GDB Tutorial

source: http://www.cs.princeton.edu/~benjasik/gdb/gdbtut.html

A debugger is a program that runs other programs, allowing the user to exercise control over these programs, and to examine variables when problems arise. The most popular debugger for UNIX systems is GDB, the GNU debugger. GDB has tons of features, however, you only need to use a few for it to be very helpful. There is complete documentation for GDB online, or you can read the man page, and the quick reference sheet is very handy.

Basic features of a debugger

When you are execute a program that does not behave as you like, you need some way to step through you logic other than just looking at your code. Some things you want to know are:

What statement or expression did the program crash on?
If an error occurs while executing a function, what line of the program contains the call to that function, and what are the parameters?
What are the values of program variables at a particular point during execution of the program?
What is the result of a particular expression in a program?

Starting GDB

You need to tell the lcc compiler that you plan to debug your program. You use the -g flag to do this. The command will now look like

lcc -g trees.c

which will create the a.out executable. To run this under the control of gdb, you type

gdb a.out

This starts up the text interface to the debugger. It's much easier to use gdb under emacs, but we'll use this interface to describe the commands

GDB commands

When gdb starts, your program is not actually running. It won't run until you tell gdb how to run it. Whenever the prompt appears, you have all the commands on the quick reference sheet available to you.

run command-line-arguments

Starts your program as if you had typed

a.out command-line arguments

a.out < somefile

break place

Creates a breakpoint; the program will halt when it gets there. The most common breakpoints are at the beginnings of functions, as in

(gdb) break Traverse

Breakpoint 2 at 0x2290: file main.c, line 20

break main

(gdb) break 20

Breakpoint 2 at 0x2290: file main.c, line 20

Breakpoint 1, Traverse(head=0x6110, NumNodes=4)

  at main.c:16

(gdb)

C-c C-b

C-x SPC

delete N

Removes breakpoint number N. Leave off N to remove all breakpoints. info break gives info about each breakpoint

help command

Provides a brief description of a GDB command or topic. Plain help lists the possible topics

step

Executes the current line of the program and stops on the next statement to be executed

next

Like step, however, if the current line of the program contains a function call, it executes the function and stops at the next line.

step would put you at the beginning of the function

finish

Keeps doing nexts, without stepping, until reaching the end of the current function

Continue

Continues regular execution of the program until a breakpoint is hit or the program stops

file filename

Reloads the debugging info. You need to do this if you are debugging under emacs, and you recompile in a different executable. You MUST tell gdb to load the new file, or else you will keep trying to debug the old program, and this will drive you crazy

where

Produces a backtrace - the chain of function calls that brought the program to its current place. The command backtrace is equivalent

print E

prints the value of E in the current framein the program, where E is a C expression (usually just a variable). display is similar, except every time you execute a next or step, it will print out the expression based on the new variable values

quit

Leave GDB. If you are running gdb under emacs,

C-x 0

The goal of gdb is to give you enough info to pinpoint where your program crashes, and find the bad pointer that is the cause of the problem. Although the actual error probably occurred much earlier in the program, figuring out which variable is causing trouble is a big step in the right direction. Before you seek help from a TA or preceptor, you should try to figure out whereyour error is occurring

GDB under Emacs

Emacs provides a much better interface to gdb that saves a lot of typing and confusion. Executing the Emacs command M-x gdb starts up a new window running gdb. If you set a breakpoint on the main function and then run the program with the correct arguments, gdb will split the window in two, with your source code on the bottom and a gdb command line in the top. Emacs intercepts output from gdb and interprets it for you. When you stop at a breakpoint, Emacs will take the file and line number reported by gdb, and display the file contents, with the point of the breakpoint market. As you step through a program, Emacs will follow your pgoress in the source file. The command C-x SPC will place a breakpoint at the current point of the file you are in.

If you have any questions, check out the online manual, or send me mail