Debugging%20Under%20Linux

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Debugging Under Linux
Sebastien Ponce Friday, 8 March 2002
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Overview

• When to use a debugger ?
• Available debuggers
• Very short introduction to gvd (GNU Visual
  Debugger)
• Some hints on debugging Gaudi Jobs
• Demo

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When to Use a Debugger ?

• If you compiled in debug mode
• When you want to trace an application execution
  (breakpoints, step in, ...)
• When you want to know where the execution crashes
  (segmentation fault is handled)
• If you compiled in optimize mode
• You can still know which functions were called
  and which one failed

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What to Do With a Core ?

• You can still debug a program after a
  segmentation fault if you have a core file
• You can learn about
• Where it failed
• What were the values of variables at this time
• What was the call stack at this time

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Available Debuggers

• GDB
• Default linux debugger
• Powerfull stable
• Installed on lxplus
• BUT for experts only
• DDD
• Based on gdb
• Installed on lxplus
• Allow Visual debugging
• BUT not stable

• GVD
• Based on gdb
• Installed in LHCb
• Allow visual debug
• Stable
• Can be found for free at
• http//libre.act-europe.fr/gvd/
• Very easy to install locally
• PREFERED CHOICE

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Launching gvd

• Launching gvd
• source setup.csh
• gvd ltexecutablegt
• Inside gvd
• set breaking points
• run ltoptionsgt
• In case of core dump
• source setup.csh
• gvd ltexecutablegt
• Inside gvd
• File?Open Core Dump ltcoregt

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gvd Basics
Menu bar
Tool bar
Data display
Call stack
Source files
Source code
Current Line
Gdb window
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Some Hints for Gaudi Jobs

• Gaudi is using shared libraries
• The source files of these do no appear in gvd
  before the libraries are loaded
• The trick is
• gvd ltexecutablegt
• break main
• run ltoptionsgt
• next ? here we loaded the ApplicationMgr
• break ApplicationMgrconfigure
• cont
• finish ? now every dll is loaded

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Demo (1)

• Launch
• tar xvf demo.tar
• cd demo
• make
• ./hello
• Segmentation Fault
• gvd hello

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Demo (2)

• gdb windows
• (gdb) run
• Starting Program /home/sponce/demo/hello
• Program received signal SIGSEGV, Segmentation
  fault
• 0x400e1630 in strcmp() from /lib/libc.so.6
• (gdb)
• In menu
• Data ? Call Stack
• In Tool Bar
• Up

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Demo (3)
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Demo (4)
Helloprint called from HellodisplayMessage
with no argument
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Demo (5)

• Edition of Hello.h
• static void print (char line 0)
• becomes
• static void print (char line "")
• In a shell
• make
• ./hello
• Segmentation Fault
• gvd hello
• In gvd
• run

There is another bug !
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Demo (6)

• In gvd
• display Call Stack
• Up"
• The bug is still in strcmp, line is still 0x0
• In gvd
• Up"
• Data?Display Any Expression ?this
• Then click on the fields of this to see the
  internal values

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Demo (7)
this?m_message is 0x0
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Demo (8)

• In gvd
• Click on the blue point in front of line 4 of
  Hello.cpp to set a break point there
• Click run
• Say you want to start from beginning
• gvd stops when the program reaches line 4
• obviously the constructor was called with message
  0x0
• Click "Up" to see where we came from

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Demo (9)
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Demo (10)
This is an example of what you get in case of non
debug mode
You still have the call stack but not the values
of the arguments
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Demo (11)
This show you what you get from a core gvd
hello File-gtOpen Core
Its equivalent to run the program inside the
debugger