path: root/doc/qtcreator/src/debugger/creator-debug-views.qdoc

// Copyright (C) 2023 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only

/*!
    \page creator-stack-view.html
    \if defined(qtdesignstudio)
    \previouspage creator-debugging-qml.html
    \else
    \previouspage creator-debug-mode.html
    \endif
    \nextpage creator-breakpoints-view.html

    \title Viewing Call Stack Trace

    When the application being debugged is interrupted, \QC displays the nested
    function calls leading to the current position as a call stack trace. This
    stack trace is built up from call stack frames, each representing a
    particular function. For each function, \QC tries to retrieve the file name
    and line number of the corresponding source file. This data is shown in the
    \uicontrol Stack view.

    \image qtcreator-debug-stack.png {Stack view}

    Since the call stack leading to the current position may originate or go
    through code for which no debug information is available, not all stack
    frames have corresponding source locations. Stack frames without
    corresponding source locations are grayed out.

    If you click a frame with a known source location, the text editor jumps to
    the corresponding location and updates the
    \l {Local Variables and Function Parameters}{Locals} and
    \l {Evaluating Expressions}{Expressions} views, making it seem like the
    application was interrupted before entering the function.

    \section1 Loading QML Stack

    To find out which QML file is causing a Qt Quick application to crash,
    select \uicontrol {Load QML Stack} in the context menu in the
    \uicontrol Stack view. The debugger tries to retrieve the JavaScript stack
    from the stopped executable and prepends the frames to the C++ frames,
    should it find any. You can click a frame in the QML stack to open the QML
    file in the editor.
*/

/*!
    \page creator-breakpoints-view.html
    \previouspage creator-stack-view.html
    \if defined(qtdesignstudio)
    \nextpage creator-locals-view.html
    \else
    \nextpage creator-threads-view.html
    \endif

    \title Setting Breakpoints

    You can associate breakpoints with:

    \list

        \li Source code files and lines

        \li Functions

        \li Addresses

        \li Throwing and catching exceptions

        \li Executing and forking processes

        \li Executing some system calls

        \li Changes in a block of memory at a particular address when an
            application is running

        \li Emitting QML signals

        \li Throwing JavaScript exceptions

    \endlist

    A breakpoint interrupts the application every time the application reaches
    its location unless you specify a boolean condition for it. The breakpoint
    evaluates the expression each time the application passes it, and the
    application stops only if the condition evaluates to \c true.

    \section1 Unclaimed and Claimed Breakpoints

    Breakpoints come in two varieties: \c unclaimed and \c claimed.
    An unclaimed breakpoint represents a task to interrupt the debugged
    application and passes the control to you later. It has two states:
    \c pending and \c implanted.

    Unclaimed breakpoints are stored as a part of a session and exist
    independently of whether an application is being debugged or not. They
    are listed in the \uicontrol {Breakpoint Preset} view and in the
    editor using the \inlineimage icons/qtcreator-unclaimed-breakpoint-icon.png
    (\uicontrol {Unclaimed Breakpoint}) icon, when they refer to a position
    in code.

    \image qtcreator-debugger-breakpoint-preset.webp {Breakpoint Preset view}

    When a debugger starts, the debugging backend identifies breakpoints
    from the set of unclaimed breakpoints that might be handled by the
    debugged application and claims them for its own exclusive use. Claimed
    breakpoints are listed in the \uicontrol {Breakpoints} view of the
    running debugger. This view only exists while the debugger is running.

    When a debugger claims a breakpoint, the unclaimed breakpoint disappears
    from the \uicontrol {Breakpoint Preset} view, to appear as a pending
    breakpoint in the \uicontrol Breakpoints view.

    At various times, attempts are made to implant pending breakpoints into
    the debugged process. Successful implantation might create one or more
    implanted breakpoints, each associated with an actual address in
    the debugged breakpoint. The implantation might also move a breakpoint
    marker in the editor from an empty line to the next line for which the
    actual code was generated, for example. Implanted breakpoint icons don't
    have the hourglass overlay.

    When the debugger ends, its claimed breakpoints, both pending and
    implanted, will return to the unclaimed state and re-appear in the
    \uicontrol{Breakpoint Preset} view.

    When an implanted breakpoint is hit during the execution of the
    debugged application, control is passed back to you.
    You can then examine the state of the interrupted application, or
    continue execution either line-by-line or continuously.

