GNU DEBUGGER / GDB

OPERATING SYSTEMS

AGH UNIVERSITY SCIENCE AND TECHNOLOGY

What is Debugger?

• A debugger is a program that is used to test and debug other programs.

There are many ways to do debugging such as• printing out messages to the screen using a debugger• just thinking about what the program is doing and

making an educated guess as to what the problem is.

WHAT IS DEBUGGER?

• Step by step running of program

• Stopping debugging when program encounter a breakpoint

• Tracking values of variables.• Debugging while program is

running.

Debuggers have

functions like

BASIC FUNCTIONS OF DEBUGGER

• To learn which statement or expression causes a error

• To specify which line/lines contains error

• To learn the instant values of variables at particular point during execution of program.

• To learn result of program at particular point

• To learn execution sequence of statements in a program.

What is GDB?

GNU Debugger which is also known as GDB is debugger for GNU operating system that provides to see and distunguish the errors inside of computer program while the program executes.

History of GDB

• Written by Richard Stallman in 1986.

• Developed by John Gilmore between 1990 and 1993.

• Now maintained by GDB Steering Committee which is part of Free Software Foundation.

Mascot of GDB

FEATURES OF GDB

GDB has mainly 4 functionality to find bugs in program

• GDB can specify any thread in program which might affect

program’s behavior

• User can monitor and modify values of programs’ variables

• GDB can be stop your program in specific conditions.

• GDB can check and analyze all program when program stops.

• GDB can correct bugs in program after detecting it.

USAGE AREAS OF GDB

• GDB is not only used in GNU operating system but also use like portable debugger which runs Unix-like systems. GDB can run on UNIX and Microsoft Windows systems

GDB Works for• Ada• C++• C• Objective C• Pascal languages

TECHNICAL DETAILS

Main Target Processors of GDB

• Alpha

• ARM

• AVR

• H8/300

• System/370

• System/390

• x86 / x86-64

• Motorola 68000

• PowerPC

• GDB is actively developing and New versions are releasing periodically. With version 7.0 It supports ‘reverse debugging’.

• Reverse debugging in gdb means gdb can allow you to "step" or "continue" your program backward in time, reverting it to an earlier execution state.

REMOTE DEBUGGING

Remote operation is when GDB runs on one machine and the program being debugged runs on another.

•  For example, you might use remote debugging on an operating system kernel or on a another system which does not have a general purpose operating system powerful enough to run a full featured debugger.

If you want to remote debugging there are 5 stages that you can do

1. Connecting: Connecting to a remote target

2. File Transfer: Sending files to a remote system

3. Server: Using the gdbserver program

4. Remote Configuration: Remote configuration

5. Remote Stub: Implementing a remote stub

REMOTE DEBUGGING

1. Connecting

GDB can communicate with the target over a serial line, or over an IP network using TCP or UDP. In each case, gdb uses the same protocol for debugging your program.The target remote command establishes a connection to the target.

For example, to connect to port 8080 on a terminal server named webpages:

target remote webpages:8080

Disconnecting:

The disconnect command disconnects the connection with remote target. After this command gdb is free to connect another target.

REMOTE DEBUGGING

2. File Transfer

Commands for sending to remote system:

remote put hostfile targetfile:

Copy file hostfile from the host system to targetfile on the target system.

remote get targetfile hostfile:

Copy file targetfile from the target system to hostfile on the host system.

remote delete targetfile:

Delete targetfile from the target system.

REMOTE DEBUGGING

3. Server• gdbserver is a control program for Unix-like systems, which allows you

to connect your program with a remote gdb via target remote but without linking in the usual debugging stub.

• To use the server, you must tell it how to communicate with gdb; the name of your program; and the arguments for your program. The usual syntax is:

target> gdbserver comm program [ args ... ]

REMOTE DEBUGGING

4. Remote Configuration

Remote Configuration commands show configuration options available when debugging remote programs.

set remoteaddresssize bits:

Set the maximum size of address in a memory packet to the specified number of bits.

show remoteaddresssize:

Show the current value of remote address size in bits.

REMOTE DEBUGGING

• 5. Remote Stub

• The stub files provided with gdb implement the target side of the communication protocol, and the gdb side is implemented in the gdb source file for example in remote.c

• The debugging stub is specific to the architecture of the remote machine; for example, use sparc-stub.c to debug programs on sparc boards.

• These working remote stubs are distributed with gdb:

• i386-stub.c

• For Intel 386 and compatible architectures.

