gdb – TheGNU

Debugger

Kurt Schmidt

Intro

Invocation

CommandsGetting Help

Essentials

Running

Listing

Breakpoints

Execution

Data

Call Stack

Trickier

Corefiles

Summary

gdb – The GNU Debugger

Kurt Schmidt

Dept. of Computer Science, Drexel University

November 28, 2018

gdb – TheGNU

Debugger

Kurt Schmidt

Intro

Invocation

CommandsGetting Help

Essentials

Running

Listing

Breakpoints

Execution

Data

Call Stack

Trickier

Corefiles

Summary

Intro

gdb – TheGNU

Debugger

Kurt Schmidt

Intro

Invocation

CommandsGetting Help

Essentials

Running

Listing

Breakpoints

Execution

Data

Call Stack

Trickier

Corefiles

Summary

The GNU Debugger

A debugger is closely tied to the compilergdb is the command-line debugger for all GNUcompilers

Language is irrelevantBack end of the compiler is the same (for a givenplatform)An executable is just a program; it’s not a “C program”,nor a “FORTRAN program”, etc.

gdb – TheGNU

Debugger

Kurt Schmidt

Intro

Invocation

CommandsGetting Help

Essentials

Running

Listing

Breakpoints

Execution

Data

Call Stack

Trickier

Corefiles

Summary

Invocation

gdb – TheGNU

Debugger

Kurt Schmidt

Intro

Invocation

CommandsGetting Help

Essentials

Running

Listing

Breakpoints

Execution

Data

Call Stack

Trickier

Corefiles

Summary

Debugging a Program

First, use the -g option, compile your program withextra (debuggin) information

$ gcc -g source files... -o prog

Then, load the executable into the debugger:

$ gdb progGNU gdb (Ubuntu 7.11.1-0ubuntu1~16.04) 7.11.1...(gdb) _

gdb – TheGNU

Debugger

Kurt Schmidt

Intro

Invocation

CommandsGetting Help

Essentials

Running

Listing

Breakpoints

Execution

Data

Call Stack

Trickier

Corefiles

Summary

Commands

gdb – TheGNU

Debugger

Kurt Schmidt

Intro

Invocation

CommandsGetting Help

Essentials

Running

Listing

Breakpoints

Execution

Data

Call Stack

Trickier

Corefiles

Summary

Using GDB

GDB is very powerfulAttach to a running processExamine a corefileDebug multi-threaded programs

Lots of commandsDon’t be intimidatedI don’t know many of themJust knowing some of the basics will get you far

gdb – TheGNU

Debugger

Kurt Schmidt

Intro

Invocation

CommandsGetting Help

Essentials

Running

Listing

Breakpoints

Execution

Data

Call Stack

Trickier

Corefiles

Summary

Getting Help

GDB commands are divided into categoriesType help to see these categories:

(gdb) helpList of classes of commands:

aliases -- Aliases of other commandsbreakpoints -- Making program stop at certain pointsdata -- Examining datafiles -- Specifying and examining filesinternals -- Maintenance commandsobscure -- Obscure featuresrunning -- Running the programstack -- Examining the stackstatus -- Status inquiriessupport -- Support facilitiestracepoints -- Tracing of program execution without ...user-defined -- User-defined commands

gdb – TheGNU

Debugger

Kurt Schmidt

Intro

Invocation

CommandsGetting Help

Essentials

Running

Listing

Breakpoints

Execution

Data

Call Stack

Trickier

Corefiles

Summary

Getting Help – Listing a Class

To see commands in a category (class):

(gdb) help runningRunning the program.

List of commands:

continue -- Continue program being debuggedfinish -- Execute until selected stack frame returnsjump -- Continue program being debugged at specified ...kill -- Kill execution of program being debuggednext -- Step programrun -- Start debugged programstart -- Run the debugged program until the beginning ...step -- Step program until it reaches a different source line

I’ve only listed some of the handier commands

gdb – TheGNU

Debugger

Kurt Schmidt

Intro

Invocation

CommandsGetting Help

Essentials

Running

Listing

Breakpoints

Execution

Data

Call Stack

Trickier

Corefiles

Summary

Getting Help on a Command

Use help cmd for help on that command:

(gdb) help breakSet breakpoint at specified location.break [PROBE_MODIFIER] [LOCATION] [thread THREADNUM] [if CONDITION]PROBE_MODIFIER shall be present if the command is to be placed in aprobe point. Accepted values are ‘-probe’ (for a generic,automatically guessed probe type), ‘-probe-stap’ (for a SystemTapprobe) or ‘-probe-dtrace’ (for a DTrace probe).LOCATION may be a linespec, address, or explicit location asdescribed below.

With no LOCATION, uses current execution address of the selectedstack frame. This is useful for breaking on return to a stackframe.

