04/19/23 S. Ponce / EP-LBC 1

Debugging Under Linux

Sebastien PonceSebastien Ponce

Friday, 8 March 2002Friday, 8 March 2002

08/03/2002 S. Ponce / EP-LBC 2

OverviewOverview

When to use a debugger ?When to use a debugger ? Available debuggersAvailable debuggers Very short introduction to gvdVery short introduction to gvd

(GNU Visual Debugger) (GNU Visual Debugger) Some hints on debugging Gaudi JobsSome hints on debugging Gaudi Jobs DemoDemo

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When to Use a Debugger ?When to Use a Debugger ?

If you compiled in debug mode :If you compiled in debug mode :• When you want to trace an application When you want to trace an application

execution (breakpoints, step in, ...)execution (breakpoints, step in, ...)• When you want to know where the When you want to know where the

execution crashes (segmentation fault is execution crashes (segmentation fault is handled)handled)

If you compiled in optimize mode :If you compiled in optimize mode :• You can still know which functions were You can still know which functions were

called and which one failedcalled and which one failed

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What to Do With a Core ?What to Do With a Core ?

You can still debug a program after a You can still debug a program after a segmentation fault if you have a core filesegmentation fault if you have a core file

You can learn aboutYou can learn about• Where it failedWhere it failed• What were the values of variables at this timeWhat were the values of variables at this time• What was the call stack at this timeWhat was the call stack at this time

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Available DebuggersAvailable Debuggers

GDBGDB Default linux debugger Powerfull & stable Installed on lxplus BUT for experts only

DDDDDD Based on gdb Installed on lxplus Allow Visual debugging BUT not stable

GVDGVD 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 CHOICEPREFERED CHOICE

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Launching gvdLaunching gvd

Launching gvd :Launching gvd :source setup.csh

gvd <executable> Inside gvd :Inside gvd :

[set breaking points]

run <options>

In case of core dump :In case of core dump :source setup.csh

gvd <executable> Inside gvd :Inside gvd :

FileOpen Core Dump <core>

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gvd Basicsgvd Basics

Menu bar

Tool bar

Call stack

Source files

Gdb window

Data display

Current Line

Source code

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Some Hints for Gaudi JobsSome Hints for Gaudi Jobs

Gaudi is using shared librariesGaudi is using shared libraries The source files of these do no appear in gvd The source files of these do no appear in gvd

before the libraries are loadedbefore the libraries are loaded The trick is :The trick is :

gvd <executable>gvd <executable>

break mainbreak main

run <options>run <options>

next next here we loaded the ApplicationMgr here we loaded the ApplicationMgr

break ApplicationMgr::configurebreak ApplicationMgr::configure

contcont

finish finish now every dll is loaded now every dll is loaded

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Demo (1)Demo (1)

Launch :Launch :tar xvf demo.tar

cd demo

make

./hello

Segmentation Fault

gvd hello

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Demo (2)Demo (2)

gdb windows :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 menuIn menu

Data Call Stack In Tool BarIn Tool Bar

Up

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Demo (3)Demo (3)

argument line was 0x0

strcmp called from Hello::print

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

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Demo (4)Demo (4)

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

when print argument is missing, the default is a 0 pointer and fails.

The default has to be changed to empty string

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Demo (5)Demo (5)

Edition of Hello.h :Edition of Hello.h :static void print (char* line = 0);

becomes :

static void print (char* line = "");

In a shell :In a shell :make

./hello

Segmentation Fault

gvd hello In gvdIn gvd

run

There is another bug !There is another bug !

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Demo (6)Demo (6)

In gvd :In gvd :display Call Stack

“Up" The bug is still in strcmp, line is still 0x0The bug is still in strcmp, line is still 0x0 In gvd :In gvd :

“Up"

DataDisplay Any Expression ”this”

Then click on the fields of this to see the internal values

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Demo (7)Demo (7)

thism_message is 0x0

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

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Demo (8)Demo (8)

In gvd :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 4gvd 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)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 !

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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

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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