GDB and it's use

What is GDB?

GDB, the GNU Project debugger, allows you to see what is going on `inside' another program while it executes -- or what another program was doing at the moment it crashed.
GDB can do four main kinds of things (plus other things in support of these) to help you catch bugs in the act:

• Start your program, specifying anything that might affect its behavior.
• Make your program stop on specified conditions.
• Examine what has happened, when your program has stopped.
• Change things in your program, so you can experiment with correcting the effects of one bug and go on to learn about another.

The program being debugged can be written in Ada, C, C++, Objective-C, Pascal (and many other languages). Those programs might be executing on the same machine as GDB (native) or on another machine (remote). GDB can run on most popular UNIX and Microsoft Windows variants.

GDB is a well known debugger library for  c and c++.  GDB is usually use for large and distributed system debugging. GDB is command line tool and has almost all function that you use in debugging using IDE.
I have explain some basic and useful methods of GDB that help up in learning GDB and debugging your programs
Installation

Before you go for installation first check either GDB is already installed or not. You check using this command
gd -help

I GDB is already installed on your machine, it shows all the available options of GDB.
If not installed it show message to install GDB.  Then you install it manually but first check these prerequisites

• An ANSI-compliant C compiler (gcc is recommended - note that gdb can debug codes generated by other compilers)
• 115 MB of free disk space is required on the partition on which you're going to build gdb.
• 
  
• 20 MB of free disk space is required on the partition on which you're going to install gdb.

Now run this command to install GDB

sudo apt-get install gdb

 GDB Command Line Arguments:

Starting GDB:

• gdb name-of-executable
• gdb -e name-of-executable -c name-of-core-file
• gdb name-of-executable --pid=process-id
  Use ps -auxw to list process id's: Attach to a process already running:
  

  [prompt]$ ps -auxw | grep myapp
  user1     2812  0.7  2.0 1009328 164768 ?      Sl   Jun07   1:18 /opt/bin/myapp
  [prompt]$ gdb /opt/bin/myapp 2812
  OR
  [prompt]$ gdb /opt/bin/myapp --pid=2812
              

Command line options: (version 6. Older versions use a single "-")

Option	Description
--help -h	List command line arguments
--exec=file-name -e file-name	Identify executable associated with core file.
--core=name-of-core-file -c name-of-core-file	Specify core file.
--command=command-file -x command-file	File listing GDB commands to perform. Good for automating set-up.
--directory=directory -d directory	Add directory to the path to search for source files.
--cd=directory	Run GDB using specified directory as the current working directory.
--nx -n	Do not execute commands from ~/.gdbinit initialization file. Default is to look at this file and execute the list of commands.
--batch -x command-file	Run in batch (not interactive) mode. Execute commands from file. Requires -x option.
--symbols=file-name -s file-name	Read symbol table from file file.
--se=file-name	Use FILE as symbol file and executable file.
--write	Enable writing into executable and core files.
--quiet -q	Do not print the introductory and copyright messages.
--tty=device	Specify device for running program's standard input and output.
--tui	Use a terminal user interface. Console curses based GUI interface for GDB. Generates a source and debug console area.
--pid=process-id -p process-id	Specify process ID number to attach to.
--version	Print version information and then exit.

GDB Commands:

Commands used within GDB:

Command	Description
help	List gdb command topics.
help topic-classes	List gdb command within class.
help command	Command description. eg help show to list the show commands
apropos search-word	Search for commands and command topics containing search-word.
info args i args	List program command line arguments
info breakpoints	List breakpoints
info break	List breakpoint numbers.
info break breakpoint-number	List info about specific breakpoint.
info watchpoints	List breakpoints
info registers	List registers in use
info threads	List threads in use
info set	List set-able option
Break and Watch
break funtion-name break line-number break ClassName::functionName	Suspend program at specified function of line number.
break +offset break -offset	Set a breakpoint specified number of lines forward or back from the position at which execution stopped.
break filename:function	Don't specify path, just the file name and function name.
break filename:line-number	Don't specify path, just the file name and line number. break Directory/Path/filename.cpp:62
break *address	Suspend processing at an instruction address. Used when you do not have source.
break line-number if condition	Where condition is an expression. i.e. x > 5 Suspend when boolean expression is true.
break line thread thread-number	Break in thread at specified line number. Use info threads to display thread numbers.
tbreak	Temporary break. Break once only. Break is then removed. See "break" above for options.
watch condition	Suspend processing when condition is met. i.e. x > 5
clear clear function clear line-number	Delete breakpoints as identified by command option. Delete all breakpoints in function Delete breakpoints at a given line
delete d	Delete all breakpoints, watchpoints, or catchpoints.
delete breakpoint-number delete range	Delete the breakpoints, watchpoints, or catchpoints of the breakpoint ranges specified as arguments.
disable breakpoint-number-or-range enable breakpoint-number-or-range	Does not delete breakpoints. Just enables/disables them. Example: Show breakpoints: info break Disable: disable 2-9
enable breakpoint-number once	Enables once
continue c	Continue executing until next break point/watchpoint.
continue number	Continue but ignore current breakpoint number times. Usefull for breakpoints within a loop.
finish	Continue to end of function.
Line Execution
step s step number-of-steps-to-perform	Step to next line of code. Will step into a function.
next n next number	Execute next line of code. Will not enter functions.
until until line-number	Continue processing until you reach a specified line number. Also: function name, address, filename:function or filename:line-number.
info signals info handle handle SIGNAL-NAME option	Perform the following option when signal recieved: nostop, stop, print, noprint, pass/noignore or nopass/ignore
where	Shows current line number and which function you are in.
Stack
backtrace bt bt inner-function-nesting-depth bt -outer-function-nesting-depth	Show trace of where you are currently. Which functions you are in. Prints stack backtrace.
backtrace full	Print values of local variables.
frame frame number f number	Show current stack frame (function where you are stopped) Select frame number. (can also user up/down to navigate frames)
up down up number down number	Move up a single frame (element in the call stack) Move down a single frame Move up/down the specified number of frames in the stack.
info frame	List address, language, address of arguments/local variables and which registers were saved in frame.
info args info locals info catch	Info arguments of selected frame, local variables and exception handlers.
Source Code
list l list line-number list function list - list start#,end# list filename:function	List source code.
set listsize count show listsize	Number of lines listed when list command given.
directory directory-name dir directory-name show directories	Add specified directory to front of source code path.
directory	Clear sourcepath when nothing specified.
Machine Language
info line info line number	Displays the start and end position in object code for the current line in source. Display position in object code for a specified line in source.
disassemble 0xstart 0xend	Displays machine code for positions in object code specified (can use start and end hex memory values given by the info line command.
stepi si nexti ni	step/next assembly/processor instruction.
x 0xaddress x/nfu 0xaddress	Examine the contents of memory. Examine the contents of memory and specify formatting. n: number of display items to print f: specify the format for the output u: specify the size of the data unit (eg. byte, word, ...) Example: x/4dw var
Examine Variables
print variable-name p variable-name p file-name::variable-name p 'file-name'::variable-name	Print value stored in variable.
p *array-variable@length	Print first # values of array specified by length. Good for pointers to dynamicaly allocated memory.
p/x variable	Print as integer variable in hex.
p/d variable	Print variable as a signed integer.
p/u variable	Print variable as a un-signed integer.
p/o variable	Print variable as a octal.
p/t variable x/b address x/b &variable	Print as integer value in binary. (1 byte/8bits)
p/c variable	Print integer as character.
p/f variable	Print variable as floating point number.
p/a variable	Print as a hex address.
x/w address x/4b &variable	Print binary representation of 4 bytes (1 32 bit word) of memory pointed to by address.
ptype variable ptype data-type	Prints type definition of the variable or declared variable type. Helpful for viewing class or struct definitions while debugging.
GDB Modes
set gdb-option value	Set a GDB option
set logging on set logging off show logging set logging file log-file	Turn on/off logging. Default name of file is gdb.txt
set print array on set print array off show print array	Default is off. Convient readable format for arrays turned on/off.
set print array-indexes on set print array-indexes off show print array-indexes	Default off. Print index of array elements.
set print pretty on set print pretty off show print pretty	Format printing of C structures.
set print union on set print union off show print union	Default is on. Print C unions.
set print demangle on set print demangle off show print demangle	Default on. Controls printing of C++ names.
Start and Stop
run r run command-line-arguments run < infile > outfile	Start program execution from the beginning of the program. The command break main will get you started. Also allows basic I/O redirection.
continue c	Continue execution to next break point.
kill	Stop program execution.
quit q	Exit GDB debugger.

 GDB Operation:

• Compile with the "-g" option (for most GNU and Intel compilers) which generates added information in the object code so the debugger can match a line of source code with the step of execution.
• Do not use compiler optimization directive such as "-O" or "-O2" which rearrange computing operations to gain speed as this reordering will not match the order of execution in the source code and it may be impossible to follow.
• control+c: Stop execution. It can stop program anywhere, in your source or a C library or anywhere.
• To execute a shell command: ! command
  or shell command
• GDB command completion: Use TAB key
  info bre + TAB will complete the command resulting in info breakpoints
  Press TAB twice to see all available options if more than one option is available or type "M-?" + RETURN.
• GDB command abreviation:
  info bre + RETURN will work as bre is a valid abreviation for breakpoints