GDB Tutorial

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.

Also, this reference card is useful when first learning the gdb commands.

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

gcc -g trees.c

which will create the default 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 an IDE, but this the real gdb interface.

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
```
or you can do the following
```
a.out < somefile
```
to pipe a file as standard input to your program
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
```
The command break main stops at the beginning of execution. You can also set breakpoints at a particular line in a source file:
```
(gdb) break 20

Breakpoint 2 at 0x2290: file main.c, line 20
```
When you run your program and it hits a breakpoint, you'll get a message and prompt like this.
```
Breakpoint 1, Traverse(head=0x6110, NumNodes=4)

  at main.c:16

(gdb)
```
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.

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.

Don't get an RTFM. Use gdb.

If you have any questions, check out the full manual.