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/****************************************************************************
**
** Copyright (C) 2020 The Qt Company Ltd.
** Contact: https://www.qt.io/licensing/
**
** This file is part of the Qt Creator documentation.
**
** Commercial License Usage
** Licensees holding valid commercial Qt licenses may use this file in
** accordance with the commercial license agreement provided with the
** Software or, alternatively, in accordance with the terms contained in
** a written agreement between you and The Qt Company. For licensing terms
** and conditions see https://www.qt.io/terms-conditions. For further
** information use the contact form at https://www.qt.io/contact-us.
**
** GNU Free Documentation License Usage
** Alternatively, this file may be used under the terms of the GNU Free
** Documentation License version 1.3 as published by the Free Software
** Foundation and appearing in the file included in the packaging of
** this file. Please review the following information to ensure
** the GNU Free Documentation License version 1.3 requirements
** will be met: https://www.gnu.org/licenses/fdl-1.3.html.
**
****************************************************************************/
/*!
//! [debugger-breakpoints]
\section1 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 a
program is running
\li Emitting QML signals
\li Throwing JavaScript exceptions
\endlist
The interruption of a program by a breakpoint can be restricted with certain
conditions.
Breakpoints come in two varieties: \c unclaimed and \c claimed.
An unclaimed breakpoint represents a task to interrupt the debugged
program 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 a program 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.png "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 program 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. Succesfull 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 program, control is passed back to you.
You can then examine the state of the interrupted program, or
continue execution either line-by-line or continuously.
\image qtcreator-debug-breakpoints.png "Breakpoints view"
\section2 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 for \macos) at a particular line you want the
program 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
program code where you want the program to stop. The other options
to specify depend on the selected location.
\image qtcreator-add-breakpoint.png "Add Breakpoints" dialog
\li In the \uicontrol Condition field, set the condition to be evaluated
before stopping at the breakpoint if the condition evaluates as
true.
\li In the \uicontrol Ignore field, specify the number of times that the
breakpoint is ignored before the program stops.
\li In the \uicontrol Commands field, specify the commands to execute
when the program stops; one command on a line. GDB executes the
commands in the order in which they are specified.
\endlist
\if defined(qtcreator)
\section2 Specifying Breakpoint Settings
You can specify settings for breakpoints in \uicontrol Tools >
\uicontrol Options > \uicontrol Debugger. For more information,
see \l{Specifying Debugger Settings}.
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 Extended 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
\section2 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
\section2 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
\section2 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 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.
With the notable exception of data breakpoints, breakpoints retain their
enabled or disabled state when the debugged program is restarted.
\section2 Setting Data Breakpoints
A \e {data breakpoint} stops the program 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 program exits, as it
is unlikely that the used addresses will stay the same at the next program
launch. If you really want a data breakpoint to be active again, re-enable
it manually.
//! [debugger-breakpoints]
//! [debugger-call-stack-trace]
\section1 Viewing Call Stack Trace
When the program 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
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 in the \uicontrol Stack view.
If you click a frame with a known source location, the text editor jumps to
the corresponding location and updates the \uicontrol {Locals} and
\uicontrol {Expressions} views, making it seem like the program
was interrupted before entering the function.
To find out which QML file is causing a Qt Quick 2 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.
//! [debugger-call-stack-trace]
//! [debugger-locals]
\section1 Local Variables and Function Parameters
The Locals view consists of the \uicontrol Locals pane and the
\uicontrol {Return Value} pane (hidden when empty).
\image qtcreator-locals.png "Locals view"
Whenever a program 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.
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.
//! [debugger-locals]
//! [debugger-expressions]
\section1 Evaluating Expressions
To compute values of arithmetic expressions or function calls, use
expression evaluators in the \uicontrol Expressions view. To insert a new
expression evaluator, either double-click on an empty part of the
\uicontrol {Expressions} or \uicontrol {Locals} view, or select
\uicontrol {Add New Expression Evaluator} from the context menu, or drag and
drop an expression from the code editor.
\image qtcreator-debugger-expressions.png
\note Expression evaluators are powerful, but slow down debugger operation
significantly. It is advisable to not use them excessively, and to remove
unneeded expression evaluators as soon as possible.
Expression evaluators are re-evaluated whenever the current frame changes.
Note that functions used in the expressions are called each time, even if
they have side-effects.
The QML debugger can evaluate JavaScript expressions.
//! [debugger-expressions]
//! [debugger-expressions-cpp]
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. Most notably, inlined functions such as most
\c{operator[]} implementations of standard containers are typically \e{not}
available.
When using GDB or LLDB as backend, a special ranged syntax can be used 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]}.
Compound variables of struct or class type are displayed as expandable in
the view. Expand entries to show all members. Together with the display of
value and type, you can examine and traverse the low-level layout of object
data.
\table
\row
\li \b{Note:}
\row
\li 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} view to show
unexpected data.
\row
\li The debug information provided by 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 contains "real data", or "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.
\endtable
\note The set of evaluated expressions is saved in your session.
//! [debugger-expressions-cpp]
//! [debugger-qt-basic-objects]
\section1 Inspecting Basic Qt Objects
The \uicontrol {Locals} and \uicontrol {Expressions} views also provide access
to the most powerful feature of the debugger: comprehensive display of data
belonging to Qt's basic objects. For example, in case of QObject, instead of
displaying a pointer to some private data structure, you see a list of
children, signals and slots.
Similarly, instead of displaying many pointers and integers, \QC's debugger
displays the contents of a QHash or QMap in an orderly manner. Also, the
debugger displays access data for QFileInfo and provides access to the
\e real contents of QVariant.
Right-click in the \uicontrol {Locals} or the \uicontrol {Expressions} view
to open a context menu that provides additional options for viewing data. The
available options depend on the type of the current items, and are provided
by the \l{Using Debugging Helpers}{Debugging Helpers}. Typically,
string-like data, such as \c{QByteArray} and \c{std::string}, offer a
selection of encodings, as well as the possibility to use a full editor
window. Map-like data, such as \c{QMap}, \c{QHash}, and \c{std::map}, offer
a compact option using the \c{name} column for keys, resulting in a concise
display of containers with short keys, such as numbers or short strings. For
example, to expand all the values of QMap, select
\uicontrol {Change Value Display Format} > \uicontrol Compact.
You can use the \uicontrol {Locals} and \uicontrol {Expressions} view to change
the contents of variables of simple data types, for example, \c int, \c float,
\c QString and \c std::string when the program is interrupted. To do so,
click the \uicontrol Value column, modify the value with the inplace editor,
and press \key Enter (or \key Return).
To change the complete contents of QVector or \c std::vector values, type
all values separated by commas into the \uicontrol Value column of the main
entry.
You can enable tooltips in the main editor displaying this information.
For more information, see \l{See the value of variables in tooltips while debugging}.
//! [debugger-qt-basic-objects]
*/
|