[ fromfile: debugging.xml id: gdb ]

Imagine you are running a program and, for some mysterious reason, it crashes.

[lazarus] app> ./playlistmgr
Segmentation fault
[lazarus] app>

When your application aborts, or crashes, it is helpful to know (as quickly as possible) exactly where it happened. You can use gdb to locate the trouble spot quickly and easily. Following is a short example of a command-line gdb session.

[lazarus] app> gdb playlistmgr
GNU gdb 6.3-debian
Copyright 2004 Free Software Foundation, Inc.
This GDB was configured as "i386-linux"...Using host libthread_db library "/lib/tls/libthread_db.so.1".

(gdb)  r  []
Starting program: ftgui/app/playlistmgr
[Thread debugging using libthread_db enabled]
[New Thread -1227622176 (LWP 17021)]
Qt: gdb: -nograb added to command-line options.
         Use the -dograb option to enforce grabbing.
This is a debug message

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread -1227622176 (LWP 17021)]
0xb7f03320 in FormDialog::createActions (this=0x80ae2a0) at formdialog.cpp:53
53          delete m_OkAction;
(gdb)

gdb shows you not only the filename, and line number, but also the corresponding line in the source code. However, you still might want to get some context for this error. The command list shows you the surrounding source code for the current file:

(gdb) list
51      void FormDialog::createActions() {
52
53          delete m_OkAction;
54          delete m_CancelAction;
55          m_OkAction = new OkAction(m_Model, m_View);
56          m_CancelAction = new CancelAction(m_Model, m_View);
57          QHBoxLayout *buttons = new QHBoxLayout(0);
(gdb)

The command where shows you the stack trace, or how you got that error spot.

(gdb) where
#0  0xb7f03320 in FormDialog::createActions (this=0x80ae2a0) at formdialog.cpp:53
#1  0xb7f03058 in FormDialog::setModel (this=0x80ae2a0, fmodel=0x80c80d0)
    at formdialog.cpp:34
#2  0x080664bd in SettingsDialog (this=0x80ae2a0, parent=0x0) at settingsdialog.cpp:14
#3  0x0805f313 in MainWindow (this=0xbfffdec8) at mainwindow.cpp:42
#4  0x08066f14 in Controller (this=0xbfffdec0, argc=1, argv=0xbfffdfe4) at controller.cpp:25
#5  0x0805a8a4 in main (argc=1, argv=0xbfffdfe4) at main.cpp:7
(gdb)

Most open source IDEs such as Eclipse and QtCreator use gdb under the hood. They each offer a user interface that makes certain features easier to learn and use. ccdebug is written in Qt and designed specifically for debugging Qt applications, including QString support.