .. index:: breakpoint

  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**
  .. index:: continue until

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

*Show Current Location*
  .. index:: current location

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

.. _The_Assembly_Window:

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`.

.. index:: screen shot
.. image:: assembly.jpg

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 :ref:`Preferences
Dialog <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
:kbd:`Page up` or :kbd:`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.

.. _The_Debugger_Console:

The Debugger Console
====================

.. index:: debugger
.. index:: 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 :kbd:`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] [at x, y] [num num]*

  .. index:: graph print
  .. index:: graph display

  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.
  This number can be specified through the `num` keyword, and will be taken
  into account if no such item already exists.  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 ...]*

  .. index:: graph enable
  .. index:: graph disable

  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*

  .. index:: graph undisplay

  This command will remove an item from the canvas

.. _Customizing_the_Debugger:

Customizing the Debugger
========================

.. index:: debugger

GPS is a high-level interface to several debugger backends, in particular gdb.
Each back end has its own strengths, but you can enhance the command line
interface to these backends through GPS, using Python.

This section will provide a small such example. The idea is to provide the
notion of "alias" in the debugger console. For example, this can be used so
that you type "foo", and this really executes a longer command, like displaying
the value of a variable with a long name.

`gdb` already provides this feature through the `define` keywords, but we will
in fact rewrite that feature in terms of python.

GPS provides an extensive Python API to interface with each of the running
debugger. In particular, it provides the function "send", which can be used to
send a command to the debugger, and get its output, and the function
"set_output", which can be used when you implement your own functions.

It also provides, through `hook`, the capability to monitor the state of the
debugger back-end. In particular, one such hook, `debugger_command_action_hook`
is called when the user has typed a command in the debugger console, and before
the command is executed. This can be used to add your own commands. The example
below uses this hook.

.. highlight:: python

Here is the code::

  import GPS

  aliases={}

  def set_alias (name, command):
     """Set a new debugger alias. Typing this alias in a debugger window
        will then execute command"""
     global aliases
     aliases[name] = command

  def execute_alias (debugger, name):
     return debugger.send (aliases[name], output=False)

  def debugger_commands (hook, debugger, command):
     global aliases
     words = command.split()
     if words[0] == "alias":
        set_alias (words[1], " ".join (words [2:]))
        return True
     elif aliases.has_key (words [0]):
        debugger.set_output (execute_alias (debugger, words[0]))
        return True
     else:
        return False

  GPS.Hook ("debugger_command_action_hook").add (debugger_commands)
  

The list of aliases is stored in the global variable `aliases`, which is
modified by `set_alias`. Whenever the user executes an alias, the real command
send to the debugger is sent through `execute_alias`.

The real part of the work is done by `debugger_commands`. If the user is
executing the `alias` command, it defines a new alias. Otherwise, if he typed
the name of an alias, we really want to execute that alias. Else, we let the
debugger back-end handle that command.

After you have copied this example in the :file:`$HOME/.gps/plug-ins`
directory, you can start a debugger as usual in GPS, and type the following in
its console::

     (gdb) alias foo print a_long_long_name
     (gdb) foo
  

The first command defines the alias, the second line executes it.

This alias can also be used within the `graph display` command, so that the
value of the variable is in fact displayed in the data window automatically,
for instance::

     (gdb) graph display `foo`
  
Other examples can be programmed. You could write complex python functions,
which would for instance query the value of several variables, and pretty print
the result. This complex python function can then be called from the debugger
console, or automatically every time the debugger stops through the `graph
display` command.