THREADNUM is the number from "info threads".CONDITION is a boolean expression....

gdb – TheGNU

Debugger

Kurt Schmidt

Intro

Invocation

CommandsGetting Help

Essentials

Running

Listing

Breakpoints

Execution

Data

Call Stack

Trickier

Corefiles

Summary

Some Essential Commands

Note, many of the commands can be abbreviated.

break b [location ] Set breakpointkill Kill running processrun [arglist ] Run your programprint p [expr ] Print exprstep s Next line, stepping into functionsnext n Next line, stepping over functionscontinue c Continue to next breakquit q Exit GDB

gdb – TheGNU

Debugger

Kurt Schmidt

Intro

Invocation

CommandsGetting Help

Essentials

Running

Listing

Breakpoints

Execution

Data

Call Stack

Trickier

Corefiles

Summary

Running Your Program

set args args Set command-line argumentsset env var val Set environment var to val (for

next run)show args Show command-line argsshow env [var ] Show environment variables [or

var ]run [args ] Run your program [with args ]start [args ] Run your program until beginning of

main procedurekill Kill running process

gdb – TheGNU

Debugger

Kurt Schmidt

Intro

Invocation

CommandsGetting Help

Essentials

Running

Listing

Breakpoints

Execution

Data

Call Stack

Trickier

Corefiles

Summary

Looking at Your Code

list or l

list

list line_no

list beg,end

list file:line_no

list func_name

gdb – TheGNU

Debugger

Kurt Schmidt

Intro

Invocation

CommandsGetting Help

Essentials

Running

Listing

Breakpoints

Execution

Data

Call Stack

Trickier

Corefiles

Summary

Setting Breakpoints

A place (and/or condition) where execution pauses,waits for a user commandCan break conditionally at a function or a line number

break func_namebreak line_nobreak file:line_nobreak ... if cond

info break show breakpointsdelete [n ] delete breakpoints [breakpoint n ]disable [n ] disable breakpoints [breakpoint n ]enable [n ] enable breakpoints [breakpoint n ]

gdb – TheGNU

Debugger

Kurt Schmidt

Intro

Invocation

CommandsGetting Help

Essentials

Running

Listing

Breakpoints

Execution

Data

Call Stack

Trickier

Corefiles

Summary

Execution Control

step s Next line, stepping into functionsnext n Next line, stepping over functionscontinue c Continue to next breakuntil loc Run until loc ; same args as breakfinish Run until frame returnsreturn [expr ] Pop frame w/out executing [using

expr ] as return value

gdb – TheGNU

Debugger

Kurt Schmidt

Intro

Invocation

CommandsGetting Help

Essentials

Running

Listing

Breakpoints

Execution

Data

Call Stack

Trickier

Corefiles

Summary

Examining Data

print p [/f ] expr Prints expr . f is a format characterdisplay [/f ] expr Prints expr each time execution

pausesinfo display Lists displayed expressionsundisplay n Removes n from display list

gdb – TheGNU

Debugger

Kurt Schmidt

Intro

Invocation

CommandsGetting Help

Essentials

Running

Listing

Breakpoints

Execution

Data

Call Stack

Trickier

Corefiles

Summary

The Call Stack

backtrace or bt Print trace of all frames in stackframe [n ] Select current frame [frame # n ]info frame Information on selected frameinfo args Arguments of selected frameinfo locals Local variables of selected frame

gdb – TheGNU

Debugger

Kurt Schmidt

Intro

Invocation

CommandsGetting Help

Essentials

Running

Listing

Breakpoints

Execution

Data

Call Stack

Trickier

Corefiles

Summary

Some Trickier (but Useful) Commands

set var VAR =expr Actually modify variables in theprogram being debugged

Assignment operator from the language(e.g., :=)Keyword var is optionalUseful when symbol name clashes with aGDB command

jump line Resume execution at line

jump *address Resume execution at address

gdb – TheGNU

Debugger

Kurt Schmidt

Intro

Invocation

CommandsGetting Help

Essentials

Running

Listing

Breakpoints

Execution

Data

Call Stack

Trickier

Corefiles

Summary

Corefiles

gdb – TheGNU

Debugger

Kurt Schmidt

Intro

Invocation

CommandsGetting Help

Essentials

Running

Listing

Breakpoints

Execution

Data

Call Stack

Trickier

Corefiles

Summary

Examining Corefiles

A corefile is a snapshot of a process (image) inmemory, when it diedTo allow corefiles on Linux (Bash)$ ulimit -c unlimited

Upon a crash, find the corefile, coreLoad the executable, along with the corefile, into thedebugger$ gdb prog -c core

Examine the program:(gdb) bt

Note, prog needn’t have been compiled with debuginformation

gdb – TheGNU

Debugger

Kurt Schmidt

Intro

Invocation

CommandsGetting Help

Essentials

Running

Listing

Breakpoints

Execution

Data

Call Stack

Trickier

Corefiles

Summary

Summary

gdb – TheGNU

Debugger

Kurt Schmidt

Intro

Invocation

CommandsGetting Help

Essentials

Running

Listing

Breakpoints

Execution

Data

Call Stack

Trickier

Corefiles

Summary

More Power

Only common commands (and uses) are shown hereThere is more functionality available

You can catch events and signalsDebuggers handle multi-threaded programsLook at machine instructions

Get comfortable with basic commandsThis much will prove quite useful

As you need more, explore