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Debugging Under Linux. Sebastien Ponce Friday, 8 March 2002. Overview. When to use a debugger ? Available debuggers Very short introduction to gvd (GNU Visual Debugger) Some hints on debugging Gaudi Jobs Demo. When to Use a Debugger ?. If you compiled in debug mode :

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debugging under linux
Debugging Under Linux
  • Sebastien Ponce
  • Friday, 8 March 2002
overview
Overview
  • When to use a debugger ?
  • Available debuggers
  • Very short introduction to gvd (GNU Visual Debugger)
  • Some hints on debugging Gaudi Jobs
  • Demo
when to use a debugger
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
what to do with a core
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
available debuggers
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

launching gvd
Launching gvd
  • Launching gvd :

source setup.csh

gvd

  • Inside gvd :

[set breaking points]

run

  • In case of core dump :

source setup.csh

gvd

  • Inside gvd :

FileOpen Core Dump

gvd basics
gvd Basics

Menu bar

Tool bar

Data display

Call stack

Source files

Source code

Current Line

Gdb window

some hints for gaudi jobs
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

break main

run

next  here we loaded the ApplicationMgr

break ApplicationMgr::configure

cont

finish  now every dll is loaded

demo 1
Demo (1)
  • Launch :

tar xvf demo.tar

cd demo

make

./hello

Segmentation Fault

gvd hello

demo 2
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

demo 3
strcmp called from

Hello::print

argument line was 0x0

Click "Up" once more to see where the line argument was set to 0x0

Demo (3)
demo 4
when print argument is missing, the default is a 0 pointer and fails.

The default has to be changed to empty string

Demo (4)

Hello::print called from Hello::displayMessage with no argument

demo 5
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 !

demo 6
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

demo 7
We'll try to place a breakpoint where the initialization of m_message is done, i.e. In the constructor of Hello.Demo (7)

thism_message is 0x0

demo 8
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

demo 9
We were suppose to go through line 20, not 22 since we gave no argument

This should have been 1 == argc !!!

Fix and run. All Right !

Demo (9)
demo 10
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

demo 11
Demo (11)

This show you what you get from a core :

gvd hello

File->Open Core

It’s equivalent to run the program inside the debugger

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