    \image qtcreator-debug-breakpoints.webp {Breakpoints view}

    \section1 Adding Breakpoints

    To add breakpoints:

    \list 1

        \li Add a new breakpoint in one of the following ways:

            \list

                \li In the code editor, click the left margin or press \key F9
                    (\key F8 on \macos) on a particular line you want the
                    application to stop.

                \li In the \uicontrol {Breakpoint Preset} view or the
                    \uicontrol Breakpoints view:

                    \list
                        \li Double-click the empty part of the view.
                        \li Right-click the view, and select
                            \uicontrol {Add Breakpoint} in the context menu.
                    \endlist
            \endlist

        \li In the \uicontrol {Breakpoint type} field, select the location in the
            application code where you want the application to stop.

            \image qtcreator-add-breakpoint.webp {Add Breakpoints} dialog
    \endlist

    Deselect the \uicontrol Enabled check box to make the breakpoint temporarily
    inoperative as if you had deleted it, but keep the information about the
    breakpoint, so that you can enable it again later.

    The other options to specify depend on the location you select, such as file
    name and line number, address, expression, or function name. The following
    table summarizes the advanced options.

    \table
    \header
        \li Option
        \li Value
    \row
        \li \uicontrol Condition
        \li The condition to evaluate before stopping at the breakpoint if the
            condition evaluates as \c true.
    \row
        \li \uicontrol {Ignore count}
        \li The number of times to ignore the breakpoint before the application
            stops.
    \row
        \li \uicontrol {Thread specification}
        \li
    \row
        \li \uicontrol Path
        \li Determines how to specify the path when setting breakpoints:
            \list
                \li \uicontrol {Use Engine Default} is the preferred setting of
                    the debugger engine.
                \li \uicontrol {Use Full Path} passes the full path to avoid
                    ambiguity if several modules contain files with the same
                    name. This is the engine default for CDB and LLDB.
                \li \uicontrol {Use File Name} passes the file name only. This is
                    useful when the location of the source tree does not match
                    the one used when building the modules. This is the engine
                    default for GDB as using full paths can be slow with it.
            \endlist
    \row
        \li \uicontrol Module
        \li Specify the module (base name of the library or executable) for
            function or file type breakpoints to speed up debugger startup
            (CDB, LLDB).
    \row
        \li \uicontrol Commands
        \li Commands to execute when the application stops. List one command per
            line. GDB executes the commands in the order in which you specify
            them.
    \row
        \li \uicontrol {Tracepoint only}
        \li A \e tracepoint lets you log information about the application
            without adding log statements or otherwise modifying your code.
            You can set conditions for the tracepoint.
    \row
        \li \uicontrol Message
        \li The tracepoint message to show in \l {Application Output}.
    \endtable

    \if defined(qtcreator)
    \section1 Specifying Breakpoint Settings

    You can specify settings for breakpoints in \uicontrol Edit >
    \uicontrol Preferences > \uicontrol Debugger. For more information,
    see \l{Debugger Preferences}.

    \image qtcreator-debugger-general-options.png {General tab in Debugger preferences}

    To use a full absolute path in breakpoints, select the
    \uicontrol {Set breakpoints using a full absolute path} check box.

    GDB and CDB enable setting breakpoints on source lines for which no code
    was generated. In such situations, the breakpoint is shifted to the next
    source code line for which the code was actually generated. To reflect
    such temporary changes by moving the breakpoint markers in the source code
    editor, select \uicontrol GDB > \uicontrol {Adjust breakpoint locations}
    or \uicontrol CDB > \uicontrol {Correct breakpoint location}.

    When using GDB as backend, you can extend the ordinary GDB
    breakpoint class by using Python. Select \uicontrol GDB >
    \uicontrol {Use pseudo message tracepoints}.

    When using CDB as backend, you can specify that the debugger should break on
    specific events, such as C++ exceptions, thread creation or exit, loading or
    unloading \l{Viewing Modules}{application modules}, or particular output.
    Select the appropriate check boxes in the \uicontrol CDB >
    \uicontrol {Break on} group. To disable first-chance break on access
    violation exceptions, select the \uicontrol {Ignore first chance access
    violations} check box. The second occurrence of an access violation will
    break into the debugger.