• m68k-stub.c

• For Motorola 680x0 architectures.

FEATURES OF GDB

GDB uses command line interface.

GDB has 3 feature which are commonly used:

1. Compiling

2. Invoking and Quitting GDB

3. Commands

COMPILING

Click icon to add pictureTo prepare program for debugging with gdb, it must be compiled it with the -g flag. So, if your program is in a source file called gizem.cpp and you want to put the executable in the file gizem;

$ g++ -g gizem.cpp

$ gdb ./a.out

$ (gdb) run

GCC’s C++ Compiler

The g++ compiler accepts both single-letter options, such as -o, and multiletter options, such as -ansi. Because it accepts both types of options you cannot group multiple single-letter options together, as you may be used to doing in many GNU and Unix/Linux programs.

• For example, the multiletter option -pg is not the same as the two single-letter options -p -g. The -pg option creates extra code in the final binary that outputs profile information for the GNU code profiler, gprof. On the other hand, the -p -g options generate extra code in the resulting binary that produces profiling information for use by the prof code profiler (-p) and causes gcc to generate debugging information using the operating system’s normal format (-g).

COMPILING AND RUNNING PROGRAM

C++ CODE EXAMPLE

• $ g++ -g gizem.cpp:

Command for compiling source file. Invoke g++ passing name of the source file. –g flag used in order to include appropriate debug information on the binary generated.

• $ gdb ./a.out:

Result on Linux and Unix systems generates executable file named a.out in the current directory. We can execute this executable file by typing ./a.out

• $ (gdb) run:

You can run the program by typing run. Program runs with current arguments.

GCC’s C++ Compiler

Despite its sensitivity to the grouping of multiple single-letter options, you are generally free to

mix the order of options and compiler arguments on the gcc command line.

$ g++ example.cpp -g –o example = $ g++ -g –o gcc example.cpp example

• g++: compiler is for c++ language.

• example.cpp: Source file name of c++ code

• -g: Flag of Compiling for given source file

• -o: Flag for specifying the name of the output file. The executable will be named a.out unless you use this option.

• example: Output file name

./example

This command used to execute output of compiled program. After executing this code output of program can be seen.

COMPILING C CODE

• Compiling a single source file, add.c, using gcc is easy—just invoke gcc, passing the name of the source file as the argument.

$ gcc add.c

$ ./a.out

• To define the name of output file that gcc produces, -o option can be used as in c++ mode.

$gcc add.c –o output

$./output

• If you are compiling multiple source files using gcc, you can simply specify them all on the gcc command line, as in the following example, which leaves the compiled and linked executable in the file named addition.

$gcc add.c helper.c –o addition

./addition

COMPILING C CODE

Example of executing predefined named output and executing default output.

Debugging

Debugging program named broken.cpp with logical error:

After completing compile and execute processes of program in order to start to debugger using output file named broken:

$ gdb broken

To set breakpoint at specific line we can use b command and number of line

(gdb) b 43

Debugging

• After setting breakpoints, we start to run program in debugger with;

(gdb) run

• Now program runs and ask us for input after entering inputs of program, program execution stops at first breakpoint.

Debugging

• If we want to investigate function’s inside where program stopped executing at breakpoint, we can step into function’s inside with

(gdb) step

• Program controls it’s first statement of the function ComputeSeriesValue (x=2, n=3)

Debugging

• To continue to debug program we can use some specific commands such as

(gdb) next

• Next command is similar to step command except it will step over functions and also we can use n and s instead of next and step respectively.

(gdb) n

(gdb) s

• If the command is simply a repeat of the previous command, you can just hit return, which will execute the last command.

(gdb)

Debugging

• If you want to know where you are in the program's execution you can view the contents of the stack using the backtrace command.

(gdb) bt

Debugging

• We can step through the program and examine the values using the print command.

• The print command reveals that the value of fact variable never changes. Note that the function is returning a value of 0 for the function call ComputeFactorial(number=0). This is an ERROR.

Debugging program with Floating Point exception error

After debugging program and executing it we see that program has floating point exception error.

Debugging program with Floating Point exception error

This means problems occur in line 17 and in return a/b expression.

Invoking and Quitting GDB

To start gdb, just type gdb at the unix prompt. Gdb will give you a prompt that looks like this:

We can quit GDB using quit command

(gdb) quit

COMMANDS

• Help

• File

• Run

• Break

• Delete

• Clear

• Continue

• Step

• Next

• Until

• List

• print

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