    You can automatically add breakpoints on some functions to catch error
    and warning messages. For more information, see \l{Specifying CDB Settings}
    and \l{Specifying GDB Settings}.

    For more information on breakpoints, see
    \l{http://sourceware.org/gdb/onlinedocs/gdb/Breakpoints.html#Breakpoints}
    {Breakpoints, Watchpoints, and Catchpoints} in GDB documentation.
    \endif

    \section1 Moving Breakpoints

    To move a breakpoint:

    \list
        \li Drag and drop a breakpoint marker to another line
            in the text editor.
        \li In the \uicontrol {Breakpoint Preset} view or the
             \uicontrol Breakpoints view, select
             \uicontrol {Edit Selected Breakpoints}, and set the
             line number in the \uicontrol {Line number} field.
    \endlist

    \section1 Deleting Breakpoints

    To delete breakpoints:

    \list

        \li Click the breakpoint marker in the text editor.

        \li In the \uicontrol {Breakpoint Preset} view or the
            \uicontrol Breakpoints view:
            \list
                \li Select the breakpoint and press \key Delete.
                \li Select \uicontrol {Delete Selected Breakpoints},
                    \uicontrol {Delete Selected Breakpoints}, or
                    \uicontrol {Delete Breakpoints of File} in the
                    context menu.
            \endlist

    \endlist

    \section1 Enabling and Disabling Breakpoints

    To temporarily disable a breakpoint without deleting it and losing associated
    data like conditions and commands:

    \list

        \li Right-click the breakpoint marker in the text editor and select
            \uicontrol{Disable Breakpoint}.

        \li Select a line that has a breakpoint and press \key Ctrl+F9
            (\key Ctrl+F8 on \macos).

        \li In the \uicontrol {Breakpoint Preset} view or the
            \uicontrol Breakpoints view:
            \list
                \li Select the breakpoint and press \key Space.
                \li Select \uicontrol {Disable Breakpoint} in the context menu.
            \endlist
    \endlist

    A hollow breakpoint icon in the text editor and the views indicates a
    disabled breakpoint. To re-enable a breakpoint, use any of the above
    methods.

    Other than data breakpoints retain their enabled or disabled state when the
    debugged application is restarted.

    \section1 Setting Data Breakpoints

    A \e {data breakpoint} stops the application when data is read or written at the
    specified address.

    To set a data breakpoint at an address:

    \list 1

        \li In the \uicontrol {Breakpoint Preset} or \uicontrol Breakpoints
            view, select \uicontrol {Add Breakpoint} in the context menu.

        \li In the \uicontrol {Breakpoint type} field, select
            \uicontrol {Break on data access at fixed address}.

        \li In the \uicontrol Address field, specify the address of the memory
            block.

        \li Select \uicontrol OK.

    \endlist

    If the address is displayed in the \uicontrol {Locals} or
    \uicontrol {Expressions} view, you can select
    \uicontrol {Add Data Breakpoint at Object's Address} in the
    context menu to set the data breakpoint.

    Data breakpoints will be disabled when the debugged application exits, as it
    is unlikely that the used addresses will stay the same at the next application
    launch. If you really want a data breakpoint to be active again, re-enable
    it manually.
*/

/*!
    \page creator-locals-view.html
    \if defined(qtdesignstudio)
    \previouspage creator-breakpoints-view.html
    \else
    \previouspage creator-source-files-view.html
    \endif
    \nextpage creator-expressions-view.html

    \title Local Variables and Function Parameters

    The \uicontrol {Locals} view consists of the \uicontrol Locals pane and the
    \uicontrol {Return Value} pane (hidden when empty).

    \image qtcreator-locals.png {Locals view}

    Whenever an application stops under the control of the debugger, it retrieves
    information about the topmost stack frame and displays it in the
    \uicontrol {Locals} view. The \uicontrol Locals pane shows
    information about parameters of the function in that frame as well as the
    local variables. If the last operation in the debugger was returning from a
    function after pressing \key {Shift+F11}, the \uicontrol {Return Value}
    pane displays the value returned by the function.

    You can expand the view contents to check that your application sets a
    local value correctly.

    \if defined(qtcreator)
    \section1 Locals View Actions

    Right-click the \uicontrol Locals view to select the following actions:

    //! [0]
    \list
        \li Add and remove expression evaluators
        \li Change \l{Changing Value Display format}{value display format}
        \li Expand and collapse view contents
        \li Copy view contents or expression values to the clipboard
        \li Open view contents in an editor
        \li Open memory editor
        \li Set data breakpoints
        \li Use \l{Using Debugging Helpers}{debugging helpers}
        \li Show and hide tooltips in the view when debugging
        \li Dereference pointers automatically
        \li Sort members of classes and structs alphabetically
        \li Use dynamic object type for display
        \li Set \l{Debugger Preferences}{debugger preferences}
    \endlist
    //! [0]
    \endif

    \section1 Selecting Object Type for Display

    When using GDB, you can specify whether the dynamic or the static type of
    objects will be displayed. Select \uicontrol {Use dynamic object type for
    display} in the context menu. Keep in mind that choosing the dynamic type
    might be slower.
*/

/*!
    \page creator-expressions-view.html
    \previouspage creator-locals-view.html
    \if defined(qtdesignstudio)
    \nextpage creator-qml-debugging-example.html
    \else
    \nextpage creator-registers-view.html
    \endif

    \title Evaluating Expressions

    To compute values of arithmetic expressions or function calls, use
    expression evaluators in the \uicontrol Expressions view.

    You can examine static variables that the debuggers don't pick up as
    \e {local variables}. For example, if you define
    \c {static int staticVar = 42;} in a source file and then add \c staticVar
    as an evaluated expression, you should see \e 42 in the view when the
    debugger stops in the source file.

    \image qtcreator-debugger-expressions.webp {Expressions view}

    \section1 Adding Expression Evaluators

    To add expression evaluators, drag an expression from the code editor
    to the \uicontrol Expressions view.

    You can also:

    \list
        \li Double-click in the \uicontrol {Expressions} or
            \l {Local Variables and Function Parameters}{Locals} view.
        \li Select \uicontrol {Add New Expression Evaluator} from the context
            menu.
    \endlist

    Enter the expression in the \uicontrol {New Evaluated Expression} dialog:

    \image qtcreator-debugger-new-evaluated-expression.webp {New Evaluated Expression dialog}

    \omit
    ## Visible in the context menu, but does not currently work.

    To insert widgets into expression evaluators, select a
    widget in the debugged application and then select
    \uicontrol {Select Widget to Add into Expression Evaluator}
    in the context menu.
    \endomit

    The set of evaluated expressions is saved in your session.

    \note Expression evaluators are powerful, but slow down debugger operation
    significantly. Use them sparingly and remove them when you no longer need
    them.

    Expression evaluators are re-evaluated whenever the current frame changes.
    The functions used in the expressions are called each time, even if
    they have side-effects.

    \if defined(qtcreator)
    \section1 Expressions View Actions

    Right-click the \uicontrol Expressions view to select the following actions:

    \include creator-debug-views.qdoc 0
    \endif

    \section1 JavaScript Expressions

    The QML debugger can evaluate JavaScript expressions.

    \if defined(qtcreator)

    \section1 C and C++ Expressions

    GDB, LLDB and CDB support the evaluation of simple C and C++ expressions.
    Functions can be called only if they are actually compiled into the debugged
    executable or a library used by the executable. Inlined
    functions such as most \c{operator[]} implementations of standard containers
    are typically \e{not} available.

    When using GDB or LLDB as backend, you can use a special ranged syntax to
    display multiple values with one expression. A sub-expression of form
    \c{foo[a..b]} is split into a sequence of individually evaluated expressions
    \c{foo[a], ..., foo[b]}.

    You can expand compound variables of struct or class type to show their
    members. As you also see the variable value and type, you can examine and
    traverse the low-level layout of object data.

    GDB and LLDB, and therefore \QC's debugger, also work for optimized
    builds on Linux and \macos. Optimization can lead to re-ordering
    of instructions or removal of some local variables, causing the
    \uicontrol {Locals} and \uicontrol {Expressions} views to show
    unexpected data.

    The debug information from GCC does not include enough
    information about the time when a variable is initialized.
    Therefore, \QC can not tell whether the contents of a local
    variable are \e {real data} or \e {initial noise}. If a QObject
    appears uninitialized, its value is reported as
    \uicontrol {not in scope}. Not all uninitialized objects,
    however, can be recognized as such.
    \endif
*/